FLOOD03276

• County: Statewide
• Basin: Statewide
• Title: Hydrologic Engineering Methods for Water Resources Development Volume 4
• Date: 10/1/1973
• Prepared For: US
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

precipitation. The area of influence of each station is obtained by

constructing polygons determined by drawing perpendicular bisectors to

lines connecting the stations, as shown in fig. 2.02b. The bisectors are

the boundaries of the effective area for each station. The area inclosed

by each polygon is measured, and weighted average precipitation for the

total area is computed by multiplying the precipitation at each station by

the proportion of the total area within its polygon and adding the products.

The results are usually considered to be more accurate than those obtained

by simple arithmetic averaging. However, this method may require exten-

sive computation, since a new Thiessen diagram is required for each change

of station network, and the wethod does not account for orographic influ-

ences or the erratic nature of precipitation.

Isohyetal Method

A more accurate method of averaging precipitation over an area and,

at the same time, showing the areal variation, is the isohyetal method.

A precipitation-depth contour map is determined by plotting station pre-

cipitation and constructing lines of equal precipitation called isohyets,

as illustrated in fig. 2.02c. Average depths are obtained by measuring

the areas between adjacent isohyets. Each increment of area is multiplied

by the estimated average precipitation depth for that area. The separate

terms are then added and the sum is divided by the total area to obtain

the average depth. The isohyetal method permits the use and interpreta-

tion of all available data and is well adapted for display and examina-

tion. In constructing an isohyetal map, the engineer can make full use

of his knowledge of orographic effects and storm morphology, and in this

way the final map should represent a more realistic precipitation estimate

than could be obtained from the gaged amounts alone. The accuracy of

the isohyeta1 method is, to a large degree, dependent upon the skill of

the engineer performing the analysis. If linear interpolation between

stations is used, the results will be essentially the same as those

obtained with the Thiessen method. In mountainous areas, systematic

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