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<br />by CDWR (California Department of Water Resources) staff (Buer, 1988). Despite great <br />advances in flood hydrology and computer technology during the past two decades, this model <br />has persisted because it is well formulated for California's variable storm regime, and it is <br />stable, efficient, and simple to use. <br /> <br />This hydrologic model is an AI (Antecedent Indexing) regression type model that is used <br />operationally by the CDWR and the NWS's CNRFC (California-Nevada River Forecast <br />Center) for basin headwater runoff and flood forecasting. (The antecedent index roughly <br />corresponds to the number of inches of rain needed to get 1 in. of runoff.) This model has <br />also been used by Reclamation's Mid-Pacific Region in California in connection with reservoir <br />operations during periods of heavy precipitation. HED71 was designed to effectively model <br />the following: <br /> <br />· Effect of precipitation input - amount and type <br />· Losses caused by evaporation, infiltration, and detention <br />· Effect of snow on the ground upon precipitation <br />· Surface runoff routing <br />· Ground-water flow (base flow) <br /> <br />Input requirements for this rainfall-runoff simulation hydrologic model are: (1) a soil <br />moisture AI, (2) an initial base flow, (3) an estimate of the rain/snow level, (4) an elevation- <br />dependent specification of preexisting snowpack water equivalent, and (5) an estimate of the <br />mean basin precipitation in 6-h increments out to any reasonable length of time into the <br />future. <br /> <br />Each new storm rainfall-runoff simulation assumes that any flow in the stream is base flow, <br />which recedes much more slowly than surface runoff. Occasionally, a new AI value must be <br />inserted in one of the simulation periods. This insertion is done during a storm if a <br />significant break occurs in the precipitation, which allows the soil to recover part of its <br />infiltration capacity while surface runoff is still draining from the basin. <br /> <br />For each simulation period, the hydrologic model program considers the effect of snowpack <br />and temperature on precipitation reaching the ground. The model analyzes each elevation <br />zone in turn, then integrates to get a basin wide average. <br /> <br />If snowpack is present on the ground in part of the basin where precipitation is falling as <br />rain, part of the snow will be melted by the combined effect of wind and rainfall. Any <br />snowpack that remains will delay the passage of rain and melt to the ground surface. The <br />deeper the pack (up to 254 mm [10 in.] of water equivalent), the greater the portion of rain <br />and melt delayed. Precipitation falling at elevations greater than the specified snow level <br />accumulates as snow on the ground and does not contribute to runoff during the storm <br />period. <br /> <br />The effective basin area and shape vary with the rain/snow level; consequently, the model <br />uses the unit hydrograph approach to convert surface runoff to streamflow. The model <br />breaks the basin into 12 elevation zones (at 1000-ft [305-m] increments for this study) and <br />computes an area-elevation curve. The model references a set of 12 hydrographs (one for <br />each elevation zone) calibrated to the basin and selects the proper unit hydrograph for the <br />snow level. <br /> <br />3 <br />