hydrology part 3 precipitation measurement
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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.
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