RainfallHomeworkSolution

RainfallHomeworkSolution - Physical Hydrology 2/e S.L...

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Physical Hydrology , 2/e S.L. Dingman ArealPptnExrSM.doc COMPUTATION OF AREAL PRECIPITATION FROM POINT VALUES Arithmetic, Thiessen, Eyeball Isohyetal Methods Attached is a map showing the boundaries of a fictitious watershed and the locations of nine rain gages. The numbers are the gage catches for a particular storm in mm. 1. Compute the average rainfall on the watershed using the following three methods: (a) arithmetic average; (b) eyeball isohyetal; (c) Thiessen polygon. Use the grid squares to estimate areas. (a) In computing the arithmetic average, one would probably use only those gages that are within the watershed boundary. Equation (4-13) then gives P ˆ = (28 + 37 + 68 + 114 + 75 + 126)/6 = 74.67 75 mm. If all gages are used, we get P ˆ = (28 + 37 + 68 + 114 + 75 + 126 + 16 + 38 + 44)/9 = 59.56 60 mm. (b) The attached figure shows a plausible isohyetal configuration drawn by eye.
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RainfallHomeworkSolution - Physical Hydrology 2/e S.L...

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