SPDSS_Task81-2_Consumptive Use and Water Budget Technical Peer Review Meeting Follow-Up

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1. General Comments and Clarifications 2. Grass Pasture High Altitude Crop Coefficients 3. South Platte Plains Crop Coefficients 1. General Clarifications Using Potential Consumptive Use as an Indicator of Actual Evapotranspiration - Several of Dr. Allen's comments are related to the use of potential consumptive use (PCU) as an indicator of actual ET. The SPDSS Task 59.1 memorandum provided recommendations for the methods and calibrated coefficients to be used to calculate potential consumptive use (PCU) under a full water supply. The actual ET is developed under other tasks by using the recommendations from Task 59 along with historical diversion records, irrigation methods, supplement sources, soil parameters, etc. to consider the amount of water historically available for crop consumption. Straight-line Diagrams -Straight-line diagrams for individual Water Districts have been developed during the SPDSS process. Straight-line diagrams were presented during the Peer Review Meeting. There was specific concern that, because water rights changes and transactions occur rapidly in the South Platte River Basin, it cannot be assumed that the straight-line diagrams available on the CDSS Web site are current. The State has adopted the following language that is being included on all straight-line diagrams: "This diagram is subject to change without notice and is a correct and authentic copy from the records and files of the State Engineer for the State of Colorado. Water rights information and structure information included in this diagram are based upon information available to the State Engineer and are believed to be accurate. However, persons seeking to use this diagram in any legal proceeding are responsible for verifying the accuracy of any information included in the diagram." Reservoir Evaporation Estimates -Peer review participants discussed alternative methods for estimating reservoir evaporation. Suggestions were considered and after further investigation, SPDSS will continue to use the approach outlined by the Office of the State Engineer. This approach is based on distributing average annual data from evaporation atlases in NOAA Technical Report NWS33 using SEO-recommended monthly distributions for above and below 6,500 feet mean sea level. This approach is standard practice and generally accepted throughout the Colorado water resources community. Effective Precipitation Estimates -Peer review participants inquired about including winter effective precipitation in the historical crop consumptive use analysis water budget. Winter effective precipitation has not been considered in previous DSS modeling efforts and it is not anticipated that it will be considered in SPDSS. This approach is generally accepted throughout the water resources community, with the exception of research applications where case winter precipitation is sometimes considered. To further address this question, LRE will perform a sensitivity analysis by estimating water supply-limited consumptive use with and without considering winter effective Page 2 of 7