SPDSS_Task58_Review Previous Estimates of Potential CU

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SPDSS Task 58 Memo August 5, 2004 Page 6 of 22 reasonable range. Where necessary, the Division Engineer's Office typically relies on the SEO for calculations that check those in applications. Both of the interviewed consultants indicated that they were considering changing to the ASCE Standardized Penman-Monteith equation. Agro Engineering indicated that the ASCE Standardized equation was found to produce similar results to their own modified Hargreaves method. Consumptive use analysis in the San Luis Valley is facilitated by the existence of 20+ years of detailed climate data, including wind, vapor pressure, and solar radiation, (not currently in the Colorado Division of Water Resource's primary database (HydroBase)) and by information learned from extensive irrigation scheduling practices. Gunnison River Basin (Division 4 The predominant consumptive use method used to support water right applications in Division 4 is the modified Blaney-Criddle method. While the Division Engineer's Office typically relies on the SEO to provide calculations to check those in water right applications, Division 4 staff has recently started using the SEO spreadsheet (see DWR TR21 spreadsheet described in the model section of this memo) to apply the modified Blaney-Criddle method with an adjustment for elevation. The Division Office is interested in investigating the validity of recent (1999-2003) lysimeter studies in the Upper Gunnison Basin in the context of previous lysimeter studies and current methods employed to estimate consumptive use in the Upper Gunnison. The Colorado River Water Conservation District and the Upper Gunnison River Water Conservancy District develop an annual report documenting the Subordination of the Wayne N. Aspinall Unit Water Rights within the Upper Gunnison Basin, based on CDSS StateCU. This annual report includes an estimate of crop consumptive use in the Upper Gunnison Basin. PCU for the annual reports are estimated using the original Blaney-Criddle methodology with coefficients developed to represent high-altitude crop growth (Kruse, 1974, "Water Use by Native Grasses in High Altitude Colorado Meadows). Colorado River Mainstem Basin (Division 5~ Representatives of two engineering firms and the Division Engineer's office in Division 5 indicate the predominant method for calculating PCU is the modified Blaney-Criddle method. For analysis of consumptive use of irrigated grasses at higher elevations, typically either an elevation adjustment is made to the modified Blaney-Criddle results or local coefficients from a Grand County lysimeter study (Carlson, 1991) or from a Gunnison lysimeter study (Kruse, 1974) are used. The use of one method over another is dependent on individual preferences of a consultant and time and budget available for a calculation. While many of the calculations of consumptive use are made at the SEO to check estimates contained in water right applications submitted in Division 5, when calculations are made at the Division Offices they principally use the IWR program (see section on models).