bfc 32002 hydrology chapter 2. precipitation

BFC 32002 Hydrology

Chapter 2. PrecipitationZarina Md Ali

Based on BFC 32002 Hydrology Module Email: [email protected]

: 074564359 / 0197722315Room: Level 6, South East Tower, FKAAS

mailto:[email protected]

Learning Objectives

At the end of the course, students should be able to:

• Define precipitation, its forms and types.

• Estimate point and areal precipitation amounts from gauge data.

BFC32002_Ch2/ZARINA'S

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Precipitation

• a major component of the hydrologic cycle.

• defined as any product of the condensation of atmosphericwater vapor (solid or liquid) that is deposited on the earth’ssurface, its form and quantity thus being influenced by theaction of other climatic factors such as wind, temperatureand atmospheric pressure

• occur in many different forms when reaches the surface ofthe earth including rain, storm, snow, hail, drizzle and sleet.


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Formation of precipitation


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Classification of precipitation


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Type DescriptionSnow Complex ice crystals. A snowflake consists of

agglomerated ice crystals. The average water content ofsnow is assumed to be about 10% of an equal volume ofwater.

Hail Balls of ice that are about 5 to over 125 mm in diameter.Their specific gravity is about 0.7 to 0.9. Thus, hailstoneshave the potential for agricultural and other propertydamage.

Sleet Results from the freezing of raindrops and is usually acombination of snow and rain.

Classification of precipitation


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Type DescriptionRain Consists of liquid water drops of a size 0.5 mm - 7 mm in

diameter.

Drizzle Very small, numerous and uniformly dispersed waterdrops that appear to float while following air currents.Drizzle drops are considered to be less than 0.5millimeter diameter. The settling velocity is slow, withthe intensity rarely exceeding 1 mm / hr. It is also knownas warm precipitation.

Cont. Precipitation

• Precipitation is important because it helps maintain theatmospheric balance.

• Precipitation helps farmers grow crops and provides afresh water supply for us to drink. Without precipitation,all of the land on the planet would be desert.

• Precipitation can also be damaging. Too much rain andsnow can cause severe flooding and lots of trafficaccidents. Hail can damage crops and cars. Freezing rainand sleet can destroy trees and power lines.


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Precipitation types


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Three major categories of precipitation are:1. Convective 2. Orographic 3. Cyclonic/Frontal

Convective precipitation


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• typical of the tropics such as in South East Asia.• it’s maybe in the form of light showers or storms of

extremely high intensity (thunderstorms).

Orographic precipitation


10• very common on the West Coast of the United States

Cylonic precipitation


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• associated with the movement of air masses from highpressure regions to low pressure regions.

• These pressure differences are created by the unequalheating of the earth’s surface.

• This precipitationmay be classifiedas frontal or non-frontal.

Measurement of precipitation• Precipitation is measured as the vertical depth that would

accumulate on a flat level surface if all the precipitationremained where it had fallen.

• These data can be defined in terms of:

• Depth (d), is the sum of rainfall

• Intensity (i), or depth of rainfall per unit time,

• Duration (t) is the duration of a storm is the time fromthe beginning of rainfall to the point where the masscurve becomes horizontal

• Frequency, it is usually called as return period (T)

• Area (A), is the area of rainfall geographic.

• Point rainfall can be plotted as accumulated total rainfall oras rainfall intensity at a particular gauge.


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Example 2.1

Time

(min)

0 10 20 30 40 50 60 70 80 90

Rainfall

(cm)

0 0.18 0.21 0.26 0.32 0.37 0.43 0.64 1.14 3.18

Time

(min)

100 110 120 130 140 150 160 170 180

Rainfall

(cm)

1.65 0.81 0.52 0.42 0.36 0.28 0.24 0.19 0.17


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From the precipitation data given, estimate cumulative rainfall and rainfall intensity.

Time (min) Rainfall (cm)

Cumulatif rainfall(cm)

Rainfall intensity(cm/hour)

0 0 0

10 0.18 0.18 1.08

20 0.21 0.39 1.26

30 0.26 0.65 1.56

40 0.32 0.97 1.92

50 0.37 1.34 2.22

60 0.43 1.77 2.58

70 0.64 2.41 3.84

80 1.14 3.55 6.84

90 3.18 6.73 19.08

100 1.65 8.38 9.90

110 0.81 9.19 4.86

120 0.52 9.71 3.12

130 0.42 10.13 2.52

140 0.36 10.49 2.16

150 0.28 10.77 1.68

160 0.24 11.01 1.44

170 0.19 11.2 1.14

180 0.17 11.37 1.02


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• Time interval, t = 10 min = 0.167 hr

• Rainfall intensity = 0.18 cm / 0.167 hour = 1.08 cm/hr

• Cumulative rainfallis a plot ofcumulative rainfallversus time (min)while

• Rainfall intensity(cm/hr) data aretypically reportedin either tabularform or graphicalform (hyetograph).


