SPDSS_Task59-1_Develop Locally Calibrated Blaney-Criddle Crop Coefficients

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SPDSS Memorandum Final To: Ray Bennett and Ray Alvarado From: LRE -Erin Wilson, Ross Bethel, and Beorn Courtney Subject: Task 59.1 -Develop Locally Calibrated Blaney-Criddle Crop Coefficients Date: March 18, 2005 (updated January 11, 2008) Introduction The modified SCSBlaney-Criddle consumptive use methodology estimates potential consumptive use (PCU) on a monthly basis using monthly average temperature and daylight hours estimated from latitude. Because this data is readily available for long study periods, the methodology is widely used and recommended for use in the SPDSS. However, the modified Blaney-Criddle crop coefficients available from the SCS publication Irrigation Water Requirements Technical Release No. 21 (TR-21) were developed to represent general conditions around the west and may not represent local conditions in the South Platte, North Platte, and Laramie River basins (SPDSS study area). Therefore locally calibrated Blaney-Criddle crop coefficients were developed for SPDSS. The SPDSS Task 58 Review Previous Estimates of Potential CU memo discusses the methods used throughout Colorado for calculating potential crop consumptive use. As summarized in Task 58, local coefficients can be developed based on lysimeter data or results of a more detailed evapotranspiration equation. Due to the relatively large expanse and varying elevations of the SPDSS study area, three different crop coefficients were developed for three different geographical regions of the study area. This memorandum presents the general approach and results from the completion of the following Task 59 subtasks: 1. Gather and review appropriate data and literature from previous lysimeter studies used to develop Blaney-Criddle crop coefficients in high altitude areas of Colorado. 2. Review the ASCE Standardized Penman Monteith methodology and determine its appropriateness for use in the calibration of Blaney-Criddle crop coefficients by comparing results to lysimeter studies. 3. Recommend methodology for the development of locally calibrated crop coefficients by geographic regions 4. Develop locally calibrated crop coefficients using the recommended methodology. Approach and Results The locally calibrated crop coefficients developed under this task will be used to estimate potential crop consumptive use under conditions of a full water supply. The potential crop consumptive use will then be compared to historical water supply, using a soil moisture balance, to estimate supply-limited crop evapotranspiration.