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STEP July, 2007: Hydrology – Page No. 1
Watershed Hydrology and Water ResourcesScience Teacher Education Program
(STEP)
Presented by
Amy Tidwell
Water and Environmental Research Center/Institute of Northern Engineering
University of Alaska Fairbanks
July 2007
STEP July, 2007: Hydrology – Page No. 2
Watershed Hydrology and Water Resources
Watershed HydrologyWatershed Delineation (Exercise)Water BudgetPrecipitationEvapotranspirationInfiltrationRunoffGroundwater (Demonstration/Activity)Wetlands (Hand out)Climate Change Considerations
Water ResourcesWater Resources Planning and Management (Hand out)Water SupplyWater as a HazardWater Management: Health, Safety, and the EnvironmentClimate Change and Water Resources
Additional ResourcesOnline Activities (time permitting)
Outline
STEP July, 2007: Hydrology – Page No. 3
Watershed Hydrology
STEP July, 2007: Hydrology – Page No. 4
Watershed Hydrology
We will apply some of what you’ve learned about the global hydrologic cycle to watersheds. As you will see, a watershed is a logical accounting unit in hydrology and water resources.
What is a watershed?
Where is a watershed? And how large is a watershed?
Delineating watersheds• Topographic maps, contour lines and slopes– where does the water flow?• Begin with a point of interest, usually along a stream.• Trace the outline of the watershed, beginning at one side of the stream, by following the steepest
slope (gradient). Recall that the steepest gradient occurs at a right angle to contour lines.• Then begin tracing the outline from the other side of the stream until your second trace meets up
with the first.• Check your work: Consider a rain drop falling over your delineated watershed. Pick several
points around and even outside of your watershed and trace the downhill flow path of the rain drop. Is it consistent with your drawn watershed boundary?
Watersheds
STEP July, 2007: Hydrology – Page No. 5
STEP July, 2007: Hydrology – Page No. 6
USGS, 2001
STEP July, 2007: Hydrology – Page No. 7
USGS, 2001
STEP July, 2007: Hydrology – Page No. 8
Where,
P = precipitationET = evapotranspirationI = InfiltrationG=GroundwaterQ= Runoff
ΔWatershed Storage = P – ET – G – R
Hydrologic Cycle and the Water Budget
P
I Q
ET
G
Watershed Hydrology
The Hydrologic Cycle and its Role in Arctic and Global Environmental Change, 2001.
STEP July, 2007: Hydrology – Page No. 9
Precipitation
STEP July, 2007: Hydrology – Page No. 10
Precipitation
• Precipitation is the primary driver for the land phase of the hydrologic cycle
• Total precipitation over land gets partitioned into different components: some soaks into the ground (infiltration), some evaporates from the surface of leaves and soil, some is taken up into plant roots and released back into the atmosphere (transpiration), some is stored at the surface as snow/ice.
• For a given watershed, how precipitation is partitioned depends on a number of environmental factors:
– Temperature– Soil moisture– Intensity of rainfall– Vegetation (seasonal effects)
• Furthermore, the state of precipitation (liquid/solid) is very important for seasonal (and sometimes interannual) partitioning.
Introduction
STEP July, 2007: Hydrology – Page No. 11
• C
USGS, 2001
STEP July, 2007: Hydrology – Page No. 12
Precipitation
•Precipitation measurements are point data; however, models often require the amount of rainfall over the watershed or area of interest.
•As a result, several methods have been developed to determine the average rainfall over the watershed - called the mean areal precipitation (MAP).
MAP = Total Precipitation VolumeWatershed Area
•Examples of methods include:– Arithmetic Average (simple average of
all stations)– Thiessen Polygon (weighted average based
on area of influence)– Hypsometric (weighted average based on
basin topography and location of stations)
G1G6
G2
G7
G5
G3
G4
Mean Areal Precipitation
STEP July, 2007: Hydrology – Page No. 13
Precipitation
• In the United States the National Weather Service (www.nws.noaa.gov) has a network of precipitation gages
– 278 primary stations - staffed full time by paid technicians (~20 AK)– 8,000 cooperative stations - mostly volunteer stations (~70 AK)
• Historical data for these stations may be downloaded at the National Climate Data Center website (www.ncdc.noaa.gov)
Where to Obtain Precipitation Data (and Other Surface Observations)
STEP July, 2007: Hydrology – Page No. 14
Precipitation
Precipitation gage networks
Primary Stations
Dingman, 2002
STEP July, 2007: Hydrology – Page No. 15
Precipitation and Rainfall Climatology
Precipitation gage networks
Cooperative Stations
Dingman, 2002
STEP July, 2007: Hydrology – Page No. 16
Evapotranspiration
STEP July, 2007: Hydrology – Page No. 17
Evapotranspiration
• Evaporation (E) occurs when water is converted into water vapor. This may occur from an open water surface or through exfiltration of soil moisture.
