comet university faculty hydrometeorology course june 2000 dennis l. johnson
TRANSCRIPT
![Page 1: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/1.jpg)
COMET University Faculty
Hydrometeorology CourseJune 2000
Dennis L. Johnson
![Page 2: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/2.jpg)
Dennis L. Johnson, Asst. ProfessorJuniata College
Environmental Science & Studies(814) 641-5335 (Phone)(814) 641 – 3685 (Fax)
[email protected] (Email)Http://www.Juniata.edu/~johnson/
![Page 3: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/3.jpg)
Usual HoughtonUsual Houghton
![Page 4: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/4.jpg)
Hydrometeorological Operations
in the “Modernized NWS”
![Page 5: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/5.jpg)
The Runoff Picture
• Hydrology is long term and short term….
• In this course we will mainly focus on the short term:
• Floods & flood flows.
• Generating runoff/high flows.
• Predicting/forecasting flows.
• Space/time scales.
![Page 6: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/6.jpg)
What’s a Flood?
• What is a flood?????• A rather elusive definition• Generally contains terms like:
– High water– High flows– Normal water course– Human impact(s)– Etc…
![Page 7: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/7.jpg)
Recipe(s) for a Flood
• What causes a flood?
• What are the conditions?
• What are the types of flooding situations?
• Your area or other areas…..
![Page 8: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/8.jpg)
My Recipes
• “BIG” heavy soaking rains…
• Low infiltration rates
• Snow melt
• Rain on snow
• Very intense precipitation
• Dam failure
• Others….??
![Page 9: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/9.jpg)
Does a Flood Have to Happen in a Defined Water Course or
Waterway?
….and If a Flood Does Occur in an Overland Situation – Does the Nearest Stream Even Feel It?
![Page 10: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/10.jpg)
FEMA - NFIP…(www.fema.gov/nfip)
Flood--A general and temporary condition of partial or complete inundation of normally dry land areas from:
Overflow of inland or tidal waters.
The unusual and rapid accumulation or runoff of surface waters from any source.
Mudslides (i.e., mudflows) which are proximately caused by flood, as defined above, and are akin to a river of liquid and flowing mud on the surface of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current.
The collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding the cyclical levels which result in flood, as defined above.
![Page 11: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/11.jpg)
What Are the Defining Characteristics of a Flood?
• Timing – rise time, recession, duration.
• Flows – peak flows, magnitude (statistical).
• Precipitation – intensity, duration, frequency….
![Page 12: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/12.jpg)
What Controls the Timing, Flow, and Precipitation?
• The hydrology – short term and long term.
• The meteorology – short term (weather/storm type) and long term (climate).
![Page 13: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/13.jpg)
Big Picture
Long term and short term
![Page 14: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/14.jpg)
Long Term(Climate and the Hydrologic Cycle)
![Page 15: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/15.jpg)
Short TermWeather (storm type) & “current hydrologic
conditions”
![Page 16: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/16.jpg)
Some of the “Right” Combinations….
• Precipitation –vs.- infiltration– Precipitation intensity > infiltration rate– Precipitation total > infiltration capacity– “Storage” in the system is full– Human induced high water or flows– Natural alterations to the watershed
![Page 17: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/17.jpg)
Our Focus
• More on the short term..
• The combination(s) of precipitation and hydrologic conditions that lead up to a potential flooding situation…
• “basin hydrology” – although basin hydrology looks at the long term hydrologic budget, as well.
![Page 18: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/18.jpg)
Let’s Take a Minute to Look at Hydrology and the Properties,
Units, Concepts, & Terminology
![Page 19: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/19.jpg)
HydrologyHydrology
… An Earth Science. It Encompasses the Occurrence, Distribution, Movement, and
Properties of the Waters of the Earth and Their Environmental Relationships." (Viessman,
Knapp, Lewis, & Harbaugh, 1977 - Introduction to Hydrology, Harper & Row
Publishers, New York)
![Page 20: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/20.jpg)
History of Hydrology
(Hydrometeorology)
![Page 21: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/21.jpg)
Early on….
• Early philosophers speculated on the hydrologic cycle:
• Homer believed that there existed large subterranean reservoirs that fed the rivers, seas, springs, and wells - was he wrong?
• Homer did understand the dependence of flow in the Greek aqueducts on conveyance and velocity!
![Page 22: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/22.jpg)
History, Cont....
• In the first century B.C., Marcus Vitruvius in the treatise de Architectura Libri Decem (the engineers chief handbook), vol. 8 hypothesized that rain and snow falling in the mountains infiltrated into the earth’s surface and appeared in the lowlands as springs and streams.....
![Page 23: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/23.jpg)
Early Success.....• 4000 b.C. The Egyptians built a dam on the Nile
to allow barren lands to again be used for agricultural purposes.
![Page 24: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/24.jpg)
More Early Successes
• 1000’s of years later, a canal to carry fresh water from Cairo to Suez was built.
• Towns in Mesopotamia were protected by flooding from high earthen walls.
![Page 25: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/25.jpg)
Early Disputes and Rules
• The cities of Lagash and Umma of Mesopotamia have documented water disputes.
• The Romans decree:
• Ne quis aquam oletato dolo malo ubi publice saliet si quis oletarit sestertiorum X mila multa esto.
• It is forbidden to pollute the public water supply; Any deliberate offender shall be punished by a fine of 10,000 sesterces!
![Page 26: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/26.jpg)
Qualitative Understanding
• Near end of 15th century, Leonardo da Vinci and Bernard Palissy independently reached conclusions on the hydrologic cycle - based on a philosophical understanding.
• There was still a lack of quantitative understanding of the hydrologic cycle.
![Page 27: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/27.jpg)
The 17th Century
• Perrault, Mariotte, and Halley began quantitative measurements and applications.
• Perrault measured rainfall and runoff over the seine river drainage basin for ~ 3 years - he illustrated that rainfall WAS adequate in quantity to account for river flows.
• Mariotte gauged the velocity of the flow in the river seine and estimated flows by also estimating river cross sectional areas.
• Halley was an astronomer! He estimated evaporation from the Mediterranean sea and correlated it to river flows into the med, concluding that river flows were sufficient enough to provide that volume of water.
![Page 28: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/28.jpg)
The 18th Century
• Bernoulli - famous for hydraulics and fluid mechanics - the piezometer, the pitot tube, and Bernoulli’s theorem.
• The Chezy formula (channel flow).
![Page 29: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/29.jpg)
The 19th Century
• Hagen-Poiseuille - capillary flow equation.• Darcy’s - flow in porous media.• Duptuit-Thien well formula.• Manning - open channel flow.• Systematic stream gaging.• Mostly empirical in nature.
![Page 30: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/30.jpg)
The 20th Century
– Government agencies began to develop programs – good or bad?
– Rational analysis begins.– Sherman - unit hydrograph theory.– Horton - infiltration theory.– Snyder - unit hydrograph.– Clark - unit hydrograph.– Etc...........
![Page 31: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/31.jpg)
Modern Day
• Very computer and data intensive
• High tech instruments
• Scale issues
• Policy issues
• Etc.................
• “Diamond edge on an old axe”……
![Page 32: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/32.jpg)
Units & Properties of WaterUnits & Properties of Water
Property Symbol Value CommentsDensity
(mass/volume)
~1.94 slugs/ft3
~ 1.0 g/cm3Slug = lb*s2/ft
Specific Weight(weight/volume)
62.4 Lbs/ft3
9.81 kN/m3
g
Specific Volume
Specific Gravity s.g. 1.0 for water@ 32.9o F
s.g.fluid =gfluid/gwater
Vapor Pressure ~0.4 psi Vapor pressure ofthe fluid - not the
atmosphere
![Page 33: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/33.jpg)
The WatershedThe Watershed• A watershedwatershed is an area of land that drains to a single outlet and is separated from other watersheds by a divide. • Every watershed has a drainage areadrainage area.• Related terms: drainage drainage basinbasin, sub-basinsub-basin, sub-areasub-area.
