Laserfiche WebLink
SPDSS Task 58 Memo <br />August 5, 2004 <br />Page 5 of 22 <br />Mountainous Area of South Platte Basin (Division 1 <br />The South Park area of the South Platte Basin is a 1,000 square mile grassland basin in the <br />geographic center of Colorado and comprises the headwaters of the South Platte River. A large <br />number of changes of irrigation based water rights in this area to municipal uses since the 1970's <br />prompted much associated research and application of consumptive use methods for irrigated <br />meadow grass (pasture grass). Research, using lysimeters, into PCU of irrigated meadow grass <br />has been performed in the South Park area from the late 1960's to the early 1990's. Due to the <br />development of crop coefficients from lysimeter work, the most common consumptive use <br />method used in South Park is a monthly form of the original Blaney-Criddle method formulation <br />with KT=1. This original (with a monthly time step) Blaney-Criddle method is included in the <br />StateCU program through definition of the crop coefficients. Though an investigation of <br />evapotranspiration in South Park (Walter, et. al., 1990) resulted in suggested crop coefficients for <br />the original Blaney-Criddle method, crop coefficients traditionally used in South Park for <br />changes of water rights have traditionally been based on the average of crop coefficients derived <br />from lysimeter studies performed prior to the 1990 study. The estimated long term average <br />(1943-1996) annual PCU (no reduction for effective precipitation) used in recent change cases <br />for irrigated pasture in the South Park area approximates 30 inches, with calibrated crop <br />coefficients (based on lysimeter results) for irrigated grass that range from .58 in April to 1.28 in <br />June (LRCWE, 1993). <br />Similar to South Park, a common practice in calculating PCU of irrigated meadow grass in other <br />mountainous areas of the South Platte River Basin is to use calibrated coefficients in the original <br />Blaney-Criddle equation such as those derived for South Park. <br />Arkansas River Basin (Division 2~ <br />Modified Blaney-Criddle, without local calibration or elevation adjustment, appears to be the <br />predominant method for calculating consumptive use in the Arkansas River Basin. The use of <br />other methods such as Penman-Monteith or Hargreaves or use of locally calibrated coefficients <br />has been very infrequent. However, the Special Master of the Colorado/Kansas Arkansas River <br />Compact litigation has recently directed Colorado to use the ASCE Standardized Penman- <br />Monteith method. This direction may influence the future calculations of consumptive use in the <br />Arkansas River Basin (where data is available for this method). The Division Engineer's Office <br />typically relies on the SEO for calculations to check PCU calculations in applications. <br />Rio Grande River Basin (Division 3~ <br />There are two primary engineering companies responsible for most of the applications to the <br />Water Courtin Water Division 3 that contain consumptive use calculations. One of those <br />companies, Agro Engineering, typically uses their modified Hargreaves method (a method used <br />in CDSS for calibration of the Rio Grande consumptive use estimates), where the other <br />company, Davis Engineering, applies modified Blaney-Criddle with elevation adjustment but no <br />local calibration. The Division Engineer's Office appears comfortable that the results from <br />either method can be used to produce estimates of historic or future consumptive use within a <br />