Arkansas - CSU Revised Lysimeter Project_Application

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Water Supply Reserve Account ? Grant Application Form Form Revised May 2007 _______________________________________ Guidance document for additional detail on information to include. This project involves direct determination of water use, evapotranspiration (ET), by the major crops in the Arkansas Valley with a weighing lysimeter. The weighing lysimeter facility was funded by the Colorado Water Conservation Board and constructed at the Arkansas Valley Research Center (CSU AVRC), Rocky Ford. The weighing lysimeter continuously monitors the amount of water consumptively (ET) used by a growing crop. A second component of the project is to utilize weather data as inputs to the Penman-Monteith equation to calculate ET. The calculated ET is multiplied by a crop coefficient to obtain a quantitative value for crop water use. This project will compare actual and calculated water use by the major crops in the Arkansas Valley. The project is a multi-year effort to characterize crop water use in the Valley and it will ultimately provide data required to address consumptive use issues inherent in the Colorado-Kansas lawsuit. In addition, the data collected will allow a comparison of ET data for Arkansas Valley with data obtained in other irrigated regions of the US with implications on the applicability of data to other regions of Colorado. The following was developed by Tom Ley as a background document for requesting funding to construct a weighing lysimeter at the AVRC (Ley, 2004). ?In the Fourth Report in Kansas v. Colorado, No. 105, Original (U.S. Supreme Court), Special Master Arthur Littleworth recommended the ASCE Standardized Penman-Monteith equation be used for determination of crop consumptive use in the H-I Model (which is used to determine depletions to usable Stateline Arkansas River flows and subsequently Arkansas River Compact compliance). The ASCE Standardized Penman-Monteith equation uses detailed electronic weather station data to compute evapotranspiration (ET) of a standardized, hypothetical reference crop. In this case, the reference crop is alfalfa. Crop evapotranspiration (consumptive use) is then computed using crop coefficients, which relate the water use of specific crops to the hypothetical alfalfa reference crop ET as the crops grow and mature through the season. A daily time step is used from which monthly crop ET is computed. To obtain the most accurate estimates of crop evapotranspiration in the Arkansas Valley of Colorado (as well as throughout the State) using the ASCE standardized Penman-Monteith methodology, two issues must be addressed: 1. The performance and predictive accuracy of this method for computing alfalfa reference crop ET must be evaluated for the growing conditions in southeastern Colorado, and, 2. Crop coefficients for the various crops grown in the Arkansas River Valley must be determined for both well- watered conditions and the typical local growing conditions; and specifically for the ASCE standardized Penman- Monteith equation (as opposed to relying on adjusted crop coefficients developed for other ET equations in other locations). ET of an alfalfa reference crop and ET of various crops of interest can be measured directly through carefully controlled use of a weighing lysimeter. Depending on the design of the lysimeter, ET can be measured: a) in time step increments ranging from 15-60 minutes to daily and longer periods, b) under various environmental crop stresses as might be caused by water shortage, poor water quality, and/or high water table conditions.? 5