• Methods for estimating evaporation include:– Water budget– Energy budget– Mass transfer techniques– Pan evaporation measurements
• Transpiration occurs when water vapor is lost to the atmosphere through small openings in the leaves of plants.
• Potential Evapotranspiration (PET) is a combined estimate of the maximum potential evaporation + transpiration over an area. When there is limited water (open surface or soil moisture) actual rates of evapotranspiration (ET) are less than the potential rate.
Overview
TranspirationTranspiration
TranslocationTranslocation
AbsorptionAbsorption
soil surface
Evaporation + Transpiration = Evapotranspiration
STEP July, 2007: Hydrology – Page No. 18
Mass Balance for Water body:ΔV = P + SWin + GWin − E – SWout – GWoutSolve for E
E = P + SWin + GWin – SWout – GWout − ΔV
Where,E = EvaporationP = PrecipitationSWin = Surface Water InflowSWout = Surface Water OutflowGWin = Groundwater InflowGWout = Groundwater OutflowΔV = Change in storage
• The water balance method is computationally simple. However, gathering the data for implementation of this method may be difficult.
• Each of the quantities in the equation above are measured or estimated, which results in uncertainty. Thus the calculation of evaporation includes the sum of the errors related to each component.
Evapotranspiration
Estimating Evaporation: Water Balance Method
SWoutSWout SWinSWinΔVΔV
EE
GWinGWin
PP
GWoutGWout
STEP July, 2007: Hydrology – Page No. 19
L = net long wave radiation input
Ta = temperature of atmosphere, in ºCTs = temperature of surface, in º Cσ=Stefan-Boltzmann const= 4.90x10-9 [MJ/m2dayK4]
εw= effective emissivity of water = 0.97εat= effective emissivity of atmosphere
va = wind speed [km/day]ra = relative humidity = ea/e*a
e*a = saturation vapor pressure at the air temperature = [kPa]
Where,Δ with Ta in ºC
γ = psychrometric constant = ca =heat capacity of air=1.0x10-3 [MJ/kgK]P = pressure [kPa]λv = latent heat of vaporization [MJ/Kg]
= 2.50 – 2.36x10-3 •Ts, T in ºCK = net short-wave radiation input
= Io • (0.803-0.34c-0.458c2) • (1- a)Io = solar insolation at the top of the atmosphere [MJ/m2day]a = albedoc = Cloud cover
KE = coefficient reflecting the efficiency of vertical transport of water vapor by turbulent eddies of the wind = 1.69 • 10-5 • AL
-0.05
AL = water surface area [km2]
ρw = density of water = 1000 [kg/m3]
EvapotranspirationEstimating Evaporation: Penman Method
Required input data1. AL – used in KE2. P or Altitude 3. Ts – used in λv and L4. Ta – used in Δ , L and ea
*
5. ea6. va7. c – used in K8. Io – used in K9. a – used in K
Required input data1. AL – used in KE2. P or Altitude 3. Ts – used in λv and L4. Ta – used in Δ , L and ea
*
5. ea6. va7. c – used in K8. Io – used in K9. a – used in K
)()1()( *
γλρλργ+Δ
−++Δ=
••
•••••••
vw
aaavwE revKLKE
⎟⎟⎠
⎞⎜⎜⎝
⎛
++=
••
3.2373.17
exp2)3.237(3.2508
aTaT
aT
)622.0()( vPac λ••
⎟⎟⎠
⎞⎜⎜⎝
⎛ •• + 3.237
3.17exp611.0aT
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44 )2.273()4.273( +−+= ••••• swaatw TT σεσεε
)22.01(2.273
72.1 27/1
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e
a
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⎞⎜⎜⎝
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•=
STEP July, 2007: Hydrology – Page No. 20
Infiltration
STEP July, 2007: Hydrology – Page No. 21
Dingman, 2002
Infiltration
Soil Properties
Sand grains
Clay particles
Dingman, 2002
• The properties of a homogeneous soil matrix include:
– Porosity, φ =
– Water content, θ =
– Field capacity, θfc = water contentat which further drainage due togravity is negligible
– Permanent wilting point, θpwp = watercontent at which plants are unable toextract additional water
• If a soil is saturated and then allowed to drain, its water content will decrease indefinitely in a quasi-exponential manner, with the drainage rate negligible within a few days to a week
tot
void
VV
MineralsWaterAirVolumeWaterAirVolume
=&,
&
ott
w
VV
MineralsWaterAirVolumeWaterVolume
=&,
STEP July, 2007: Hydrology – Page No. 22
Dingman, 2002
Infiltration
• Ground-water zone: Saturated, positive pressure; in absence of ground-water flow pressure is hydrostatic , where p is the pressure, z is the height above the datum, and γw is the specific weight of water
• Tension-saturated zone (capillary fringe):Saturated zone above the water table due to capillary rise through the pore spaces; pressure is zero at the top of the water table and negative in the capillary fringe
• Intermediate zone: Water enters as percolation from above and leaves by gravity drainage
