SPDSS_Task58_Review Previous Estimates of Potential CU

Home Browse Search

SPDSS Task 58 Memo August 5, 2004 Page 15 of 22 reference crop coefficients to this grass based reference method (IDS contain grass based crop coefficients for only potatoes and small vegetables - but it is unclear if these are used by the Hargreaves method). 7. Regional Studies on Consumptive Use A purpose of the literature search and interviews was to identify studies where determinations had been made of PCU on a regional basis that could be used for comparisons and verifications of SPDSS consumptive use determinations. During the interview process for this Task, inquiries were made of regional consumptive use studies used in consumptive use determinations. The interviewed parties generally indicated that they were either not aware of regional consumptive use studies and/or did not use such studies. The following documents, gained from the literature review, provide estimates of PCU or irrigation water requirements but, as discussed, are of limited value for comparison and verification of SPDSS estimates. Colorado Irrigation Guide The Colorado Irrigation Guide was revised by the Soil Conservation Service in 1988 and is currently out of print. This guide divided the State into seven climatic zones and for each zone estimated (based on the modified Blaney-Criddle method) the net average monthly irrigation requirements by crop. The values excluded apre-irrigation requirement. It is our understanding that the Colorado Irrigation Guide was never accepted by the engineering community who could easily calculate similar PCU with generally available programs that implemented the modified Blaney-Criddle Method. While several of the parties interviewed indicated they might use the Colorado Irrigation Guide as a crude estimate of irrigation water requirements, none of the parties interviewed indicated that they consider it for a serious determination of PCU or irrigation water requirements. Colorado's Net Irrigation Requirements for Agriculture, 1995 In 1999, personnel of the Colorado Department for Agriculture estimated the net irrigation requirements by county for the year 1995. The normal year net irrigation requirements were prepared by the Natural Resources Conservation Services (MRCS) for up to 20 crops at 64 weather stations in 44 counties. The MRCS calculated PCU by the modified Blaney-Criddle method, average monthly temperature, total monthly precipitation, typically planting and harvesting dates, elevations and crop growth-stage water requirement curves. Table 1 of this report summarizes the normal year net irrigation requirements in inches of water. The scale of this data ("normal year") reduces its value for comparison and validation of SPDSS estimates, which estimates will generally be computed on a monthly, multiyear basis.