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<br />sampling errors usually result because of the preponderance of stations <br /> <br /> <br />at low elevation. Their average usually represents less precipitation <br /> <br /> <br />than occurs on the basin as a whole. In such regions, an isohyetal map <br /> <br /> <br />can still be employed to advantage if the effect of topography is taken <br /> <br />into consideration in locating isohyetal lines. <br /> <br /> <br />Where average annual precipitation varies appreciably over a basin <br /> <br /> <br />and storm preeipitation usually reflects such variation (as in mountainous <br /> <br /> <br />regions), an adjustment by use of long-term averages can be made. The <br /> <br /> <br />average storm precipitation for a basin is determined by multiplying the <br /> <br /> <br />weighted storm precipitation ,at each station by the ratio of the average <br /> <br /> <br />annual precipitation of the basin to the weighted average annual precipi- <br /> <br /> <br />tation at that station. The long-term average for the basin is determined <br /> <br /> <br />by one of the methods previously discussed, using all stations having <br /> <br />appreciable record. Where average annual precipitation is not a goo~ indi- <br /> <br /> <br />cator of the areal variation of storm precipitation, average precipitation <br /> <br /> <br />for the rainy season, average storm precipitation, or storm precipitation <br />. <br /> <br />amounts exceeded with a specified frequency (such as once in 10 years) can <br /> <br /> <br />be used for this adjustment. The principal advantage of this type of adjust- <br /> <br /> <br />ment is that some information on normal area patterns is obtained from stations <br /> <br />that do not have records for a particular storm. <br /> <br /> <br />Computer program number 723-Gl-L2260, "Basin Rainfall and Snowmelt <br /> <br /> <br />Computation," described in Appendix 2, is designed to accept preCipitation <br /> <br /> <br />gage rainfall data for a specific storm period and time interval for an en- <br /> <br /> <br />tire region, and to compute basin-mean values for any number of basins in <br /> <br /> <br />the region with output time intervals that are specified multiples or <br /> <br /> <br />fractions of the input time interval. These routines are also incorporated <br /> <br /> <br />into computer program, HEC-l, "Flood Hydrograph Package, n which is described <br /> <br /> <br />in Volume 1 of this report. <br /> <br />2-07 <br />