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III/ I \I ~ I :1 Methodology Definitions Potential crop consumptiH' use. The amount of water required to meet the evapotranspiration needs of a crop at the field level so that its production is not limited from lack of\~ater. Altemath'cly. the amoulll of waler, if available. thai would be removed from the soil and plant surfaces of a vegetati..,c area. Potential Crop ConSUmpll\e u~ r-.;ellrrigation Rcquirem\.'1lt Effcetin' precipitation. Precipitation received during the gro\~ing period that is available to meet the potential consumptive use requirements of a crop. Alternatively. the portion of precipitation remaining on the foliage or in the soil thai is available for evapotranspiration, } ElTecthc Precipitation Net ir..-igation requirement. The amount of irrigation water--.exclusivc of precipitation. soil moisture. or ground.....atcr-that is required to meet potential crop consumptive use requirements. Net irrigation requirement excludes conveyance losses and leaching requirements, Description of Ihe j\lelhod TIle Natural Resources Conservation Service {NRCSI has calculated net irrigation requirements for up to 20 crops at 64 weather stations in 44 counties. using a modification of the Blaney.Criddle method and weather data.4 The NRCS calculated potentia] crop consumptive use by considering average monthly temperature. total monthly precipitation. typical planting and harvesting dates. e1ev"atioll. and crop growth-stage water requirement curves. Steps taken to derive Colorado' 50 1995 net irrigation water requirements of5.35 million acre-feet are listed below. First. in consultation with NRCS statT. normal year net irrigation requirements for each crop in each county were estimated from NRCS's report covering 44 counties (Table 1), Normaf.....ear is defined by the NRCS as a year in which the grov.'ing season elTecti\e precipitation will be equaled or exceeded five out of 10 years. In some cases. estimates \\ere based upon data from two locations within the same county. In counties with no NRCS locations. estimates v.cre derived from NRCS data from appropriate adjacent coullties. Second. county-Ievcl harvcsted acreage for 1995 for Colorado's major irrigated crops was obtained from the Colorado Agricultural Statistics Service (CASS)~. which updates this infonnation annually (see Table 2. columns 1-13). Harvested acreage for 1992 for Colorado's minor and specialty' irrigated crops. induding pasture1and. was obtained from the federal Bureau of the Censust> (see Table 2. columns 14-26), This data is updated once e\'er)' five years. Third. the total nct irrigation requirement for each crop and county was then estimated by "multiplying" its net irrigation requirement per acre [Tablc I J by its haf\C'sted irrigated acreage [Table 2] and dividing the result by 12 [Table 31. The resulting table was then simplified by combining some columns containing minor and specialty crops. producing Table 3. This "multiplication" is not straightfornard. as the column heading for Tables I and 2 are not idcntical. Table 4 displays the relationship uscd between these sets of column headings before cell-by-cell multiplication of Tables I and 2. Table I 1\et Irrigation Requirements per acre for 20 crops in 63 counties. (acre-inches/acre) x Table 2 Irrijl:lI.ted Uane!itro Cropllind for 26 crops in 63 counties. (acres) Table 3 Totlil Net Irrigation Requirements for each crop and CounI)' in Colorado, (acre. feet) x 1 fOOl 12 inches . Natural ReSl.1urces Consenalion Sen ice-USDA, EsII/PIaltJ 'rTigatedCrop Barer ReqtllremttllSfor Selec/l'd Crops and Locarions In Colorado. Unpublishcrlr.:port. 1990. (Lalc"ood. Col.,raJo 1990), S Colorado Agricultural Statistics Service-USDA. Colorado Agrkulrural Slatisrics 1996. (Lake-wood. Colorado 1996). ~ Bureau orm..: Censu~.U.S" D.:panmcnl ofC.,mrTll:rce. '99~ Cel1SU$ oj .--lgrICU{tUrt? Volume I. Pan 6: Colorado and Count) Data. (WlI!ihinglon.DC.. 1m).