• Root zone: Layer from which plant roots can extract water, bounded by the surface above and an indefinite and irregular lower bound; water enters by infiltration and leaves via transpiration and gravity drainage
Hydrologic Horizons
2121 ;)()( zzzzzp w >−⋅= γ
STEP July, 2007: Hydrology – Page No. 23
Infiltration
• Infiltration is the process by which water arriving at the soil surface enters the soil column. The maximum rate that a soil can accept water is called the infiltration capacity, f(t)*.
• At a given point the infiltration rate, f(t), changes systematically with time and is influenced by:– The rate at which water arrives from above, w(t), or the depth of ponding on the surface, H(t)– The hydraulic conductivity of the soil, Kh
*
– Antecedent soil moisture
• Three general conditions during infiltration may be distinguished– No ponding: In this case the infiltration rate equals the water-input rate and is less than or
equal to the infiltrability
– Saturation from above: Ponding is present because the water-input rate exceeds the infiltrability in which case the infiltration rate equals the infiltrability
– Saturation from below: Ponding is present because the water table has risen to or above the surface in which case the infiltration rate is zero
The Infiltration Process
)()()(,0)( * tftwtftH ≤==
)()()(,0)( * twtftftH ≤=>
0)(,0)( =≥ tftH
STEP July, 2007: Hydrology – Page No. 24
Runoff
STEP July, 2007: Hydrology – Page No. 25
Runoff
Definitions• Watershed response to an input event is characterized by
stream discharge at a single point that defines the outlet of the watershed
• A graph of water input vs. time can be constructed from spatially averaged precipitation measurements and is called a hyetograph
• A graph of stream discharge vs. time is a streamflow hydrograph
• A storm hydrograph is the time trace made by an observer at a fixed point of a flood wave moving downstream
Basic Aspects of Stream Response
Rai
n (d
epth
/tim
e)D
isch
arge
(vol
ume/
time)
Hyetograph
Hydrograph
Dingman, 2002
STEP July, 2007: Hydrology – Page No. 26Dingman, 2002
Runoff
Streamflow• Streamflow is a spatially and temporally integrated response
determined by– Spatially and temporally varying input rates (precipitation, snow
melt, glacial melt)– Time required for each drop of water to travel from where it
strikes the watershed surface to the stream network (determined by length, slope, vegetative cover, soils, and geology of hillslopes)
– Time required for water to travel from its entrance into the channel to the point of measurement
• Flow may enter the stream at the surface, from overland flow and channel precipitation, and as subsurface flow, from groundwater and interflow
• Flow in the stream takes the form of a flood wave that moves downstream through the stream network
Basic Aspects of Stream Response
STEP July, 2007: Hydrology – Page No. 27
Dingman, 2002
Runoff
Effective Rainfall• Only a fraction of water input to the watershed actually appears in the response hydrograph, with
the remainder leaving the watershed as– Evapotranspiration– Streamflow that is realized too long after
the input event to be associated with that event (baseflow)
– Groundwater outflow (other than baseflow)• Depending on the type of model, it is often
necessary to estimate the effective rainfall from the hyetograph of water input
• There are several approaches used for this estimation as shown here
– a) Losses equal to a constant fraction of water input for each time period
– b) Losses equal a constant rate throughout event– c) Losses given by an initial abstraction followed by a constant rate– d) Losses given by an approximation to an infiltration-type curve
Response Hydrographs
STEP July, 2007: Hydrology – Page No. 28
Runoff
Hydrograph Separation• Event flow is streamflow resulting from the effective rainfall
• Hydrograph separation divides the hydrograph into a portion attributed to event flow and a portion attributed to baseflow
• Gaging station measurements of streamflow cannot distinguish event flow from flow originating from a previous event
• Therefore, graphical hydrograph separation is often used as a convenient delineation in order to analyze and model event responses and the factors influencing them
• Graphical separation does not actually identify flow from different sources
Response Hydrographs
After Linsley and Franzini, 1979
STEP July, 2007: Hydrology – Page No. 29
Flow statistics of three rivers near the headwaters of the Yukon River.