![Page 34: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/34.jpg)
AreaArea
• 1 acre = 43,560 ft2
• 1 mi2 = 640 acres
• 1 hectare = 100m x 100m = 2.471 acres = 10,000 m2
• 1 km2 = 0.386 mi2
![Page 35: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/35.jpg)
VolumeVolume
• 1 acre-foot = 1 ac-ft = 1 acre of water x 1 foot deep = 43,560 x 1 = 43,560 ft3.
• 1 ac-inch = 1 acre x 1 inch deep = 43,560 x 1/12 = 3,630 ft3.
• 1 ft3 = 7.48 gallons.
• 1 gallon H2O ~ 8.34 lbs.
![Page 36: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/36.jpg)
Runoff VolumeRunoff Volume
• 1-inch of runoff over 1 square mile :
• 1/12 feet x 1 mi2 x 640 acres/mi2 x 43,560 ft2/mi2 = 2,323,200 ft3
![Page 37: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/37.jpg)
DischargeDischarge
• 1 cfs = 1 cubic foot per second
• 1 cfs x 7.48 gal/ft3 x 3600 sec/hr x 24 hrs/day = 646,272 gpd = 0.646 MGD
• 1 cfs x 3600 sec/hr x 24 hrs/day = 86,400 cfs/day
• 86,400 cfs/day x 1 ac-ft/43,560 ft3 = 1.983 ac-ft/day (~ 2 ac-ft/day)
• 1.983 ac-ft/day x 12 inches/ft x 1 day/24 hrs = 0.992 ac-in/hr
• 1 ac-in/hr x 43,560 ft3/ac-ft x 1 hr/3600 sec x 1 ft/12 inches = 1.008 cfs
![Page 38: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/38.jpg)
PowerPower
• Hp = HQ/550
• 1 hp = 550 ft*lb/sec = 0.7547 kilowatts
![Page 39: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/39.jpg)
Hydrology TerminologyHydrology Terminology•StreamflowStreamflow is the movement of water through a channel.•The cross-sectional areacross-sectional area of a stream is the region bounded by the walls of the stream and the water surface. The cross-sectional area is illustrated below.•See also Manning’s “n”.Manning’s “n”.
Stream Flow
Cross-sectional Area
![Page 40: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/40.jpg)
Hydrology TerminologyHydrology Terminology•Manning’s “n”Manning’s “n” is a measure of the roughness of a surface, and in streamflow it is the roughness of the channel bottom and it’s sides.
Diagram 2 will have a higher Manning’s “n”Manning’s “n” because it has rougher surface due to the jagged bottom and pebbles.
Diagram 1 Diagram 2
![Page 41: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/41.jpg)
Hydrology TerminologyHydrology Terminology
HydrologicHydrologic HydraulicHydraulic
RoutingRouting
•RoutingRouting is used to account for storage and translation effects.
t
SSOOII
122121 2
1
2
1
![Page 42: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/42.jpg)
Hydrology TerminologyHydrology Terminology
0.0000
100.0000
200.0000
300.0000
400.0000
500.0000
600.0000
700.0000
0.0000 1.0000 2.0000 3.0000 4.0000 5.0000 6.0000 7.0000 8.0000 9.0000 10.0000
Generalized effect of routing
![Page 43: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/43.jpg)
Hydrology TerminologyHydrology Terminology• SnowfallSnowfall is a form of precipitation that comes down in white or translucent ice crystals. •SnowmeltSnowmelt is the excess water produced by the melting of snow. This leads to flooding possibilities in the spring when temperatures begin to rise. There is generally a delay in the snowmelt response of a basin due to the melting process and travel times.•SnowpackSnowpack is the amount of annual accumulation at
higher elevations.
![Page 44: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/44.jpg)
Hydrology TerminologyHydrology Terminology
•RunoffRunoff is the excess precipitation and is often considered a “fast” response.•Overland flowOverland flow is the flow of water across the land surface.•Sub-surface flowSub-surface flow is the flow of water through the soil layers to the stream.•BaseflowBaseflow is the flow in a channel due to ground water or subsurface supplies. The baseflow is generally increased by precipitation events that produce enough infiltration.
![Page 45: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/45.jpg)
Hydrology TerminologyHydrology Terminology
• InfiltrationInfiltration is the movement of water from the surface into the soil.•The rate of infiltration is based on a number of factors, including but not limited to:
•soil types•current conditions•precipitation intensity
![Page 46: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/46.jpg)
Hydrology TerminologyHydrology Terminology
•The velocityvelocity of the flow is very dependent on the slope of the stream bottom. The greater the slope the greater the potential velocity of the flow.
•The “wave” speed“wave” speed is the velocity of the flood wave down the channel. The speed of this wave affects how quickly the downstream area will effected.
![Page 47: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/47.jpg)
Hydrology TerminologyHydrology Terminology
Energy Grade Line
Hydraulic Grade Line(water surface)
Channel Bottom
headloss
g
v
2
22
g
v
2
21
Elevation Head
Depth1
Depth2
Datum
•The energy grade lineenergy grade line represents the depth of the water surface and the velocity component of the Bernoulli equation. •The hydraulic grade linehydraulic grade line represents the depth of the water surface.
![Page 48: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/48.jpg)
Hydrology TerminologyHydrology Terminology•Karst hydrology Karst hydrology is caused by pores and holes in limestone formations. This increases the infiltration into the limestone, reducing the runoff potential.
•The slopeslope changes the speed of runoff and therefore effects collection times.
![Page 49: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/49.jpg)
Hydrology TerminologyHydrology Terminology
•The frequencyfrequency of a storm event is described by its return periodreturn period. For example a two year storm event has a 1 in 2 chance of occurring in any given year.
•The probabilityprobability is also affected by the return period. Thus the probability of a 2 year storm occurring is 50%. The probability of a 100-year
event occurring is 1/100 or 1%
![Page 50: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/50.jpg)
Basin Hydrology
![Page 51: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/51.jpg)
PrecipitationPrecipitation• ... primary "input" for the hydrologic
cycle (or hydrologic budget). • … The patterns of the precipitation are
affected by large scale global patterns, mesoscale patterns, "regional" patterns, and micro-climates.
• … In addition to the quantity of precipitation, the spatial and temporal distributions of the precipitation have considerable effects on the hydrologic response.
![Page 52: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/52.jpg)
SnowSnow• … response mechanisms of snow are at a much slower
time scale than for most of the other forms of precipitation.
• … The melt takes place and the runoff is "lagged" due to the physical travel processes.
• … Items to consider in the snowmelt process are the current "state" of the pack and the snow water equivalent of the snow pack., as well as the melt potential of the current climate conditions.
• … A rain-on-snow event may produce very high runoff rates and is often a difficult situation to predict due to the integral nature of the runoff and melt processes. The timing of these events is often very difficult to predict due to the inherent "lag" in the responses.
![Page 53: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/53.jpg)
Snow HydrologySnow Hydrology
Special Thanks, Credit, and Recognition to Special Thanks, Credit, and Recognition to Don ClineDon Cline
And the And the
National Operational Hydrologic Remote Sensing Center
![Page 54: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/54.jpg)
Why is Snow Important?Why is Snow Important?
![Page 55: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/55.jpg)
Why is Snow Important?Why is Snow Important?
