SPDSS_Task58_Review Previous Estimates of Potential CU

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SPDSS Memorandum To: Ray Alvarado and Ray Bennett From: Ross Bethel Subject: Task 58 -Review Previous Estimates of Potential CU Date: August 5, 2004 INTRODUCTION In preparation for modifications to the methods used to determine potential consumptive use for the South Platte Decision Support System (SPDSS) and associated applications to estimate SPDSS consumptive use, it is important to understand how consumptive use calculations have historically been made and may be transitioning in the SPDSS study area. General investigations have been made into consumptive use methods and tools around the State such that the Colorado Decision Support System (CDSS) consumptive use tool modifications will be applicable to the full state. Investigations have been made of the methods and tools traditionally used for potential consumptive use analyses, other methods that may gain favor in the future, basic consumptive use data obtained from lysimeters, and regional studies of consumptive use that could be used to verify and/or compare consumptive use estimates developed for the SPDSS. Potential Consumptive Use (PCU) is a term used in CDSS to indicate the amount of water (from any source, including irrigation or precipitation) consumed by the plants evaporation and transpiration process that would occur if a full water supply was provided to those plants. PCU is also referred to as evapotranspiration (ET) in CDSS. A computer program called "StateCU" and an associated interface has been developed as part of CDSS to perform the calculations of PCU. The following excerpt, from the StateCU documentation, describes the computational process implemented for estimating irrigation water requirements in the Water Divisions (3, 4, 5, 6 and 7) previously included in the CDSS. "Monthly crop evapotranspiration (ET), or potential consumptive use, can be determined by structure using the Soil Conservation Service (SCS) Blaney-Griddle method outlined in Irrigation Water Requirements Technical Release 21 (1970) (SCS TR-21). Daily ET can be determined using either the Penman-Monteith or Modified Hargreaves approach. The potential crop consumptive use estimates are reduced by an amount of monthly or daily precipitation considered effective at serving crop needs to determine the irrigation water requirement. The potential consumptive use for a structure is determined based on the acreage and corresponding crop types associated with the structure." Where to find more information ^ A companion SPDSS memorandum (Restructured SPDSS Task 61) on methods used to estimate the effectiveness of precipitation in serving potential consumptive use is being prepared. Further discussions of effective precipitation are deferred to that memo.