USGS, 2001
STEP July, 2007: Hydrology – Page No. 30
Runoff
US Geological Survey•http://water.usgs.gov/ Select Alaska Select “Real Time Data Table”
Select station Select data product (Daily data is usually best)
Where to Obtain Streamflow Data (and Groundwater Data)
STEP July, 2007: Hydrology – Page No. 31
Runoff
Simple Runoff Model
Rational Method• Regional equations suitable for assessing the impact of developing on peak discharge are not
generally available for small watersheds
• One widely used method, intended for use on small watersheds, is the Rational Method, which relates the peak discharge of an area, qp(ft3/s), to
– Drainage area, A (acres), – Rainfall intensity, i (in/hr),– Runoff coefficient, C
• Rainfall intensity is obtained from an intensity-duration-frequency (IDF) curve using a specified return period
• Primary use of Rational Method: design problems for small urban areas (small drainage areas, short times of concentration)
q = CiA
STEP July, 2007: Hydrology – Page No. 32
Runoff
• The model states consist of the contents of various conceptual reservoirs identified in the upper and lower soil zones
• Water fills and spills over in a cascade of reservoirs based on parameters that represent average soil characteristics in each reservoir
• This movement of water between compartments is governed by the precipitation rate, the capacities of each reservoir, evapotranspiration, and the rates at which water can transfer between compartments (infiltration, interflow, or percolation)
• While an infinite number of layers could be established, the goal of parameterization is to use no more than necessary to effectively describe the physical system
Sophisticated Model: Sacramento Soil Moisture Accounting Model
STEP July, 2007: Hydrology – Page No. 33
Runoff
Sophisticated Runoff Model
STEP July, 2007: Hydrology – Page No. 34
Groundwater
STEP July, 2007: Hydrology – Page No. 35
Groundwater
Basic Groundwater Characteristics
Cone of depression
Ocean
Pumped well
Unsaturated zone
Perched water table
Spring
Lake
Perched aquifer
Spring
Marsh
Influent stream(seepage from stream)
Effluent stream(seepage into stream)
Ground water flow
Water table
Artesian well
Bedrock
Piezometricsurface
Water table
Zone of saturation
Confined (artesian) aquifer
Confining layer
Unconfined aquifer
Infiltration
Percolation
Saltwater intrusion
Snow
Cone of depression
Ocean
Pumped well
Unsaturated zone
Perched water table
Spring
Lake
Perched aquifer
Spring
Marsh
Influent stream(seepage from stream)
Effluent stream(seepage into stream)
Ground water flow
Water table
Artesian well
Bedrock
Piezometricsurface
Water table
Zone of saturation
Confined (artesian) aquifer
Confining layer
Unconfined aquifer
Infiltration
Percolation
Saltwater intrusion
Snow
•The groundwater portion of the hydrologic cycle is rather complex– Water enters at the surface (infiltration),– Redistributes under forces of gravity, energy gradients, capillary rise, and evapotranspiration– Water percolates to lower water reservoirs (aquifers)– Groundwater flows under the influence of energy gradients– Groundwater may flow into or receive recharge from surface water bodies– Pumping of groundwater alters the region around the well by drawing down either the water
table or the piezometric surface (cone of depression)
Reproduced from McCuen, 1998
STEP July, 2007: Hydrology – Page No. 36
Bouwer, 1978
Groundwater
Definitions•Water that enters the soil is considered soil moisture while in the unsaturated zone and is called groundwater once in the saturated zone.•Within the saturated zone water occupies all pore space and is under hydrostatic pressure
•Aquifer- groundwater-bearing formations sufficiently permeable to transmit and yield usable quantities of water•Unconfined Aquifer- permeable underground formation having a surface at atmospheric pressure•Confined Aquifer- confined (or artesian) aquifers form between layers of very low permeability material
– If the layers are essentially impermeable they are called aquicludes– If the layers are permeable to transmit water vertically to or from the confined aquifer, but not permeable enough for lateral transport, they are called aquitards
Basic Groundwater Characteristics
STEP July, 2007: Hydrology – Page No. 37
Wetlands
STEP July, 2007: Hydrology – Page No. 38USGS, 2001
Handout: What are wetlands and why are they important?
