SPDSS_Task58_Review Previous Estimates of Potential CU

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SPDSS Task 58 Memo August 5, 2004 Page 4 of 22 o Colorado Water Conservation Board -Ray Alvarado o Northern Colorado Water Conservancy District -Ton Altenhofen o Denver Water Department -Larry Dirks • Collect and initially review the lysimeter program and associated results conducted in Division 6 (including North Park), in South Park, and in the Gunnison River Basin. • Review of consumptive use method references in Colorado Statutes. • Collect computer models and associated documentation for crop consumptive use estimation applied in the South Platte Basin study area. • Review documents related to consumptive use in the library of Leonard Rice Engineers. • Prepare this memorandum summarizing findings from research and providing recommendations for future SPDSS consumptive use activities. RESULTS 1. "Traditional" Calculations of PCU in Colorado Calculations of PCU are frequently made by water resource or water rights engineers, hydrologists and planners in support of historic water use or future use calculations needed in changes of water rights, augmentation plans, temporary use plans, future water supply plan development, depletion studies, basin modeling, etc. As indicated in the approach description a number of persons from the private and public sectors, familiar with consumptive use calculations, have been interviewed. Based on these discussions, the "traditional" calculations of PCU have been generalized by the State's Water Divisions as discussed below. The calculation of lawn grass consumptive use is addressed in a separate section from the Division-based discussions. Plains Area of South Platte River Basin (Division 1 Discussions with engineering consultants from Leonard Rice Engineers, Spronk Water Engineers, Bishop-Brogden Associates, Tetratech RMC and Helton and Williamson have indicated primary reliance for recent historic PCU calculations in the South Platte River Basin (excluding the foothills and mountains) on the SCS TR21 method (without local calibration or elevation adjustment). However, examples were found of consultants' use of elevation adjustments to modified Blaney-Criddle results for the South Platte Basin in the mid-1980's to early 1990's. Discussions with Mr. Jon Altenhofen of the Northern Colorado Water Conservancy District also indicated similar reliance on the SCS TR21 method in his development of PCU values (where required) for augmentation planning in the lower South Platte River Basin. Representatives of the Colorado State Engineer's Office have indicated that the State predominantly uses the modified Blaney-Criddle method (without local calibration but typically with an elevation adjustment) in their review or preparation of engineering estimates of PCU in the South Platte Basin. To check the reasonableness of consumptive use reported in water right applications, the Division Offices typically use a combination of in-house calculations (using the IWR or DWR TR21 programs -see computer model section) and reliance on the SEO.