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PD-PT-PQ-PE-PS <br />Where: <br />(I) <br />PD =Precipitation recharge to the aquifer (deep percolation) <br />PT =Total precipitation <br />PQ =Precipitation to surface water runoff <br />PE =Precipitation that directly satisfies growing season evapotranspiration (ET) and non- <br />growing season evaporation/sublimation <br />Ps =Precipitation stored in the soil moisture reservoir <br />The soil moisture balance approach described in Equation 1 was implemented in the following <br />steps. <br />1) Estimate Runoff (PQ) from Irrigated and Non-Irrigated Lands During the <br />Growing Season (April through October) <br />The SCS Curve Number approach (described in the SPDSS memorandum for Task 61 - <br />Effective Precipitation Estimates for Determining Crop Irrigation Water Requirements) <br />was used to estimate the surface water runoff (P~ component of precipitation during the <br />irrigation season. This approach uses daily total precipitation and a curve number to <br />calculate daily surface water runoff; daily runoff estimates were cumulated on a monthly <br />basis. <br />Curve numbers are a function of soil type, land use, and antecedent moisture condition. <br />Curve numbers for this analysis were selected from the National Engineering Handbook <br />Volume 4, NEH-4 (SCS, 1985). <br />The NEH classifies soil into the following four classes: <br />A. High infiltration rates even when thoroughly wetted and consisting chiefly of <br />deep, well to excessively drained sands or gravels. These soils have a high rate <br />of water transmission (low runoff potential). <br />B. Moderate infiltration rates when thoroughly wetted and consisting chiefly of <br />moderately deep to deep, moderately well to well drained soils with moderately <br />fine to moderately coarse textures. These soils have a moderate rate of water <br />transmission. <br />C. Slow infiltration rates when thoroughly wetted and consisting chiefly of soils <br />with a layer that impedes downward movement of water, or soils with <br />moderately fine to fine texture. These soils have a slow rate of water <br />transmission. <br />D. Soils having very slow infiltration rates when thoroughly wetted and consisting <br />chiefly of clay soils with a high swelling potential, soils with a permanent high <br />water table, soils with a claypan or clay layer at or near the surface, and shallow <br />soils over nearly impervious material. These oils have a very slow rate of water <br />transmission. <br />Task 64.doc 7 of 13 <br />