• Water Resources
• Flooding
• Economics
• Transportation
![Page 56: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/56.jpg)
Snow HydrologySnow Hydrology
• Understanding and predicting the physical processes of:
• Snow Accumulation
• Ablation
• Melt Water Runoff
![Page 57: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/57.jpg)
Snow HydrologySnow Hydrology
• 4 Simultaneous Estimation Problems
– the quantity of water held in snow packs– the magnitude and rate of water lost to the
atmosphere by sublimation– the timing, rate, and magnitude of snow melt– the fate of melt water
![Page 58: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/58.jpg)
Snow Cover DistributionSnow Cover Distribution
![Page 59: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/59.jpg)
Snow Cover DistributionSnow Cover Distribution• Three Spatial Scales
– Macroscale• Areas up to 106 km2
• Characteristic Distances of 10-1000 km• Dynamic meteorologic effects are important
– Mesoscale• Characteristic Distances of 100 m to 10 km• Redistribution of snow along relief features due to wind• Deposition and accumulation of snow may be related to terrain
variables and to vegetation cover– Microscale
• Characteristic Distances of 10 to 100 m• Differences in accumulation result from variations in air flow patterns
and transport
![Page 60: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/60.jpg)
Snow Cover DistributionSnow Cover Distribution• Effect of Topography
– The depth of seasonal snow cover usually increases with elevation if other influencing factors do not vary with elevation
• This trend is generally due to:– increase in the number of snowfall events
– decrease in evaporation and melt
• The rate of increase with elevation may vary widely from year-to-year
– However, elevation alone is not a causative factor in snow cover distribution
• Many other factors must be considered:– slope, aspect, vegetation, wind, temperature, and characteristics of the
parent weather systems
![Page 61: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/61.jpg)
Snow Cover DistributionSnow Cover Distribution• Effect of Vegetation
– Snow falling into a vegetation canopy is influenced by two phenomena:
• Turbulent air flow above and within the canopy
– may lead to variable snow input rates and microscale variation in snow loading on the ground
• Direct interception of snow by the canopy elements
– may either sublimate or fall to the ground
– Processes are related to vegetation type, vegetation density, and the presence of nearby open areas
![Page 62: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/62.jpg)
Snow Cover DistributionSnow Cover Distribution• Forested Environments
– Differences in snow accumulation between different species of conifers is usually small compared to between coniferous and deciduous stands
• coniferous stands are all relatively efficient snow interceptors
• Once intercepted, cohesion between snow particles helps keep snow in the canopy for extended time periods
– snow is more susceptible to sublimation losses in the canopy than on the forest floor
» High surface area to mass ratio
![Page 63: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/63.jpg)
Snow Cover DistributionSnow Cover Distribution• Forested Environments
– Most studies show greater snow accumulation in clearings than in the forest
– Most of the difference develops during storms, not between storms
• redistribution of intercepted snow by wind to clearings is not typically a significant factor
– Interception and subsequent sublimation are the major factors contributing to the difference
20-45%Greater SnowAccumulation
![Page 64: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/64.jpg)
Snow Cover DistributionSnow Cover Distribution
• Open Environments– Over highly exposed terrain, the effects of meso- and micro-
scale differences in vegetation and terrain features may produce wide variations in accumulation patterns.
![Page 65: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/65.jpg)
Snow Cover DistributionSnow Cover Distribution
• Open Environments– Relative accumulation on
various landscapes in an open grassland environment
• Normalized to snow accumulation on level plains under fallow
Landscape RelativeAccumulation
Level Plains Fallow 1.00 Stubble 1.15 Pasture (grazed) 0.60Gradual Hill and Valley Slopes Fallow 1.0 – 1.10 Stubble, hayland 1.0 – 1.10 Pasture (ungrazed) 1.25Steep Hill and Valley Slopes Pasture (ungrazed) 2.85 Brush 4.20Ridge and Hilltops Fallow, ungrazed pasture 0.40 – 0.50 Stubble 0.75Small Shallow Drainageways Fallow, stubble, pasture (ungrazed) 2.0 – 2.15Wide Valley Bottoms Pasture (grazed) 1.30Farm Yards Mixed Trees 2.40
![Page 66: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/66.jpg)
Blowing SnowBlowing Snow
• Sublimation Losses– Snow particles are more exposed to atmosphere during wind transport– Sublimation losses can be very high as a result
• depends on transport rate, transport distance, temperature, humidity, wind speed, and solar radiation
![Page 67: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/67.jpg)
Blowing SnowBlowing Snow
• Sublimation Losses
30
25
252216
225020
Mean Annual Blowing Snow Sublimation
CANADA, 1970-1976Loss in mm SWE over 1 km
![Page 68: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/68.jpg)
Snow Pack CharacteristicsSnow Pack Characteristics
![Page 69: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/69.jpg)
Snow Pack CharacteristicsSnow Pack Characteristics
• What is a Snow Pack?– Porous Medium
• ice + air (+ liquid water)
– Generally composed of layers of different types of snow
• more or less homogeneous within one layer
– Ice is in form of crystals and grains that are usually bonded together
• forms a texture with some degree of strength
![Page 70: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/70.jpg)
Snow Pack CharacteristicsSnow Pack Characteristics
• Snow Water Equivalent (SWE)– The height of water if a snow cover is
completely melted, on a corresponding horizontal surface area.
• Snow Depth x (Snow Density/Water Density)
![Page 71: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/71.jpg)
Density of Snow CoverDensity of Snow Cover
Snow Type Density (kg/m3)
Wild Snow
Ordinary new snow immediatelyafter falling in still air
Settling Snow
Average wind-toughened snow
Hard wind slab
New firn snow
Advanced firn snow
Thawing firn snow
10 to 30
50 to 65
70 to 90
280
350
400 to 550
550 to 650
600 to 700
Snow Depth for One Inch Water
98” to 33”
20” to 15”
14” to 11”
3.5”
2.8”
2.5” to 1.8”
1.8” to 1.5”
1.6” to 1.4”
![Page 72: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/72.jpg)
Snow Pack CharacteristicsSnow Pack Characteristics• Liquid Water Content
– Wetness, Percentage by volume
Term Remarks
Moist
Wet
Very Wet
Slush
Dry
Approximate RangeUsually T < 0oC, but can occur at any temperature up to 0oC. Little tendency for snow grains to stick together.T = 0oC. The water is not visible even at 10x magnification. Has a distinct tendency to stick together.T = 0oC. The water can be seen at 10x magnification by its miniscus between grains, but cannot be pressed out by squeezing snow (pendular regime).
T = 0oC. The water can be pressed out by squeezing snow, but there is an appreciable amount of air (funicular regime).T = 0oC. The snow is flooded with water and contains a relatively small amount of air.
<3%
3-8%
8-15%
>15%
0%
![Page 73: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/73.jpg)
Snow CharacteristicsSnow Characteristics• Diurnal Temperature Gradients
0 -5 -10
0
20
40
60
80
100
120
140
Temperature (oC)
EveningDay
TemperatureProfile
Snow Surface
Snow Pack
Ground Surface
![Page 74: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/74.jpg)
Water Flow Through Snow
![Page 75: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/75.jpg)
Water Flow through Snow• Wide Range of Flow Velocities
– 2 - 60 cm/min– Depends on several factors
• internal snow pack structure
• condition of the snow pack prior to introduction of water
• amount of water available at the snow surface
![Page 76: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/76.jpg)
Water Flow Through Snow• Flow through Homogeneous
Snow– At melting temperature, a thin film of
water surrounds each snow grain• Much of the water can flow through this
film
– Once pores are filled, laminar flow can occur
• Very efficient mechanism for draining the snow pack
![Page 77: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/77.jpg)
Water Flow through Snow• Four Liquid Water Regimes
• Capillary: < 1% free water– water doesn’t drain due to capillary tension
• Unsaturated: 1-14% free water– water drains by gravity, but air spaces are continuous– Pendular Regime
• Saturated: > 14% free water– water drains by gravity, but air spaces are discontinuous– Funicular Regime
• Melt/Freeze– water melts and refreezes, possible several times, before it drains from
the snow pack
![Page 78: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/78.jpg)
Water Flow Through Snow• Flow through Heterogeneous
Snow– Preferential Flow Paths
• Dye studies reveal vertical channels or macropores in most natural snowpacks
– Ice Layers• Develop from surface melt or refreezing
• Relatively impermeable
• Forces ponding of water and lateral flow
Ice Lens
Water Flow
Ice Lenswith Ponding
Preferential Flow Paths
![Page 79: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/79.jpg)
Water Flow Through Snow• Liquid Water Transmission
Melt and rain water arelagged and attenuated as they move through the snow cover.
