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budget, monthly precipitation data will be used to estimate precipitation inflow over the <br />ground water model area. By assigning key climate stations to geographic areas within <br />the ground water model area, precipitation inflow can be estimated at any location. For <br />the average annual basin-wide water budget, the average annual precipitation GIS <br />coverage, developed by the Colorado Climate Center, will be used to estimate the basin- <br />wide average annual precipitation. <br />Reservoir evaporation rates and water surface areas will be used in preparing the Consumptive <br />Use and Losses Summary, the water budget, and for the potential future surface water modeling <br />efforts. Reservoir evaporation data exist at only a few reservoirs in the SPDSS study area and <br />only for a short period during the 1950 through current year study period. <br />Approach and Results <br />1. Assign Key Climate Stations to Geographic Areas <br />Several interpolation methodologies commonly used to spatially distribute data to a subset of a <br />given area were investigated including: Thiessen polygon weighting, linear interpolation, and <br />kriging. The advantages and limitations of each method were investigated and are summarized <br />below. The criteria used to evaluate the methods were as follows: <br />1. Data Centered Approach -The method must be able to respond to changes in the input <br />data sets. For example, if the irrigated area associated with a structure is adjusted, the <br />method must be able to adjust the associated climate stations and weights in a straight <br />forward manner, preferably by just rerunning a procedure with a new input data set. <br />2. Applicability -The method must be applicable for the use intended. For example, the <br />climate station weights should be applied for those portions of the models where the <br />elevation of the respective stations is similar to the data set. <br />3. Engineering Judgment -The method must be able to be adjusted based on an engineering <br />review of the results. For example, the automated procedure may develop climate station <br />weights for a structure from six or seven climate stations while the practical limit may be <br />to use only the highest three or four stations. <br />4. Compatibility with Existing Models -The method must be developed with the existing <br />CDSS models in mind. For example, StateCU requires that each structure have a fixed <br />number of climate stations and fixed weights throughout the study period. Variable <br />weights by year would not be practical nor needed for SPDSS. <br />Standard Tools -The method should be developed with existing software currently in use <br />in the CDSS. This includes the ArcView GIS software package with standard extensions. <br />Page 2 of 11 <br />