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Rain gauge• Rain gauge is an instrument used to measure how much

rain has fall.

• There are several different types of rain gauges that are grouped by how they operate:

• non recording rain gauge

• recording rain gauge,


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Non recording rain gauge

010

020

030

040

050

8²

24²

20²

8²

2.53²

Support (wood) Overflow can Measuring

tubeReceiver

Measuring stick

(to road directly in .01 of an inch of precipitation)

One - tenth inch

One - hundredth inch division


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Non-recording gauge, 8-inch-dia opening

• the standard rain gauge is a standard8-inch-dia rain gauge (203 mm)

• A smaller metal tube may be locatedin this larger overflow can.

• An 8-inch-diameter receiver cap maybe on top of the overflow can and isused to funnel the rain into thesmaller tube until it overflows.

• The receiver cap has a knife edge tocatch rain falling precisely in thesurface area of an 8-inch-diameteropening.

• Measurements are made using a special measuring stick with graduations devised to account for the 8-inch receiver cap opening, funneling water into the smaller tube.

• When the volume of the smaller tube is exceeded, the volume from the smaller tube is dumped into the larger overflow can.

Recording rain gauge


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• Non recording gauge which requires an observer to manually measure the rain at regular intervals (i.e. every 24 hours).

• While, recording gauges does not require constant observation.

• There are at least three types of gauges commonly in use to record depth:• Weighing gauge• Tipping bucket• Float type

Weighing gauge


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• Consists of a storage bin, which isweighed to record the mass.

• Certain models measure the massusing a pen on a rotating drum, or byusing a vibrating wire attached to adata logger.

• The advantages: it does not underestimate intense rain, & itcan measure other forms of precipitation.

• These gauges are more expensive and require moremaintenance than tipping bucket gauges.

• This gauge also contains a device to measure the quantity ofchemicals contained in the locations atmosphere.

• This is extremely helpful for scientists studying the effects ofgreenhouse gases released into the atmosphere and theireffects on the levels of the acid rain.

Tipping Bucket


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• Consists of a large copper cylinder set into theground & a funnel (at cylinder top) that collectsand channels the precipitation.

• The precipitation falls onto one of two smallbuckets or levers which are balanced in samemanner as a scale.

• After an amount of precipitation equal to 0.2 mm(0.007 in) falls the lever tips and an electricalsignal is sent to the recorder.

• The recorder consists of a pen mounted on an armattached to a geared wheel that moves once witheach signal sent from the collector.

• When the wheel turns the pen arm moves eitherup or down leaving a trace on the graph and atthe same time making a loud click.

Tipping Bucket


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• The chart is measured in 10 minuteperiods (vertical lines) and 0.4 mm(0.015 in) (horizontal lines) and rotatesonce every 24 hours and is powered by aclockwork motor that must be manuallywound.

• This gauge is not as accurate as thestandard rain gauge because the rainfallmay stop before the lever has tipped.

• When the next period of rain begins it may take no morethan one or two drops to tip the lever.

• This would then indicate that 0.2 mm (0.007 in) has fallenwhen in fact only a minute amount has.

• The advantage: the character of the rain (light, medium orheavy) may be easily obtained.

Float type


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• In this type of instrument, the rain passes into a float chamber containing a light float.

• As the level of the water within the chamber rises, the vertical movement of the float is transmitted, by a suitable mechanism, to the movement of a pen on a chart or a digital transducer.

• By suitably adjusting the dimensions of the collector orifice, the float, and the float chamber, any desired chart scale can be used.

Location for rain gauge• Buildings, landscaping and trees, and even the wind can

affect the amount of precipitation arriving at the raingauge.

• Proper placement is critical to ensure that rain sensorreadings are an accurate representation of the actualrainfall.

• The ideal site for a rain gage is in an open area that isprotected from the wind in all directions.

• A good guideline is to allocate the rain gauge at a minimumdistance of twice the height of building/tree away. B

FC32002_Ch2/ZARINA'S

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Missing data• Precipitation measuring stations sometimes fail in providing

a continuous record of precipitation.• Instruments do malfunction and back-up systems may not

always provide accurate data.• A tipping – bucket may not function for a short period of

time and the back-up volume gage may not provide time –related data.

• For a non automatic recording gage, an individual may failto record the data and miss a visit to the site.

• Thus, there are generally missing data, the values of whichmust be estimated.

• The procedure for estimating daily totals relies on the datafrom adjacent stations. The locations of the adjacentstations are such that they are close to and approximatelyevenly spaced around the site with the missing data.


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Missing Data: Point precipitation


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• Precipitation events are recorded by gauges at specificlocations.