Function of depth, density, ice layers, grain size, and refreezing.
122 123 124 125 126 127 128 129 1300
2
4
6
130 131 132 133 134 135 136 137 1380
2
4
6
138 139 140 141 142 143 144 145 1460
2
4
6
146 147 148 149 150 151 152 153 1540
2
4
6
Snow Melt at SurfaceOutflow from Base
Niwot Ridge, ColoradoMay 2-30, 1995
Day of Year
Rain
![Page 80: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/80.jpg)
Snow Measurement
![Page 81: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/81.jpg)
Snow Measurement• Snow Water Equivalent (SWE)
– Ground Observations• Snow Pillows
– SNOTEL Sites (Western U.S.)
• Snow Courses– Transects with snow depth and density
• Snow Tubes– measure volume and mass of snow cores
• Snow Pits– Measure vertical profiles of SWE, and other snow pack
variables.
![Page 82: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/82.jpg)
Snow Measurement• Airborne Snow Survey Program (SWE)
Natural Gamma Sources
238U Series, 232Th Series, 40K SeriesSoil
Snow
Atmosphere
Radon Daughtersin Atmosphere
Cosmic Rays
Uncollided
Gamma RadiationAbsorbed by Waterin the Snow Pack
Gamma Radiationreaches
Detector in Aircraft
Scattering
0
1000
2000
3000
4000
5000
6000
400 800 1200 1600 2000 2400 2800
K40
Tl208
ENERGY (keV)
Background(No Snow)
Over-Snow
![Page 83: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/83.jpg)
Snow Measurement• Satellite Areal Extent of Snow Cover
![Page 84: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/84.jpg)
Snow Measurement• NOAA-16 1.6 Micron Channel
![Page 85: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/85.jpg)
Snow Measurement• NOAA-16 1.6 Micron Channel
Visible Channel 1.6 micron Channel
SNOW
Snake River Valley, Idaho
![Page 86: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/86.jpg)
EvaporationEvaporation• … Evaporation is a process that allows water to
change from its liquid phase to a vapor. • … Hydrologists are mostly interested in the
evaporation from the free water surface of open water or subsurface water exposed via the capillary action; however, precipitation that is intercepted by the vegetative canopy may also be evaporated and may be a significant amount in terms of the overall hydrologic budget.
• … Factors that affect evaporation are temperature, humidity and vapor pressure, radiation, and wind speed.
• … A number of equations are used to estimate evaporation. There are also a number of published tables and maps providing regional estimates of annual evaporation.
![Page 87: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/87.jpg)
TranspirationTranspiration
• … Water may also pass to the atmosphere by being "taken up" by plants and passed on through the plant surfaces.
• … Transpiration varies greatly between plants or crops, climates, and seasons.
• … Evaporation and transpiration are often combined in a term - evapotranspiration.
• … In many areas of the country and during certain seasons evapotranspiration is a major component of the hydrologic budget and a major concern in water supply and yield estimates.
![Page 88: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/88.jpg)
Storage - SurfaceStorage - Surface
• ... Storage - Surface is used to describe the precipitation that reaches the ground surface; however, is not available for runoff or infiltration.
• … It is instead, held in small quantities on the surface in areas, such as the leafy matter and small depressions.
• … In general, surface storage is small and only temporary in terms of the overall hydrologic budget; however, it may have an effect on a storm response as it is effectively "filled" early on a storm event.
![Page 89: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/89.jpg)
InfiltrationInfiltration
• … Soils, depending on current conditions, have a capacity or ability to infiltrate precipitation, allowing water to move from the surface to the subsurface.
• ... "physically based” -> soil porosity, depth of soil column, saturation levels, and soil moisture.
• … The infiltration capacity of the soil column is usually expressed in terms of length per time (i.e. inches per hour).
• … As more water infiltrates, the infiltration generally decreases, thus the amount of water that can be infiltrated during the latter stages of a precipitation event is less than that at the beginning of the event.
![Page 90: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/90.jpg)
Infiltration cont.Infiltration cont.• … Storms that have high intensity levels may
also cause excess precipitation because the intensity (inches per hour) may exceed the current infiltration capacity (inches per hour).
• … periods of low rainfall or no rainfall will allow the soil to "recover" and increase the capacity to infiltrate water.…
• Infiltrated water replenishes soil moisture and groundwater reservoirs. Infiltrated water may also resurface to become surface flow.
• … attempt to account for infiltration by estimating excess precipitation (the difference between precipitation and excess being considered infiltration), for example, the Soil Conservation Service (SCS) runoff curve number method
![Page 91: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/91.jpg)
Subsurface FlowSubsurface Flow
• …water may move via several paths.
• …subsurface flow can be evaporated if there is a well maintained transfer mechanism to the surface. This is particularly true for areas of high ground water table (the free water surface of the groundwater) which is within the limits of the capillary action or transport abilities.
• …Vegetation may also transpire or use the water.
• …The subsurface flow may also continue to move with the groundwater table as a subsurface reservoir, which the natural system uses during periods of low precipitation.
![Page 92: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/92.jpg)
Storage - SubsurfaceStorage - Subsurface
• … The infiltrated water may continue downward in the vertical, may move through subsurface layers in a horizontal fashion, or a combination of the two directions.
• … Movement through the subsurface system is much slower than the surface and thus there are storage delays. The water may also reach an aquifer, where it may be stored for a very long period of time.
![Page 93: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/93.jpg)
RunoffRunoff• … runoff will be used to collectively describe the
precipitation that is not directly infiltrated into the groundwater system.
• … is generally characterized by overland, gully and rill, swale, and channel flows.
• … is that portion of a precipitation event that "quickly" reaches the stream system. The term "quickly" is used with caution as there may be great variability in response times for various flow mechanisms.
• … Runoff producing events are usually thought of as those that saturate the soil column or occur during a period when the soil is already saturated. Thus infiltration is halted or limited and excess precipitation occurs. This may also occur when the intensity rate of the precipitation is greater than the infiltration capacity.
![Page 94: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/94.jpg)
Overland FlowOverland Flow•… Overland flow or surface flow is that precipitation that either fails to penetrate into the soil or that resurfaces at a later point due to subsurface conditions.
•… often referred to as "sheet" flow.
•… for the purposes of this discussion, overland flow (sheet and surface flow, as well) is considered to be the flow that has not had a chance to collect and begin to form gullies, rills, swales
![Page 95: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/95.jpg)
Overland Flow (cont.)Overland Flow (cont.)•… will eventually reach defined channels and the stream system.
•… may also be infiltrated if it reaches an area that has the infiltration capacity to do so.
•… Overland flow distances are rather limited in length - National Engineering Handbook (1972) - overland flow will concentrate into gullies in less than 1000 feet.
•… Other (Seybert, Kibler, and White 1993) recommend a distance of 100 feet or less.
![Page 96: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/96.jpg)
Gullies & RillsGullies & Rills
• ... sheet flow or overland flow will soon concentrate into gullies and rills in the process of flowing towards the stream network. The location of these gullies and rills may vary from storm to storm, depending on storm patterns, intensities, current soil and land use conditions.
![Page 97: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/97.jpg)
SwalesSwales• … swales are of a more constant or permanent
nature.
• … do not vary in location from storm to storm.
• … Swales are a natural part of the landscape or topography that are often more apparent than gullies and rills.
• … Flow conditions and behaviors in swales are very close to that which is seen in channels.
![Page 98: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/98.jpg)
Channel FlowChannel Flow
• … Excess precipitation ultimately reaches the stream channel system.
