SPDSS_Task59-1_Develop Locally Calibrated Blaney-Criddle Crop Coefficients

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2. Review the ASCE Standardized Penman Monteith Methodology and Determine its Appropriateness for Use in the Calibration of Blaney-Criddle Crop Coefficients Under Task 58, it was recommended that the ASCE Standardized Penman-Monteith equation be used to develop locally calibrated coefficients for the modified Blaney-Criddle method in areas where lysimeter data are not available. Similar to the original Penman-Monteith equation, the ASCE Standardized method can be applied using a set of short reference crop (grass) or tall reference crop (alfalfa) coefficients. Alfalfa has been suggested as the preferable reference crop for arid climates (ASCE, 1990) and therefore, the ASCE Standardized equation for tall reference crop was added to the State's consumptive use model, StateCU. ASCE provides a set of mean crop coefficients and basal crop coefficients for a tall reference crop (ASCE, 1990). The mean coefficients include ET from both the crop and the soil whereas the basal coefficients require an additional set of coefficients to account for ET from the soil. Site-specific information is needed to develop the basal coefficients and while this may be appropriate for afarm-level analysis, the mean coefficients were determined to be more appropriate for the SPDSS basin-scale analyses. The mean crop coefficients for tall reference crop provided by ASCE were originally developed by J.L. Wright in 1982 for the 1982 Kimberly Penman equation (see ASCE, 1990, Table 6.9 - "Mean Et Crop Coefficients, Kcm, for Normal Irrigation and Precipitation Conditions, for Use with Alfalfa Reference Et, Etr"). According to the ASCE Environmental and Water Resources Research Institute, these coefficients can be used in the ASCE Standardized method without any adjustment. The growing season parameters outlined in TR-21 and typically used with the modified Blaney- Criddle method are commonly applied in water rights analyses throughout the SPDSS study area. These parameters define the beginning and ending of the growing season, maximum root zone depth, maximum application depth, and cutting parameters for each crop type. While more site- specific information might be applied for a specific parcel analysis, these parameters are believed to sufficiently represent the SPDSS area for this basin wide analysis. Therefore, the TR-21 growing season parameters were used with the Standardized ASCE method. 3. Recommend Methodology for the Development of Locally Calibrated Crop Coefficients by Geographic Regions Consistent with the recommendations made under Task 58, locally calibrated crop coefficients for irrigated grasses at high altitude will be based on lysimeter data. Locally calibrated coefficients for the major crop types irrigated throughout the plains (from the foothills east) of the SPDSS study area will be developed through calibration with the ASCE Standardized method, due to the unavailability of lysimeter data. This calibration will be performed by comparing PCU (gross potential consumptive use prior to reducing for effective precipitation) estimates between the modified Blaney-Criddle and ASCE Standardized methods. For this task and other SPDSS tasks, the high altitude portion of the SPDSS study area is defined as areas west of the foothills (generally above 6,500 feet) including Water Districts 23, 47, 48, 76, and 80 and the upper portions of Water Districts 3, 4, 5, 6, 7, 8, 9 as depicted in Figure 4. Figure 4 and the division between upper and lower portions of water districts were developed using digital elevations obtained from the State, originally derived from data obtained from the USGS. Task59_l.doc (updated January 11, 2008) 6 of 32