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Modeling in Support of AWDI Case <br />In 1991 American Water Development Corporation made application to the Division 3 Water Court <br />to pump water from the confined aquifer underlying the San Luis Valley, and export this water to the <br />Front Range of Colorado. As part of the application process, an extensive effort was made to model <br />the groundwater conditions. Included in the groundwater modeling efforts were representation of <br />the Rio Grande River, the Conejos River, and numerous surface water diversions. These surface <br />features were included in the groundwater modeling in order to simulate the impact on surface <br />streams from groundwater withdrawal. <br />In addition to the groundwater modeling related to the AWDI case, an effort was made to determine <br />exchange potential along the Rio Grande River at several points of the mainstem. This effort <br />consisted of amass-balance approach that considered historical flows at several gaging stations and <br />diversion structures. <br />An in-depth review of the modeling efforts associated with AWDI were not performed as part of <br />Task 3 of the Surface Water Modeling Scope of Work because 1) a very simplistic approach was <br />used to represent surface diversions in the groundwater model; 2) the surface water modeling relied <br />only on historical information (gage records); 3) the efforts provided no additional information that <br />is expected to be useful for the RGDSS effort. <br />U.S.G.S. Groundwater Modeling <br />In the late 1960, Phillip Emery led an effort by the U.S. Geological Survey to develop an electric <br />analog model of the San Luis Valley groundwater aquifers. The effort was specifically directed at <br />assessing the effects of changes resulting from a plan to salvage water consumed by non-beneficial <br />vegetation in the central part of the Valley. The analog model simulates the boundaries, storage <br />coefficient and transmissivity of the unconfined aquifer and represents pumping from a network of <br />shallow wells. The Rio Grande and Conejos River were represented as fully penetrating, constant <br />head sources, which assumes a constant hydraulic connection between the streams and the aquifer. <br />The model predicted little impact to surface waters resulting from pumping from the unconfined <br />aquifer. This effort does not explicitly represent any surface water drainage features and does not <br />contribute information expected to be useful for the surface water modeling component of the <br />RGDSS effort. <br />Modeling in Support of Other Legal Change Cases <br />A variety of other modeling efforts have occurred over the years in relation to numerous water right <br />change cases in the San Luis Valley. These modeling efforts have ranged from simple accounting <br />tables to fairly sophisticated computer spreadsheets. The purposes of these simple models have <br />generally been to support changes in water rights such as amount, place, and time of use. <br />An in-depth review of modeling efforts to support other legal change cases was not performed as <br />part of Task 3 of the Surface Water Modeling Scope of Work because 1) they are generally very <br />localized in geographic scope; and 2) they were developed for the very specific purpose of <br />evaluating the impacts associated with particular water right changes. <br />Comments and Concerns <br />None. <br />C:\cdss\Tsk3aMem.doc Previous Modeling Efforts April 6, 1999 -Page 4 of 4 <br />