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.. JOINT STATEMENT Date: November 27, 1996 To: Mr. Michael Clinton General Manager Imperial Irrigation District P.O. Box 937 Imperial, CA 92251 From: Dr. Richard G. Allen Dr. Charles M. Burt Dr. Albert 1. Clemmens Dr. Kenneth H. Solomon Subject: Statement Regarding Certain Technical Issues Related to IID Water Use We have all been involved in preparing and reviewing a variety of reports regarding water usage and efficiency in the Imperial Irrigation District. Most recently we reviewed "Water Use Assessment of the Imperial Irrigation District, Final Report, 22 August 1995" by Dr. Marvin E. Jensen, issued by the Water Management and Conservation Team of the US Bureau of Reclamation (USBR) in Boulder City, Nevada. In conducting our technical review of the Jensen report, we consulted and met with Dr. Jensen concerning proper interpretation of the technical analysis and the validity of some conclusions drawn in that report. As a result, the views of Dr. Jensen have been considered in this statement. It is our hope that this statement clarifying some controversial issues will be of benefit to all interested parties. We offer the following points, both as a guide to the interpretation of past work and as a basis for further work in this area. 1. Water Balance Approach. Given the hydrology of the Imperial Valley and current water measurement practices and technologies, the water balance approach is the most accurate method for a historical assessment of district-wide evapotranspiration (ET). The accuracy of these estimates is also quantifiable. 2. Crop Factor-Weather Approach. Techniques to estimate IID ET from weather, crop type, and acreages (Kc-ETo approach) are more appropriate for an analysis of the future effects of changing cropping patterns or weather conditions on ET, especially when calibrated against the water balance estimates.