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Oro~raphic adjustment <br />The ground water model requires monthly precipitation estimates for recharge values. Since <br />some areas of the proposed ground water model are at elevations not represented by nearby <br />climate stations, an orographic adjustment will be applied to precipitation values from climate <br />stations. This basis of the adjustments will the Colorado Average Annual Precipitation Map <br />(1951 - 1980) published by the Colorado Climate Center. At each climate station, the average <br />annual precipitation value will be estimated from the map. Other locations in the vicinity of each <br />climate station will then be assigned a value equal to the estimated annual precipitation at the <br />location of interest divided by the annual precipitation value at the nearby climate station. For <br />example, if the average annual precipitation value at a climate station is 15 inches, then every <br />location affected by that station with a precipitation value of 14 inches will be assigned a value <br />of 15/14 or 0.93. This process will create a continuous grid of weights for each station. The <br />product of the spatial grid weights and the orographic weights can then be used to automate the <br />estimate of precipitation values throughout the groundwater model area. It is important to note <br />that using this method, the total weights can be greater than or less than 1.0. <br />Results - SW Model and WB Model <br />As noted above, average annual precipitation data is required for the SW model and the basin- <br />wide WB model. An automated approach will be developed within ArcGIS, as part of SPDSS <br />Task 18, to determine the annual precipitation for a defined area using the GIS average annual <br />precipitation coverage developed by the Colorado Climate Center. For the basin-wide average <br />annual WB model, the defined area will be the entire basin. For the SW model, defined areas <br />will represent drainage areas. <br />2. Estimate Average Monthly Reservoir Evaporation Rates for Geographic Areas <br />There are no continuous reservoir evaporation stations located in the SPDSS study area. The <br />Denver Water Department and Northern Colorado Water Conservancy District (NCWCD) have <br />developed average monthly reservoir evaporation rates for their respective models, which they <br />provided to SPDSS for review. Denver Water also provided a memorandum documenting their <br />evaporation rates ("Estimation of Net Evaporation Rates", December 11, 2003). Documentation <br />of the development of evaporation rates for NCWCD was not available and there were <br />discrepancies between Denver Water and NCWCD evaporation rates used in near proximity. A <br />review of the data used in these models shows that the methods used to estimate net evaporation <br />rates (net evaporation =gross evaporation -effective precipitation) are inconsistent. For <br />example, some of the average monthly rates were based on State Engineer's Office data, which <br />has applied a 70 percent factor when calculating effective precipitation (net evaporation =gross <br />evaporation - 0.70 x total precipitation) while others were based on Denver Water Department <br />and Bureau of Reclamation data which consider 100 percent of precipitation to be effective. In <br />addition, Denver Water estimates winter evaporation rates at upper reservoirs to be zero, whereas <br />our analysis shows that reservoirs in these areas can experience net winter evaporation. <br />The State Engineer's Office is calculating evaporation for a different purpose than Denver Water <br />and the Bureau of Reclamation. The State Engineer's Office administers reservoir storage based <br />on decreed storage rights, regardless of evaporation. However, when reservoirs are required to <br />Page 6 of 11 <br />