STEP July, 2007: Hydrology – Page No. 39
Watershed Hydrology
• Ways that climate change might affect hydrology: (class suggestions- recall the components of the water budget)
• Example from the Nile basin
• How are Alaska and the Arctic different from lower latitudes?
• Evapotranspiration- temperature, soil moisture, vegetation
• Glacial fed streams
• Continuous permafrost regions
• Discontinuous permafrost
• Groundwater
• Storm frequency/intensity
Climate Change Considerations
30-Year Monthly Mean Flow
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
1 2 3 4 5 6 7 8 9 10 11 12
Month
Flow
(mcm
)
Base MaxBase MinBase MeanFuture MaxFuture MinFuture Mean
Blue Nile
30-Year Monthly Mean Flow
0
1000
2000
3000
4000
5000
6000
1 2 3 4 5 6 7 8 9 10 11 12
Month
Flow
(mcm
)
Base MaxBase MinBase MeanFuture MaxFuture MinFuture Mean
Atbara
30-Year Monthly Mean Flow
0
500
1000
1500
2000
2500
3000
1 2 3 4 5 6 7 8 9 10 11 12
Month
Flow
(mcm
)
Base MaxBase MinBase MeanFuture MaxFuture MinFuture Mean
Sobat
STEP July, 2007: Hydrology – Page No. 40
USGS, 2001
STEP July, 2007: Hydrology – Page No. 41
Water Resources
STEP July, 2007: Hydrology – Page No. 42
Handout: The development of Dryville
Water Resources Planning and Management
Introduction to Water Resources
STEP July, 2007: Hydrology – Page No. 43
Purpose Economic Development
Environmental Quality
Social Well-Being
Hydrologic Analysis
Public water supply X X WS, D, Q
Industrial water supply X X WS, D, Q
Irrigation X X WS, D, Q
Hydroelectric power X X WS
Navigation X X WS
Waste transport and treatment X X X WS, Q
Recreation X X WS, Q
Wildlife habitat X X WS
Reduction of flood damages X X F
WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency.
Goals
Water Resources Planning and Management
Hydrologic analysis for water resources management can be categorized according to the following assessments:1. Present and future supply of water available from surface and/or ground water sources;2. Present and future quality of surface and/or ground water;3. Present and future frequency with which human activities will be subject to floods; and4. Present and future frequencies of low streamflows and drought.
Relationships among water resources management goals, purposes, and types of analyses:
Role of Hydrologic Analysis
Purpose Economic Development
Environmental Quality
Social Well-Being
Hydrologic Analysis
Public water supply X X WS, D, Q
Industrial water supply X X WS, D, Q
Irrigation X X WS, D, Q
Hydroelectric power X X WS (D, F)
Navigation X X WS
Waste transport and treatment X X X WS, Q
Recreation X X WS, Q
Wildlife habitat X X WS (Q, F)
Reduction of flood damages X X F
WS = water supply; D = drought; Q = water quality; F = flood magnitude-frequency.
Goals
After Dingman, 2002
STEP July, 2007: Hydrology – Page No. 44
Water Supply
• Water Users
• Water Uses
• Infrastructure
• Resource allocation and competing objectives
Water in Our Communities
Environmental needs-users, uses, minimum requirements?
STEP July, 2007: Hydrology – Page No. 45
Water as a Hazard
STEP July, 2007: Hydrology – Page No. 46
USGS, 2001
STEP July, 2007: Hydrology – Page No. 47
Water as a Hazard
There’s not enough…• Storage: Reservoirs, tanks, groundwater
– Redistribute water for year round availability, according to demand– Mitigate the effects of drought (period of consecutive dry years)
• Emergency supply: fire flows require greater water volumes and much higher pressures
There’s too much…• Storms and runoff• Seasonal high flows • Excessive wet years
(storage plays an important role here as well)• Wetlands??
