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Rio Grande River Basin <br />Under the RGDSS, the approach to estimating recharge from precipitation varied based on land <br />use (irrigated versus non-irrigated lands) and season (irrigation season versus non-irrigation <br />season). The percentage of total precipitation that was estimated to recharge the aquifer was as <br />follows: <br />• Irrigated Lands/Irrigation Season - 10 percent <br />• Irrigated Lands/Non-Irrigation Season - 3 percent <br />• Non-irrigated Lands/Year Round - 3 percent <br />In addition to the above generalized approach, a soil moisture balance approach was used at the <br />Great Sand Dunes in order to address the unique soil properties at that location. <br />South Platte River Basin <br />When compared to average annual precipitation, the recharge estimates presented in the USGS <br />Open File Report 03-311 (depicted in Figure 2) indicate recharge in the South Platte River basin <br />ranges from approximately 15 percent along the foothills down to as low as 2 percent in the <br />lower South Platte River basin. These estimates were based on a nationwide study. Two studies <br />that focused specifically on the South Platte River basin were identified as described below. <br />A ground water model of the Denver Basin aquifer was prepared by the State Engineers Office in <br />support of Senate Bill 5. The results showed that less than one percent of total precipitation <br />recharges the Denver Basin aquifer. The model concentrated this precipitation recharge in the <br />region of the Palmer Divide, therefore much of the model area had no recharge from <br />precipitation. The USGS is currently updating this model. <br />The results from a study conducted in Akron, Colorado have been used by several water <br />resources professionals for water rights related work in Colorado. Based on data collected <br />during twelve winter seasons from 1965 through 1979, the measured snowmelt intake efficiency <br />on ungrazed native grass pasture averaged 38 percent (Greb, 1980). This estimate has been used <br />in engineering analyses throughout the South Platte basin to model precipitation available to the <br />soil moisture zone during the non-growing season, with excess amounts being attributed to <br />ground water recharge. <br />2. Recommend an Approach to Estimate Precipitation Recharge <br />Based on research of previous studies that developed precipitation recharge estimates, the <br />following parameters should be considered when estimating precipitation recharge: <br />• Soil type, <br />• Land use (irrigated agricultural lands versus native areas), <br />• Season (growing/irrigation season versus non-growing/non-irrigation season). <br />Effects of these parameters can be represented using a soil moisture balance to estimate recharge <br />from precipitation as follows: <br />Task 64.doc 6 of 13 <br />