SPDSS_Task58_Review Previous Estimates of Potential CU

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SPDSS Task 58 Memo August 5, 2004 Page 18 of 22 2. Lysimeter studies were principally found to be conducted for high altitude irrigated grasses. Lysimeter studies were identified in South Park, the Gunnison River Basin, the Yampa River Basin and North Park. Data from these studies will assist the SPDSS consumptive use team in developing calibrated crop coefficients for the original Blaney-Criddle equation for use in consumptive use estimates for high altitude grasses. Several networks (principally NCWCD and COAGMET), consisting of more than 40 climate stations, exist in the SPDSS study area and are suitable for making accurate estimates of daily PCU for recent years with aPenman-Monteith method. 4. Few regional (SPDSSS study area) assessments of PCU were found through the literature searches/reviews and interviews. The few assessments that were found were not believed to be of value for comparing and validating estimates of consumptive use developed for the SPDSS. A number of non-State supported models (Ref-ET, IDSCU, spreadsheets developed by others) exist that generally provide trusted results of PCU derived from the daily Penman-Monteith and other methods. These models can be used to compare to and validate new implementations of consumptive use methods (i.e. ASCE Standardized P-M Method) in the StateCU program. The SPDSS scope of work suggested that the modified Blaney-Criddle methodology be used for estimates during a long study period but with the generation of locally calibrated Blaney-Criddle crop coefficients. Given the predominance of the modified Blaney-Criddle method applications and the means for improving estimates developed with modified Blaney-Criddle through local calibration, identified in this task, this suggestion remains viable. The strategy for development of local calibrated crop coefficients is dependent on whether a site has sufficient local climate data to allow calibrations with the more detailed consumptive use methods. Based on the investigations performed for this Task, we also suggest two additional strategies (to local calibration of crop coefficients using a more detailed PCU method) below for the local calibration of crop coefficients, one strategy suggested for areas where local calibration is not possible, and one strategy suggested for lawn grass. Calibration with More Accurate CU Method It is appropriate that local calibration be accomplished, where sufficient data exists, through comparisons with calculations of PCU performed with a more accurate method such as Penman- Monteith, ASCE Standardized Penman-Monteith, or Kimberly-Penman. Given the growing acceptance of the ASCE Standardized Penman-Monteith equation as indicated by the interviews, we believe that the ASCE Standardized Penman-Monteith equation (see Appendix) should be considered for local calibration of crop coefficients in areas where sufficient climate data exists. Therefore, it is recommended that the ASCE Standardized Penman-Monteith consumptive use methodologies be added to the CDSS's StateCU program (and interface). It is also