It’s dirty…• Sediment and mineral loads (water quality, water treatment)• Naturally occurring and introduced contaminants (suitability, pollution)• Flooding and water quality (contamination)
Planning, Management and Risk
STEP July, 2007: Hydrology – Page No. 48
Water as a Hazard
Impacts of Water Management
Activity Magnitude-Frequency Timing Duration Rate of Change
Damming Reduced variability (WS, FC); Reduced peak flows (FC)
Altered (WS, FC, HP)
Reduced periods of inundation (FC)
Rapid fluctuations (HP)
Diversion Reduced flows; Reduced variability Altered
Urbanization and drainage Increased variability; Increased peak flows
Reduced periods of floodplain inundation due to stream entrenchment
Levees and channelization May increase downstream peak flows
Reduced periods of floodplain inundation
Groundw water pumping Reduced low flows
Deforestation Increased variability; Increased peak flows; Reduced low flows
WS = water supply; FC = flood control; HP = hydropowerAfter Dingman, 2002
STEP July, 2007: Hydrology – Page No. 49
Water Management: Health, Safety, and the Environment
• Point source versus Non-point source (NPS)
• Types of pollutants:– Oxygen-demanding material– Nutrients– Pathogens– Suspended solids– Toxic metals and organic compounds– Heat
• Typical water quality concerns for types of water resources:
• Pollution management: “control the discharge of pollutants so that water quality is notdegraded to an unacceptable extent below the natural background level.” Davis and Cornwell
• To do so, we need to: Measure pollutants, predict impacts of pollutants, know background (natural) water quality, decide on acceptable water quality level for a water body.
Pollution
Water Resource pH Dissolved
SolidsSuspended
SolidsDissolved Oxygen
Organics and Petrleum
Compounds
Pathogenic Organisms Excess Heat
Precipitation X X
Ground Water X X X
Streams X X X X X X X
Lakes X X X X
"X" Indicates that a given type of water-quality constituent is typically of concern in a given type of water resource. After Dingman, 2002
STEP July, 2007: Hydrology – Page No. 50
Water Management: Health, Safety, and the Environment
Biological Oxygen Demand (BOD): dissolved oxygen consumed during the oxidation of an organic compound. Consumption of dissolved oxygen poses a threat to fish and other higher forms of aquatic life that must have oxygen to live.
Pollution
Davis and Cornwell, 1998
STEP July, 2007: Hydrology – Page No. 51
Climate Change and Water Resources
• Tools for understanding potential consequences of change (for better or worse)– Global Climate Models (GCMs), historical climate records– Hydrologic Models (& other environmental models)– Water Resources Systems Models– Verification requires observations and an understanding of uncertainty
• Climate Change Impact Assessments:– Climate change and water supply– Climate change and infrastructure– Climate change and ecology– Sustainability– …
• What are the appropriate time and spatial scales for these assessments?– How do we experience the change?– At what scales do we have confidence in projections and models?
• Hand-out: “Climate Change and Wetlands”
Potential Impacts of Climate Change
STEP July, 2007: Hydrology – Page No. 52
Water Science
The Water Source Books by the Environmental Protection Agency http://www.epa.gov/safewater/kids/wsb/index.html
Water Science for Schools by the US Geological Surveyhttp://ga.water.usgs.gov/edu/
Watershed Game at the Bell Museum site http://www.bellmuseum.org/distancelearning/watershed/watershed2.html
Rivers 2001 by the National Geographic Society http://www.nationalgeographic.com/geographyaction/rivers/ga22.html
Alaska Wildlife Curriculum Teacher’s Guide by the Alaska Dept of Fish and Game http://www.wildlife.alaska.gov/index.cfm?adfg=education.awc
Additional Resources
STEP July, 2007: Hydrology – Page No. 53
References
Davis, M. and D. Cornwell, 1998. Environmental Engineering. 3rd Ed. McGraw-Hill, Boston, 919 pp.
Dingman, L., 2002. Physical Hydrology. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 646 pp.
McCuen, R., 1998. Hydrologic Analysis and Design. 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey, 814 pp.
Thompson, S., 1999. Water Use, Management, and Planning in the United States. Academic Press, San Diego, 371.
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