• … the stream system is generally more defined, it is by no means a constant or permanent entity.
• … The stream bed is constantly changing and evolving via aggredation and degradation.
• … Stream channels convey the waters of the basin to the outlet and into the next basin.
• … attenuation of the runoff hydrograph takes place.
• … Stream channel properties (flow properties) also vary with the magnitude of the flow.
![Page 99: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/99.jpg)
Stream ChannelsStream Channels• … Channels are commonly broken into main
channel areas and overbank areas.
• … overbank areas are often referred to as floodplains.
• … Stream gaging stations are used to determine flows based on elevations in the channel and/or floodplain.
• … Bank full is often thought of as flood stage although more rigorous definitions are more applicable as they pertain to human activity and potential loss of life and property.
• … It is worth noting that the 2-year return interval flow is often thought of as "bank-full".
![Page 100: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/100.jpg)
StreamflowStreamflow• … in the public eye -> the most important aspect
of flooding and hydrology. • … flooding from streams and rivers have the
greatest potential to impact human property and lives; although overland flow flooding, mudslides, and landslides are often just as devastating.
• … Subsurface flow also enters the stream; although in some instances and regions, stream channels lose water to the groundwater table - regardless, this must be accounted for in the modeling of the stream channel.
• … Channels also offer a storage mechanism and the resulting effect is most often an attenuation of the flood hydrograph.
![Page 101: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/101.jpg)
Storage - ReservoirsStorage - Reservoirs• … Lakes, reservoirs, & structures, etc. are given
a separate category in the discussion of the hydrologic cycle due to the potential impact on forecasting procedures and outcomes.
• … provide a substantial storage mechanism and depending on the intended purpose of the structure will have varying impacts on the final hydrograph, as well as flooding levels.
• … This effect can vary greatly depending on the type of reservoir, the outlet configuration, and the purpose of the reservoir.
![Page 102: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/102.jpg)
Storage - Reservoirs (cont.)Storage - Reservoirs (cont.)• … Flood control dams are used to attenuate
and store potentially destructive runoff events.
• … Other structures may have adverse effects. For example, bridges may cause additional "backwater" effects and enhance the level of flooding upstream of the bridge.
• … a catastrophic failure of a structure often has devastating effects on loss of life and property.
![Page 103: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/103.jpg)
Simulating the Hydrologic Simulating the Hydrologic ResponseResponse
Model TypesPrecipitationLossesModeling LossesModel Components
![Page 104: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/104.jpg)
Model TypesModel Types
• Empirical
• Analytical
• Lumped
• Distributed
Model TypesModel TypesPrecipitationLossesModeling LossesModel Components
![Page 105: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/105.jpg)
General Goal of Most Models
Basin Process Representation
Infiltration
Excess Precip.
Interception
Storage
Time Series
Time Series
We must begin to think of the basin as a “whole”
![Page 106: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/106.jpg)
The Basic Process
Excess Precip. Model
Excess Precip.
Excess Precip. Basin “Routing”Runoff
Hydrograph
Runoff Hydrograph
Stream “Routing”
Downstream Hydrograph
![Page 107: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/107.jpg)
From A Basin View
Excess Precip. Model
Excess Precip.
Basin “Routing”
Runoff HydrographStream
“Routing”
![Page 108: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/108.jpg)
Precipitation Input
• Precipitation is generally “pre-processed
• Uniform in space and time – never!
• Gages
• Radar
• satellite
![Page 109: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/109.jpg)
PrecipitationPrecipitation• … magnitude, intensity, location, patterns, and
future estimates of the precipitation.
• … In lumped models, the precipitation is input in the form of average values over the basin. These average values are often referred to as mean aerial precipitation (MAP) values.
• … MAP's are estimated either from 1) precipitation gage data or 2) NEXRAD precipitation fields.
![Page 110: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/110.jpg)
Precipitation (cont.)Precipitation (cont.)• … If precipitation gage data is used, then the
MAP's are usually calculated by a weighting scheme.
• … a gage (or set of gages) has influence over an area and the amount of rain having been recorded at a particular gage (or set of gages) is assigned to an area.
• … Thiessen method and the isohyetal method are two of the more popular methods.
![Page 111: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/111.jpg)
ThiessenThiessen
•Thiessen methodThiessen method is a method for areally weighting rainfall through graphical means.
![Page 112: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/112.jpg)
IsohyetalIsohyetal
•Isohyetal methodIsohyetal method is a method for areally weighting rainfall using contours of equal rainfall (isohyets).
![Page 113: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/113.jpg)
NEXRADNEXRAD
•NexradNexrad is a method of areally weighting rainfall using satellite imaging of
the intensity of the rain during a storm.
![Page 114: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/114.jpg)
Excess Precip. Models
• Physically Based
• Empirical
• Analytical
• Conceptual
• Generally Lumped
![Page 115: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/115.jpg)
LossesLosses
• … modeled in order to account for the destiny of the precipitation that falls and the potential of the precipitation to affect the hydrograph.
• … losses include interception, evapotranspiration, depression storage, and infiltration.
• … Interception is that precipitation that is caught by the vegetative canopy and does not reach the ground for eventual infiltration or runoff.
• … Evapotranspiration is a combination of evaporation and transpiration and was previously discussed.
• … Depression storage is that precipitation that reaches the ground, yet, as the name suggests, is stored in small surface depressions and is generally satisfied during the early portion of a storm event.
![Page 116: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/116.jpg)
Modeling LossesModeling Losses
• … simplistic methods such as a constant loss method may be used.
• … A constant loss approach assumes that the soil can constantly infiltrate the same amount of precipitation throughout the storm event. The obvious weaknesses are the neglecting of spatial variability, temporal variability, and recovery potential.
• Other methods include exponential decays (the infiltration rate decays exponentially), empirical methods, and physically based methods.
• … There are also combinations of these methods. For example, empirical coefficients may be combined with a more physically based equation. (SAC-SMA for example)
![Page 117: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/117.jpg)
SCS Curve NumberSCS Curve Number
0.8S)+(P
)0.2S-(P = Q
2
![Page 118: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/118.jpg)
Estimating “S”Estimating “S”
• The difficult part of applying this method to a watershed is the estimation of the watershed’s potential maximum retention, S.
• SCS developed the concept of the dimensionless curve number, CN, to aid in the estimation of S.
• CN is related to S as follows :
10 - CN
1000 = S
CN ranges from 1 to 100 (not really!)
![Page 119: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/119.jpg)
Determine CNDetermine CN
• The Soil Conservation Service has classified over 8,500 soil series into four hydrologic groups according to their infiltration characteristics, and the proper group is determined for the soil series found.
• The hydrologic groups have been designated as A, B, C, and D.
• Group A is composed of soils considered to have a low runoff potential. These soils have a high infiltration rate even when thoroughly wetted.
• Group B soils have a moderate infiltration rate when thoroughly wetted,
• while group C soils are those which have slow infiltration rates when thoroughly wetted.
• Group D soils are those which are considered to have a high potential for runoff, since they have very slow infiltration rates when thoroughly wetted (SCS, 1972).
![Page 120: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/120.jpg)
Determine CN, cont….Determine CN, cont….
• Once the hydrologic soil group has been determined, the curve number of the site is determined by cross-referencing land use and hydrologic condition to the soil group - SAMPLE
Land use and treatment Hydrologic soil group or Hydrologic practice condition A B C D
FallowStraight row ---- 77 86 91 94Row CropsStraight row Poor 72 81 88 91Straight row Good 67 78 85 89Contoured Poor 70 79 84 88
![Page 121: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/121.jpg)
Initial ConditionsInitial Conditions
5-day antecedent rainfall, inches Antecedent moisture
Dormant Season Growing Season
I Less than 0.5 Less than 1.4
II 0.5 to 1.1 1.4 to 2.1
III Over 1.1 Over 2.1
![Page 122: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/122.jpg)
Adjust CN’sAdjust CN’s
CN for AMC II Corresponding CN’s
AMC I AMC III
100 100 100
95 87 98
90 78 96
85 70 94
80 63 91
75 57 88
70 51 85
65 45 82
60 40 78
55 35 74
50 31 70
![Page 123: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/123.jpg)
Sort of the other end of the Sort of the other end of the scale….scale….
