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• Assign Soil Moisture Capacities to Parcels and Structures. Soil moisture capacity <br />estimates were determined for each ditch system or aggregated ditch system based on <br />Colorado STATSGO mapping and irrigated acreage parcel locations. The approach <br />and results are provided in a memorandum attached as Appendix C. <br />• Select and Fill Key Climate Stations. Key climate stations were selected for the Rio <br />Grande Basin based on their period of record and location. Precipitation, <br />temperature, and frost data were filled as needed to create monthly data for the study <br />period. The approach and results are provided in a memorandum attached as <br />Appendix D. <br />• Assign Climate Stations to Irrigated Parcels. Climate station weights were assigned <br />to each County/HUC combination, based on climate station and irrigated acreage <br />locations. The approach and results are provided in a memorandum attached as <br />Appendix E. <br />• Calibration of Blaney-Criddle Coefficients. San Luis Valley calibrated crop <br />coefficients for the Blaney-Criddle method were developed through comparisons of <br />potential ET results using the Modified Hargreaves ET method for thirteen years. <br />The approach and results are provided in a memorandum attached as Appendix F. <br />• Execute Calibrated Model -Unit Irrigation Water Requirement. StateCU was <br />exercised with calibrated crop coefficients and parameters to develop unit irrigation <br />water requirements at climate stations in the basin for the four major crops grown in <br />the basin. The approach and results are provided in a memorandum attached as <br />Appendix G. <br />2.4 Definitions <br />Several terms used in this report have been broadly used in other studies. The following <br />definitions are consistent with the American Society of Civil Engineers Manuals and <br />Reports on En ing eerin~ Practice No. 70 -Evapotranspiration and Irrigation Water <br />Requirements. <br />Potential Evapotranspiration (ET) The total amount of water that would be <br />used for crop growth if provided with an ample water supply. Also called <br />potential consumptive use. <br />Effective Precipitation The portion of precipitation falling during the crop- <br />growing season that is available to meet the evapotranspiration requirements of <br />the crop. <br />Irrigation Water Requirement The amount of water required from surface or <br />ground water diversions to meet crop consumptive needs. Calculated as potential <br />evapotranspiration less effective precipitation. <br />Water Supply-Limited Consumptive Use The amount of water actually used by <br />the crop, limited by water availability. Also called actual consumptive use. <br />cureport_6-2004.doc 11 of 48 June, 2004 <br />