Hydrology Part 3 Precipitation Measurement

8/10/2019 Hydrology Part 3 Precipitation Measurement

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Mean Areal Depth of

Precipitation Method

Keep it si


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Precipitation Units

Average precipitation is measured by the totaof precipitation falling over a given catchmenevery hour.

A.P. = (Volume/Area)/Hour

Volume per Area is equal to the Average depth.

A.P. = Average Depth/Hour

Usually expressed in inches/millimetres per hour.


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Methods of Measurement

Arithmetic Mean

Thiessen Polygon Method

Isohyetal Method

Distance Weighting


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Arithmetic Mean

The Mean Areal depth of precipitation is comtaking the average of the readings.

Easiest method but accuracy depends on theof stations where reading is taken.


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Arithmetic Mean Procedure

The Average precipitation average of the station read

. . =

. . =.55:.7:.:5.:.

5

. . = 2.21 .


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Thiessen Polygon Method

In this method, each reading is assumed to bethroughout an influence area, the average pis then computed by taking the total volume odivided by the total area.

More accurate than Arithmetic Mean.


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Thiessen Polygon Method Proce

The first step is to costations with straightshow in figure a.


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Thiessen Polygon Method Proce

Perpendicular bisecthen drawn in everybetween two statio


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Distance Weighting Method

In this method, stations closer to the centroid oarea are given more emphasis than the farthe


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Distance Weighting Method Pro

The distance of the the centroid is com

Compute the k for estation.

=1

The weight of the stcomputed using:

=


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Distance Weighting Method Pro

Station distance k w Precipitation

A 1.1 0.826 0.096 0.55

B 0.5 4.000 0.462 0.87

C 0.6 2.778 0.322 2.33

D 1.4 0.510 0.059 5.40

E 1.4 0.510 0.059 1.89

= 8.624


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Isohyetal Method

The most accurate of the methods, but also thlaborious.

Isohyets are drawn and the area between twis multiplied by the average precipitation in thsum up all the results and dividing by the total

will get the average precipitation.

Isohyets = Iso + Hye

Equal Precipi


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Isohyetal Method

Draw the isohyets.

Estimate the precipevery grid within the

Sum up the precipitdivide by the total a

A.P.=

. . = 1.90


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Graphical Representation of Rai

Keep it simple.

Hydrology Part 3 Precipitation Measurement

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