![Page 124: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/124.jpg)
SAC-SMASAC-SMA
• … The Sacramento Soil Moisture Accounting Model (SAC-SMA) is a conceptual model of soil moisture accounting that uses empiricism and lumped coefficients to attempt to mimic the physical constraints of water movement in a natural system.
Tension Free
Tension Free - Primary
Free - Supplemental
Upper Zone
Lower Zone
![Page 125: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/125.jpg)
RunoffRunoff
• … Runoff is essentially the excess precipitation - the precipitation minus the losses.
• … Runoff must be transformed to streamflow at the basin outlet via a unit hydrograph.
• … In actuality, all forms of surface and subsurface flow that reach a stream channel and eventually the outlet are modeled through the use of the unit hydrograph for the general hydrologic model…
![Page 126: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/126.jpg)
126
Unit Hydrograph TheoryUnit Hydrograph Theory
• Sherman - 1932
• Horton - 1933
• Wisler & Brater - 1949 - “the hydrograph of surface runoff resulting from a relatively short, intense rain, called a unit storm”
• The runoff hydrograph may be “made up” of runoff that is generated as flow through the soil (black, 1990)
![Page 127: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/127.jpg)
Linearity of Unit HydrographLinearity of Unit Hydrograph• … In addition, when unit hydrograph theory is applied, it is
assumed that the watershed responds uniformly.
• … Meaning that peak flow from 2 inches of excess will be twice that of 1 inch of excess
0.0000
100.0000
200.0000
300.0000
400.0000
500.0000
600.0000
700.0000
0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000
![Page 128: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/128.jpg)
Unit Hydrograph “Lingo”Unit Hydrograph “Lingo”
• Duration
• Lag Time
• Time of Concentration
• Rising Limb
• Recession Limb (falling limb)
• Peak Flow
• Time to Peak (rise time)
• Recession Curve
• Separation
• Base flow
![Page 129: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/129.jpg)
Graphical RepresentationGraphical Representation
Lag time
Time of concentration
Duration of excess precipitation.
Base flow
![Page 130: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/130.jpg)
Methods of Developing UHG’sMethods of Developing UHG’s
• From Streamflow Data
• Synthetically– Snyder– SCS– Time-Area (Clark, 1945)
• “Fitted” Distributions
![Page 131: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/131.jpg)
Unit HydrographUnit Hydrograph
• The hydrograph that results from 1-inch of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration.
• The key points :1-inch of EXCESS precipitationSpread uniformly over space - evenly over the watershedUniformly in time - the excess rate is constant over the time
intervalThere is a given duration
![Page 132: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/132.jpg)
Derived Unit HydrographDerived Unit Hydrograph
0.0000
100.0000
200.0000
300.0000
400.0000
500.0000
600.0000
700.0000
0.00
00
0.16
00
0.32
00
0.48
00
0.64
00
0.80
00
0.96
00
1.12
00
1.28
00
1.44
00
1.60
00
1.76
00
1.92
00
2.08
00
2.24
00
2.40
00
2.56
00
2.72
00
2.88
00
3.04
00
3.20
00
3.36
00
3.52
00
3.68
00
Baseflow
Surface Response
![Page 133: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/133.jpg)
Derived Unit HydrographDerived Unit Hydrograph
0.0000
100.0000
200.0000
300.0000
400.0000
500.0000
600.0000
700.0000
0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000
Total Hydrograph
Surface Response
Baseflow
![Page 134: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/134.jpg)
Derived Unit HydrographDerived Unit Hydrograph
Rules of Thumb :… the storm should be fairly uniform in nature and the excess precipitation should be equally as uniform throughout the basin. This may require the initial conditions throughout the basin to be spatially similar. … Second, the storm should be relatively constant in time, meaning that there should be no breaks or periods of no precipitation. … Finally, the storm should produce at least an inch of excess precipitation (the area under the hydrograph after
correcting for baseflow).
![Page 135: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/135.jpg)
Deriving a UHG from a StormDeriving a UHG from a Stormsample watershed = 450 mi2sample watershed = 450 mi2
0
5000
10000
15000
20000
25000
0 8 16 24 32 40 48 56 64 72 80 88 96 104
112
120
128
Time (hrs.)
Flo
w (
cfs)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pre
cip
itat
ion
(in
ches
)
![Page 136: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/136.jpg)
Separation of BaseflowSeparation of Baseflow
... generally accepted that the inflection point on the recession limb of a hydrograph is the result of a change in the controlling physical processes of the excess precipitation flowing to the basin outlet.
In this example, baseflow is considered to be a straight line connecting that point at which the hydrograph begins to rise rapidly and the inflection point on the recession side of the hydrograph.
the inflection point may be found by plotting the hydrograph in semi-log fashion with flow being plotted on the log scale and noting the time at which the recession side fits a straight line.
![Page 137: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/137.jpg)
Semi-log PlotSemi-log Plot
1
10
100
1000
10000
100000
29 34 39 44 49 54 59 64 69 74 79 84 89 94 99 104
109
114
119
124
129
134
Time (hrs.)
Flo
w (
cfs)
Recession side of hydrograph becomes linear at approximately hour
64.
![Page 138: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/138.jpg)
Hydrograph & BaseflowHydrograph & Baseflow
0
5000
10000
15000
20000
25000
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
112
119
126
133
Time (hrs.)
Flo
w (
cfs)
![Page 139: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/139.jpg)
Separate BaseflowSeparate Baseflow
0
5000
10000
15000
20000
25000
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
112
119
126
133
Time (hrs.)
Flo
w (
cfs)
![Page 140: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/140.jpg)
Sample CalculationsSample Calculations• In the present example (hourly time step), the flows are summed and
then multiplied by 3600 seconds to determine the volume of runoff in cubic feet. If desired, this value may then be converted to acre-feet by dividing by 43,560 square feet per acre.
• The depth of direct runoff in feet is found by dividing the total volume of excess precipitation (now in acre-feet) by the watershed area (450 mi2 converted to 288,000 acres).
• In this example, the volume of excess precipitation or direct runoff for storm #1 was determined to be 39,692 acre-feet.
• The depth of direct runoff is found to be 0.1378 feet after dividing by the watershed area of 288,000 acres.
• Finally, the depth of direct runoff in inches is 0.1378 x 12 = 1.65 inches.
![Page 141: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/141.jpg)
Obtain UHG OrdinatesObtain UHG Ordinates
• The ordinates of the unit hydrograph are obtained by dividing each flow in the direct runoff hydrograph by the depth of excess precipitation.
• In this example, the units of the unit hydrograph would be cfs/inch (of excess precipitation).
![Page 142: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/142.jpg)
Final UHGFinal UHG
0
5000
10000
15000
20000
25000
0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105
112
119
126
133
Time (hrs.)
Flo
w (
cfs)
Storm #1 hydrograph
Storm#1 direct runoff hydrograph
Storm # 1 unit hydrograph
Storm #1 baseflow
![Page 143: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/143.jpg)
Determine Duration of UHGDetermine Duration of UHG• The duration of the derived unit hydrograph is found by examining the
precipitation for the event and determining that precipitation which is in excess.
• This is generally accomplished by plotting the precipitation in hyetograph form and drawing a horizontal line such that the precipitation above this line is equal to the depth of excess precipitation as previously determined.
• This horizontal line is generally referred to as the -index and is based on the assumption of a constant or uniform infiltration rate.
