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land conditions. Land use types of "pasture or range" and "straight row crop" with good <br />quality hydrologic conditions were selected to represent irrigated land conditions. <br />Curve numbers are adjusted based on soil types. Soil types for this analysis were selected <br />from the Hydrologic Group described in the Water Features Table in STATSGO. Curve <br />numbers are also adjusted based on the preceding 5-day antecedent moisture condition <br />(AMC). The NEH uses the following AMC levels (SCS, 1985), determined using the <br />ranges shown in Table 1 below: <br />AMGI. Lowest runoff potential. The watershed soils are dry enough for <br />satisfactory plowing or cultivation to take place. <br />AMC-H. The average condition. <br />AMC-III. Highest runoff potential. The watershed is practically saturated from <br />antecedent rains. <br />Table 1 <br />Seasonal Rainfall Limits for AMC <br />AMC G Total 5-Da Antecedent Rainfall inches <br />roup Dormant Season Growin Season <br />I Less than 0.5 Less than 1.4 <br />II 0.5 to 1.1 1.4 to 2.1 <br />III Over 1.1 Over 2.1 <br />AMC values were calculated from daily precipitation data and used to adjust the curve <br />numbers for the analysis of non-irrigated lands. An AMC of III (wettest condition) was <br />used for irrigated lands, to reflect that irrigation water is being applied. <br />2) Estimate Precipitation to ET (PE) and Soil Storage (Ps) on Irrigated Lands <br />During the Growing Season (April through October) <br />The surface water runoff was subtracted from the total precipitation (PT_P~. The <br />remaining precipitation was estimated to go to the following uses: (a) satisfy crop ET and <br />soil moisture reservoir storage and (b) deep percolation: <br />a. The SCS Effective Precipitation Method outlined in Technical Release 21 <br />(described in the SPDSS memorandum for Task 61 -Effective Precipitation <br />Estimates for Determining Crop Irrigation Water Requirements) was used to <br />estimate the component of precipitation that directly satisfies ET on a monthly <br />time step. Consistent with accepted engineering analyses, this monthly approach <br />considers precipitation that may be held in soil moisture storage and made <br />available for subsequent use by the crops (PE + Ps). <br />b. Equation 1 was used to calculate deep percolation (P~). <br />Root zone depths of 3.3 feet and 4.9 feet were used to represent irrigated row-crop and <br />irrigated alfalfa (corresponding to root zone depth recommendations in ASCE Manual <br />70). <br />Task 64.doc 9 of 13 <br />