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amounts corresponding to specified frequencies and durations. Such maps have been prepared by the U.S. National Weather Service for the United States. and a portion of one of those maps is shown in figure 2.03. Such point-rainfall estimates are often converted to area-rainfall estimates by leans of general relationships developed between point rainfall and area rainfall. The proper IIlethod to do this is to perfom frequency studies of average rafnfall recorded simultaneously at a number of stations and to compare point-rainfall amounts ~th area-rafn- fall amounts for various specified frequencies. This should be done for various station groupings and frequencies. In each case. the area size represented by the stations whose amounts are averaged should correspond to an area circumscribing the stations a distance beyond outlying stations equal to about half of the average distance between stations in the group. Relationships developed for this purpose are illustrated fn figure 2.04. Depth-area-duration diagrams representing precipitation for a specified frequency at a river basin can be determined by averaging map values of point precipitation over the river basfn for each of various pertinent durations. Relatfonshfps sfmilar to those fllustrated in figure 2.04 can then be applied to these point values in order to develop the full depth-area-duration relationship. This relationship can be illustrated as shown in figure 2.01. but the frequency relationship will be different quantitatively from the relationship for a single storm. particularly in that amounts will not decrease as rapidly with increasing area size. Section 2.07. Snowpack and snowmelt In regions where snowmelt is a major contribution to flood runoff. snow accumulates on the ground during the winter season and melts during the spring. The primary parameter of snowpack of interest from a snow- 2-10