SPDSS_Task58_Review Previous Estimates of Potential CU

Home Browse Search

SPDSS Task 58 Memo August 5, 2004 Page 9 of 22 As part of the Arkansas River Compact interstate litigation, the Special Master accepted the method for calculating crop consumptive use based on the ASCE Standardized Penman- Monteith Method. The State is implementing future calculations of PCU estimates in the Arkansas River Basin, for use for testing of future Compact compliance, based on the ASCE Standardized Penman-Monteith method. The State Engineer indicated in the April 8, 2004 conference that the State will be investigating a potential broader use (i.e. statewide) of the Penman-Monteith as it investigates its application in the Arkansas River Basin. The ASCE Standardized Penman-Monteith method appears to be gaining favor over other consumptive use methods when data is available to enable the calculation. A number of the consultants interviewed for this task indicated they were considering the ASCE Standardized Penman-Monteith method for future determinations of consumptive use. It is significant that even the company that developed the Modified Hargreaves equation in the Rio Grande Basin expects to transition to the ASCE Standardized P-M equation within several years. Detailed climate data availability for the last 20+ years enables the ASCE Standardized P-M equation to be used for historic consumptive use calculations. Hargreaves Method The analysis and conclusions associated with an ongoing five year lysimeter program in the Upper Gunnison Basin provides additional valuable information on PCU of irrigated meadow grasses at higher elevations. As is often the case at higher elevations, estimation of PCU of irrigated grasses in the Upper Gunnison Basin is complicated by the existence of few climate stations that collect the detailed data required by the more data intensive consumptive use methods. Therefore, with limited climate data availability for estimating consumptive use at higher elevations, initial analysis of the lysimeter program suggests that a Hargreaves method that relies principally on temperature data (maximum and minimum daily temperatures) provides reasonable estimates of PCU. In comparisons recently completed by staff at the Soil and Crop Sciences Department of Colorado State University (Smith, 2004), Mr. Smith offered the opinion, based on the lysimeter results and additional literature research, that application of the Hargreaves method (without the need for local calibration or altitude adjustment) provides a reasonable basis for estimating PCU of irrigated grasses at higher elevations. Estimates using the Hargreaves method are believed to be much improved over estimates derived from the modified Blaney-Criddle method without local calibration or elevation adjustment. Calibration with another more Detailed Method In the development of the Rio Grande Basin Decision Support System (a component of the State's CDSS), a consumptive use method (Modified Hargreaves) that uses detailed climate data (temperature, wind, radiation) was applied in recent years when that data existed and compared to modified Blaney-Criddle method results for the same time period. This allowed the generation of a "calibrated" (to Modified Hargreaves estimates) set of crop coefficients for use in the modified Blaney-Criddle equation. Given the limited amount of climate data available, climate data for the historic period of record was used in the generation of calibrated coefficients rather than segregation of that data into calibration/validation periods.