• The uniform infiltration necessary to cause 1.65 inches of excess precipitation was determined to be approximately 0.2 inches per hour.
![Page 144: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/144.jpg)
Estimating Excess Precip.Estimating Excess Precip.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time (hrs.)
Pre
cip
itat
ion
(in
ches
)
Uniform loss rate of 0.2 inches per hour.
![Page 145: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/145.jpg)
Excess PrecipitationExcess Precipitation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time (hrs.)
Exc
ess
Pre
c. (
inch
es)
Small amounts of excess precipitation at beginning and end may
be omitted.
Derived unit hydrograph is the result of approximately 6 hours
of excess precipitation.
![Page 146: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/146.jpg)
Average Several UHG’sAverage Several UHG’s• It is recommend that several unit hydrographs be derived and averaged.
• The unit hydrographs must be of the same duration in order to be properly averaged.
• It is often not sufficient to simply average the ordinates of the unit hydrographs in order to obtain the final unit hydrograph. A numerical average of several unit hydrographs which are different “shapes” may result in an “unrepresentative” unit hydrograph.
• It is often recommended to plot the unit hydrographs that are to be averaged. Then an average or representative unit hydrograph should be sketched or fitted to the plotted unit hydrographs.
• Finally, the average unit hydrograph must have a volume of 1 inch of runoff for the basin.
![Page 147: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/147.jpg)
One Step Shy of a Full Derivation?
• You could part of the previous analysis for a very useful tool.
• Take a storm
• Plot streamflow
• Determine volume of runoff
• Divide by basin area
• Get depth of runoff
• Estimate total basin (mean) precipiation
• Compare!
• Do this for a variety of storm over a variety of conditions and seasons.
![Page 148: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/148.jpg)
Synthetic UHG’sSynthetic UHG’s
• Snyder
• SCS
• Time-area
![Page 149: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/149.jpg)
SnyderSnyder
• Since peak flow and time of peak flow are two of the most important parameters characterizing a unit hydrograph, the Snyder method employs factors defining these parameters, which are then used in the synthesis of the unit graph (Snyder, 1938).
• The parameters are Cp, the peak flow factor, and Ct, the lag factor.
• The basic assumption in this method is that basins which have similar physiographic characteristics are located in the same area will have similar values of Ct and Cp.
• Therefore, for ungaged basins, it is preferred that the basin be near or
similar to gaged basins for which these coefficients can be determined.
![Page 150: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/150.jpg)
Basic RelationshipsBasic Relationships3.0)( catLAG LLCt
5.5LAG
durationtt
)(25.0 .. durationdurationaltLAGlagalt tttt
83 LAG
baset
t
LAG
ppeak t
ACq
640
![Page 151: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/151.jpg)
What are the L & Lca Doing?
![Page 152: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/152.jpg)
Final ShapeFinal ShapeThe final shape of the Snyder unit hydrograph is controlled by the
equations for width at 50% and 75% of the peak of the UHG:
![Page 153: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/153.jpg)
SCSSCS
SCS Dimensionless UHG Features
0
0.2
0.4
0.6
0.8
1
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
T/Tpeak
Q/Q
pe
ak
Flow ratios
Cum. Mass
![Page 154: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/154.jpg)
Dimensionless RatiosDimensionless RatiosTime Ratios
(t/tp)Discharge Ratios
(q/qp)Mass Curve Ratios
(Qa/Q)0 .000 .000.1 .030 .001.2 .100 .006.3 .190 .012.4 .310 .035.5 .470 .065.6 .660 .107.7 .820 .163.8 .930 .228.9 .990 .300
1.0 1.000 .3751.1 .990 .4501.2 .930 .5221.3 .860 .5891.4 .780 .6501.5 .680 .7001.6 .560 .7511.7 .460 .7901.8 .390 .8221.9 .330 .8492.0 .280 .8712.2 .207 .9082.4 .147 .9342.6 .107 .9532.8 .077 .9673.0 .055 .9773.2 .040 .9843.4 .029 .9893.6 .021 .9933.8 .015 .9954.0 .011 .9974.5 .005 .9995.0 .000 1.000
![Page 155: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/155.jpg)
Triangular RepresentationTriangular RepresentationSCS Dimensionless UHG & Triangular Representation
0
0.2
0.4
0.6
0.8
1
1.2
0.0 1.0 2.0 3.0 4.0 5.0
T/Tpeak
Q/Q
pea
k
Flow ratios
Cum. Mass
Triangular
Excess Precipitation
D
Tlag
Tc
TpTb
Point of Inflection
![Page 156: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/156.jpg)
Triangular RepresentationTriangular Representationpb T x 2.67 T
ppbr T x 1.67 T - T T
)T + T( 2
q =
2
Tq +
2
Tq = Q rp
prppp
T + T
2Q = q
rpp
T + T
Q x A x 2 x 654.33 = q
rpp
The 645.33 is the conversion used for delivering 1-inch of runoff (the area under the unit hydrograph) from 1-square
mile in 1-hour (3600 seconds). T
Q A 484 = q
pp
SCS Dimensionless UHG & Triangular Representation
0
0.2
0.4
0.6
0.8
1
1.2
0.0 1.0 2.0 3.0 4.0 5.0
T/Tpeak
Q/Q
pea
k
Flow ratios
Cum. Mass
Triangular
Excess Precipitation
D
Tlag
Tc
TpTb
Point of Inflection
![Page 157: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/157.jpg)
484 ?484 ?
Comes from the initial assumption that 3/8 of the volume under the UHG is under the rising limb and the remaining 5/8
is under the recession limb.
General Description Peaking Factor Limb Ratio (Recession to Rising)
Urban areas; steep slopes 575 1.25 Typical SCS 484 1.67
Mixed urban/rural 400 2.25 Rural, rolling hills 300 3.33 Rural, slight slopes 200 5.5
Rural, very flat 100 12.0
T
Q A 484 = q
pp
![Page 158: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/158.jpg)
Time of ConcentrationTime of Concentration
• Regression Eqs.
• Segmental Approach
![Page 159: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/159.jpg)
A Regression EquationA Regression Equation
TlagL S
Slope
08 1 0 7
1900 05
. ( ) .
(% ) .
where : Tlag = lag time in hoursL = Length of the longest drainage path in feetS = (1000/CN) - 10 (CN=curve number)%Slope = The average watershed slope in %
![Page 160: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/160.jpg)
Segmental ApproachSegmental Approach
• More “hydraulic” in nature
• The parameter being estimated is essentially the time of concentration or longest travel time within the basin.
• In general, the longest travel time corresponds to the longest drainage path
• The flow path is broken into segments with the flow in each segment being represented by some type of flow regime.
• The most common flow representations are overland, sheet, rill and
gully, and channel flow.
![Page 161: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/161.jpg)
A Basic ApproachA Basic Approach 2
1
kSV
McCuen (1989) and SCS (1972) provide values of k for several flow situations
(slope in %)
K Land Use / Flow Regime
0.25 Forest with heavy ground litter, hay meadow (overland flow)0.5 Trash fallow or minimum tillage cultivation; contour or strip
cropped; woodland (overland flow)0.7 Short grass pasture (overland flow)0.9 Cultivated straight row (overland flow)1.0 Nearly bare and untilled (overland flow); alluvial fans in
western mountain regions1.5 Grassed waterway2.0 Paved area (sheet flow); small upland gullies
Flow Type KSmall Tributary - Permanent or intermittent
streams which appear as solid or dashedblue lines on USGS topographic maps.
2.1
Waterway - Any overland flow route whichis a well defined swale by elevation
contours, but is not a stream section asdefined above.
1.2
Sheet Flow - Any other overland flow pathwhich does not conform to the definition of
a waterway.
0.48
Sorell & Hamilton, 1991
![Page 162: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/162.jpg)
Time-AreaTime-Area
![Page 163: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/163.jpg)
Time-AreaTime-Area
Time
Q % Area
Time
100%
Timeof conc.
