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Input requirements for the orographic precipitation model are (1) an actual or predicted profile of temperature, humidity, and winds aloft entered in 50-mb (millibar) intervals, (2) a set of topographic grids (obtained for every 10-degree azimuth relative to true north) with a grid interval of 10 km or less, and (3) the "period of representativeness" of the input sounding (usually set as the time interval between input soundings, observed or predicted). For this study, a 5-km horizontal topographic grid interval and a 12-h period of representativeness werE~ used. Fast running time and usage of upper-air soundings routinely available every 12 h as input data were key considerations in constructing this operationally-oriented computational method. Therefore, no mesoscale modeling of the flow field over complex terrain was attempted. Rather, the air was assumed to flow along gridlines, with the topographic grid x-axis aligned with the 700-mb wind direction and with the x-component wind speeds for each 50-mb layer computed accordingly. Thus, a topographic grid of different orientation may be needed for each model run that uses a different input upper-air sounding, depending upon the amount of change in the 700-mb wind direction. A key feature of the orographic precipitation model is its simulation of upstream barrier "precipitation-shadowing" effects. Some model features, such as precipitation efficiency, can be varied if desirable, when adapting the model for use in an area. A weakness of the model is that although precipitation quantities in mountainous areas are obviously highly controlled by topographic features, non orographic influences, which are not modeled, are also important. The orographic precipitation model keeps track of the condensate or evaporation caused by forced vertical displacements as the air flows over the underlying topography. (Condensate is the product of condensation, which in meteorology is the physical process by which water vapor becomes liquid; evaporation is the opposite of condensation.) For rising air at a given grid point, part of the condensate precipitates. The rest moves downstream to the next grid point, where a fraction (precipitation efficiency) of it and the condensate generated by additional orographic lift precipitates. For sinking motion, part or all of the parcel cloud water evaporates. Precipitation falling into a layer from above partially (or totally) evaporates when encountering subsaturated conditions. Eventually, precipitation generated in each layer reaches the ground, provided it does not totally evaporate. Orographic precipitation model computations are made at the pressure mid-points of 50-mb- thick layers, up to as high as the 450-mb level, depending on where the top of the moisture is found. The model's "moisture top" is defined as the highest level with at least 65 pct relative humidity which is not undercut by any lower layer(s) of less than 50-pct relative humidity. Over a given grid interval, computations are made for the highest layer first and proceed downward. When computations are completed for all layers over that grid interval, a step forward (downwind) along the grid line is made by incrementing location indices. Thus, computations proceed one grid line at a time. A printout of precipitation for each grid point gives the resulting map of amounts. Specific measurement site amounts and area averages for desiIied watersheds can also be calculated. 2.2 Hydrologic Model Description The HED71 rainfall-runoff simulation hydrologic model briefly described in this section has been a key tool for flood forecasting in northern California for over two decades. A more . complete description of this hydrologic model is contained in the Program Manual prepared 2