• Precipitation measured at a rain gauge is called pointrainfall.

• Methods:• Arithmatic Mean Method• Normal Ratio Method• Quadrant Method

Arithmetic Mean Method


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Example 2.2


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Rain gauge X was out of operation for a month during which therewas a storm. The rainfall amounts at 3 adjacent stations A, B, and Cwere 37, 42, and 49 mm, respectively. The average annualprecipitation amounts for the gauges are X = 694, A = 726, B = 752and C = 760 mm. Using the arithmatic method, estimate the amountof rainfall for gauge X.

Stations Amounts of precipitation (mm)

Normal annual precipitation (mm)

A 37 726B 42 752C 49 760X ? 694

Solution:

Normal Ratio Method


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Example 2.3


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The average annual precipitation amounts for the gauges A, B, C and D are 1120, 935, 1200 and 978 mm. In year 1975, station D was out of operation, while stations A, B and C recorded rainfall amounts of 107, 89 and 122 mm, respectively. Estimate the amount of precipitation for station D in year 1975.

Station Normal annual precipitation

Amounts of precipitation year

1975A 1120 107B 935 89C 1200 122D 978 X

PX = 95.3 mm

Solution:

Quadrant Method


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• This is station weighting technique.• A grid of point estimates is made based on a distance

weighting scheme.• Each observed point value is given a unique weight for

each grid point based on the distance from the gridpoint in question.

• The grid point precipitation value is calculated based onthe sum of the individual station weight multiplied byobserved station value.

• Once the grid points have all been estimated they aresummed and the sum is divided by the number of gridpoints to obtain the areal average precipitation.

Quadrant Method


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X

P

Q

R

S

U

T

I II

III IV

y

xx

y

4

2

4

2

424 2

Procedure:1. Consider that rainfall is to be

calculated for point X.2. Establish a set of axes running

through X and determine theabsolute coordinates of thenearest surrounding points P, Q,R, S, T and U.

3. The estimated precipitation at X is determined as aweighted average of the other six points. The weights arereciprocals of the sums of the squares of distance X and Y;that is,

X

P

Q

R

S

U

T

I II

III IV

y

xx

y

4

2

4

2

424 2

Example 2.4


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Stations A, B, C, D, E, F and G are the gauge stations. Raingauge at station A was out of operation. Precipitationamounts for other stations were 40, 45, 37.5, 50, 47.5 and42.5 mm. Calculate the rainfall depth at station A withcoordinates (0,0) using the quadrant method.

Solution:


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Quadrant Station Precipitation, p

(mm)

Coordinate Li2 Wi WiPi

X Y

IB 40.0 4 2 20 - - -G 42.5 2 1 5 0.200 0.601 25.5

II F 47.5 -5 2 29 0.034 0.102 4.85

IIID 37.5 -3 -2 13 0.077 0.231 8.66

E 50.0 -3 -3 18 - - -IV C 45.0 1 -6 37 0.027 0.081 3.65

Total 0.333 Total 42.66

2iL

1

From above table precipitation at station A is 42.66 mm

Gage Consistency • When the catch at rain gauges is inconsistent over a

period of time and adjustment of the measured data isnecessary to provide a consistent record.

• Double Mass Curve analysis is a technique commonlyemployed to detect changes in data-collectionprocedures or conditions at a given location.

• The changes may result from changes in instrumentation,changes in observation procedures or changes in gaugelocation or surrounding conditions.

• A double mass curve is a plot of the accumulation of theobserved element over time for one location (teststation) versus the accumulation over time for areference location (base station).


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Gage Consistency • The mass curve is approximately a straight line if the

variations at both test and base stations are quiteconsistent.

• Any break point in the curve suggests a possible changeat the test station in relation to the base station.

• If a change in slope is evident, then the record needs tobe adjusted, with either the early or later period ofrecord adjusted.


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Procedure of Double Mass Curve 1. Add the annual precipitation of base stations.

2. Cummulate the sums of Step 1.

3. Cummulate the annual precipitation for station X.

4. Plot graph accumulated annual precipitation Station Xversus accumulated precipitation of Base stations and compute the slope Mo and Ma.

5. Adjust the measured precipitation of gauge X using the general equation:


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P

Pox

PaxMa

Mo

Accu

mulated

total p

recipitatio

n at S

tation X

Accumulated precipitation of base stations

Example

../../../BFC_3092_0708/Examples Chapter 2.doc

Example 2.5


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Measured annual precipitation gauge for five stations (A, B,C, D and E) from 1926 until 1942 are given below. After 5years, gauge A was relocated at a new location due tochanges in land use that make it impractical to maintain thegauge at the old location. You are required to adjust therecord for the period from 1926 to 1930 using the recordsat gauges B, C, D and E.