![Page 164: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/164.jpg)
Time-AreaTime-Area
![Page 165: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/165.jpg)
Hypothetical ExampleHypothetical Example
• A 190 mi2 watershed is divided into 8 isochrones of travel time.
• The linear reservoir routing coefficient, R, estimated as 5.5 hours.
• A time interval of 2.0 hours will be used for the computations.
WatershedBoundary
Isochrones
2
345
66
7
8
6
6
5
7
7
1
0
![Page 166: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/166.jpg)
Rule of ThumbRule of Thumb
R - The linear reservoir routing coefficient can be estimated as approximately 0.75
times the time of concentration.
![Page 167: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/167.jpg)
Basin BreakdownBasin Breakdown
MapArea #
BoundingIsochrones
Area(mi2)
CumulativeArea (mi2)
CumulativeTime (hrs)
1 0-1 5 5 1.02 1-2 9 14 2.03 2-3 23 37 3.04 3-4 19 58 4.05 4-5 27 85 5.06 5-6 26 111 6.07 6-7 39 150 7.08 7-8 40 190 8.0
TOTAL 190 190 8.0
WatershedBoundary
Isochrones
2
345
66
7
8
6
6
5
7
7
1
0
![Page 168: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/168.jpg)
Incremental AreaIncremental Area
0
5
10
15
20
25
30
35
40
Incr
emen
tal
Are
a (s
qau
re m
iles
)
1 2 3 4 5 6 7 8
Time Increment (hrs)
WatershedBoundary
Isochrones
2
345
66
7
8
6
6
5
7
7
1
0
![Page 169: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/169.jpg)
Cumulative Time-Area CurveCumulative Time-Area Curve
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100 120 140 160 180 200
Time (hrs)
Cu
mu
lati
ve A
rea
(sq
aure
mil
es)
WatershedBoundary
Isochrones
2
345
66
7
8
6
6
5
7
7
1
0
![Page 170: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/170.jpg)
Trouble Getting a Time-Area Trouble Getting a Time-Area Curve?Curve?
0.5) Ti (0for 414.1 5.1 ii TTA
1.0) Ti (0.5for )1(414.11 5.1 ii TTA
Synthetic time-area curve - The U.S. Army Corps of Engineers (HEC 1990)
![Page 171: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/171.jpg)
Instantaneous UHGInstantaneous UHG)1(
)1( iii
IUHccIIUH
tR
tc
2
2
t = the time step used n the calculation of the translation unit hydrograph
The final unit hydrograph may be found by averaging 2 instantaneous unit hydrographs that are a t time step apart.
![Page 172: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/172.jpg)
ComputationsComputationsTime(hrs)
(1)
Inc.Area(mi2)(2)
Inc.TranslatedFlow (cfs)
(3)
Inst. UHG
(4)
IUHGLagged 2
hours(5)
2-hrUHG(cfs)(6)
0 0 0 0 02 14 4,515 1391 0 7004 44 14,190 5333 1,391 3,3606 53 17,093 8955 5,333 7,1508 79 25,478 14043 8,955 11,50010 0 0 9717 14,043 11,88012 6724 9,717 8,22014 4653 6,724 5,69016 3220 4,653 3,94018 2228 3,220 2,72020 1542 2,228 1,89022 1067 1,542 1,30024 738 1,067 90026 510 738 63028 352 510 43030 242 352 30032 168 242 20034 116 168 14036 81 116 10038 55 81 7040 39 55 5042 26 39 3044 19 26 2046 13 19 2048 13
![Page 173: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/173.jpg)
Incremental AreasIncremental Areas
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10
Time Increments (2 hrs)
Are
a In
crem
ents
(sq
uar
e m
iles
)
![Page 174: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/174.jpg)
Incremental FlowsIncremental Flows
0
5000
10000
15000
20000
25000
30000
1 2 3 4 5 6
Time Increments (2 hrs)
Tra
nsl
ated
Un
it H
ydro
gra
ph
![Page 175: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/175.jpg)
Instantaneous UHGInstantaneous UHG
0
2000
4000
6000
8000
10000
12000
14000
16000
0 10 20 30 40 50 60
Time (hrs)
Flo
w (
cfs/
inch
)
![Page 176: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/176.jpg)
Lag & AverageLag & Average
0
2000
4000
6000
8000
10000
12000
14000
16000
0 10 20 30 40 50 60
Time (hrs)
Flo
w (
cfs/
inch
)
![Page 177: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/177.jpg)
Let’s talk about Modeling IssuesLet’s talk about Modeling Issues
Weaknesses, strengths, etc…Weaknesses, strengths, etc…
![Page 178: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/178.jpg)
Factors Affecting the Factors Affecting the Hydrologic ResponseHydrologic Response
• Current Conditions• Precipitation Patterns• Land Use• Channel Changes• Others…..
![Page 179: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/179.jpg)
Channel ChangesChannel Changes
• Slopes• Storage• Rating Curve
![Page 180: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/180.jpg)
Variable Source Area ConceptVariable Source Area Concept
Not all of the watershed is contributing during an event......
![Page 181: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/181.jpg)
ExampleExample
![Page 182: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/182.jpg)
And so on...And so on...
![Page 183: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/183.jpg)
And the recession...And the recession...
![Page 184: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/184.jpg)
Small Basin HydrologySmall Basin Hydrology
and and
Distributed ModelsDistributed Models
![Page 185: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/185.jpg)
Why do we need Why do we need DISTRIBUTED MODELS?DISTRIBUTED MODELS?
Non-homogeneity!Non-homogeneity!
![Page 186: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/186.jpg)
Causes of Non-homogeneityCauses of Non-homogeneity
• Small scale precipitationSmall scale precipitation
• Spatially diverse precipitation patternsSpatially diverse precipitation patterns
• Small scale basin changes – i.e. soil Small scale basin changes – i.e. soil moisture, slope, etc….moisture, slope, etc….
• Sub-basin changes – urbanizationSub-basin changes – urbanization
• Others????Others????
![Page 187: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/187.jpg)
Hydrology TerminologyHydrology Terminology
•Precipitation can fall in many different patterns, Precipitation can fall in many different patterns, which influences the hydrologic response.which influences the hydrologic response.
•For example, a storm may be:For example, a storm may be:•Uniform over the entire watershedUniform over the entire watershed•A storm may move up the watershedA storm may move up the watershed•A storm may move down the watershedA storm may move down the watershed•A storm may only rain on a portion of A storm may only rain on a portion of the watershed. the watershed.
![Page 188: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/188.jpg)
ApproachesApproaches
• Many sub-basins – at least more than you Many sub-basins – at least more than you currently have…currently have…
• Hillslope processesHillslope processes
• TIN’sTIN’s
• Grids - rasterGrids - raster
![Page 189: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/189.jpg)
Common with LumpedCommon with Lumped
• Still must compute excessStill must compute excess
• Can still use empirical, analytical, Can still use empirical, analytical, conceptual, etc….conceptual, etc….
![Page 190: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/190.jpg)
ComputationallyComputationally
• Huge demands computationallyHuge demands computationally
• Must now keep track of flow, precip, Must now keep track of flow, precip, moisture, etc.. On hundreds to thousands moisture, etc.. On hundreds to thousands of pixels, sub-basins, etc….of pixels, sub-basins, etc….
![Page 191: COMET University Faculty Hydrometeorology Course June 2000 Dennis L. Johnson](https://reader034.vdocuments.net/reader034/viewer/2022042718/56649f295503460f94c4257e/html5/thumbnails/191.jpg)
Moving water off basinMoving water off basin
• Lumped we tended to use the UHGLumped we tended to use the UHG
• Now we tend to be more physically Now we tend to be more physically based:based:– Hydraulic equationHydraulic equation– Hydrologic routingHydrologic routing– Etc….Etc….