Solution:


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Year Annual precipitation (mm) Total Cummulative precipitation

(mm)

A B C D E B+C+D+E A B+C+D+E

1926 32.9 39.8 45.7 30.7 37.4 153.6 33 1541927 28.1 39.6 38.5 41.0 30.9 150 61 3041928 33.5 42.0 48.3 40.4 42.0 172.7 95 4761929 29.6 41.4 34.6 32.5 39.9 148.4 124 6251930 23.8 31.6 45.1 36.7 36.3 149.7 148 7741931 58.4 56.5 53.3 62.4 36.6 208.8 206 9831932 46.3 48.1 40.1 47.9 38.6 174.7 253 11581933 30.8 39.9 29.6 32.7 26.9 129.1 283 12871934 46.8 45.4 41.7 36.1 32.4 155.6 330 14431935 38.1 44.9 48.1 30.7 41.6 165.3 368 16081936 40.8 32.6 39.5 35.4 31.3 138.8 411 17471937 37.9 45.9 44.1 39.2 44.1 173.3 449 19201938 50.7 46.1 38.9 43.3 50.6 178.9 499 20991939 46.9 49.8 41.6 49.9 41.1 182.4 546 22811940 50.5 47.3 49.7 47.9 39.0 183.9 597 24651941 34.4 37.1 31.9 32.2 34.5 135.7 631 26011942 47.6 45.9 38.2 52.4 47.3 183.8 679 2785


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Mean Areal Precipitation


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• The representative precipitation over a defined areais required in engineering application, whereas thegaged observation pertains to the point precipitation.

• The areal precipitation is computed from the recordof a group of rain gages within the area by thefollowing methods:• Arithmatic - mean Method• Thiessen Polygon Method• Isohyetal Method

Arithmatic - mean Method


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The arithmetic average method uses only those gaging stations within the topographic basin and is calculated using:

where;P = average precipitation depth (mm) Pi = precipitation depth at gage (i) within the topographic

basin (mm)n = total number of gaging stations within the

topographic basin


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Example 2.6Calculate average precipitation

Station Depth of rainfall (mm)

A 1.46

B 1.92

C 2.69

D 4.50

E 2.98

F 5.00

Total 18.55

Average precipitation = 18.55 /6 = 3.09 mm

Thiessen Polygon Method


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• This technique is quick to apply for multiple stormsbecause it uses fixed sub-areas.

• It is based on the hypothesis that, for every point in thearea, the best estimate of rainfall is the measurementphysically closest to that point.

• This concept is implemented by drawing perpendicularbisectors to straight lines connecting each two rain-gages.

• This procedure is not suitable for mountainous areasbecause of orographic influences.



45Thiessen Polygon Method



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The procedure involves:1. Connecting each precipitation station with straight lines; 2. Constructing perpendicular bisectors of the connecting

lines and forming polygons with these bisectors; 3. The area of the polygon is determined.


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Station Area (km2) Precipitation (mm)

Area x precipitation (km2.mm)

A 72 90 6480B 34 110 3740C 76 105 7980D 40 150 6000E 76 160 12160F 92 140 12880G 46 130 5980H 40 135 5400I 86 95 8170J 6 70 420

Total 568 1185 69210

Example 2.7Using data given below, estimate the average precipitation using Thiessen method.

Calculated

Isohyetal Method


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• based on interpolation between gauges and closelyresembles the calculation of contours in surveying andmapping.

• The isohyetal method is the most accurate approach fordetermining average precipitation over an area.

• Procedure:1. plot the rain gauge locations on a suitable map & record

the rainfall amounts.2. interpolation between gauges is performed and rainfall

amounts at selected increments are plotted.3. Identical depths from each interpolation & then connected

to form isohyets (lines of equal rainall depth).4. The areal average is the weighted average of depths

between isohyets, that is, the mean value between theisohyets.


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Isohyetal Method


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Use the isohyetal method to determine the averageprecipitation depth within the basin for the storm.

Example 2.8

Isohyetalinterval(mm)

Average precipitation

(mm)

Area (km 2)

Area x Average

precipitation (km2.mm)

<10.0 10 0 010 - 20 15 84 126020 – 30 25 75 187530 - 40 35 68 238040 - 50 45 60 270050 - 60 55 55 302560 - 70 65 86 5590Total 428 16830

Summary


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• Precipitation input is the main driver of the hydrologiccycle, as it relates to river flow, water supply and urbandrainage.

• Too much or too little can mean the difference betweenprosperity and disaster.

• In between these extremes are the normal precipitationevent that are experienced with a frequency andintensity related mainly to geographic position andtopographic features.

• At the end of this chapter you should be able to estimatepoint and areal precipitation amounts from gauge dataand conceptualize simple hydrologic process models.

THANK YOU


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bfc 32002 hydrology chapter 2. precipitation

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