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Arkansas River Basin <br />The Hydrological Institutional (HI) model was developed during the Arkansas Basin litigation <br />between Kansas and Colorado. The model primarily covers irrigated areas served by ditch <br />systems. It uses a soil moisture balance approach (described further below) to estimate <br />precipitation recharge over irrigated lands during the irrigation season. Monthly total <br />precipitation is provided from NOAA data. The Bureau of Reclamation effective rainfall method <br />is used to determine the portion of the total precipitation that is effective at meeting crop <br />demands. The remainder (total precipitation -effective precipitation) is estimated as surface <br />water runoff. The effective precipitation, along with irrigation water, is then applied to a <br />consumptive use algorithm with a soil moisture balance approach. After crop demands are met, <br />the remaining portion of the effective precipitation and irrigation water is used to first fill the soil <br />moisture reservoir, and then finally to recharge the aquifer (lagged using unit response curves). <br />According to Dale Straw, because the effective precipitation and irrigation water are not <br />explicitly tracked separately, it is difficult to distinguish the portion of the ground water recharge <br />from precipitation versus applied irrigation water. <br />The model includes both the irrigation and non-irrigation seasons, however a nominal ET <br />requirement is produced during the non-irrigation season. The model also estimates precipitation <br />runoff from ungaged tributaries with non-irrigated lands using a rainfall runoff method that uses <br />a uniform annual distribution of precipitation. <br />Republican River Basin <br />A ground water model of the Republican River Basin was developed as part of the final <br />settlement stipulation in the Kansas v. Nebraska and Colorado litigation. Precipitation recharge <br />to the aquifer was one of several model input variables (also including irrigation recharge from <br />ground water, irrigation recharge from surface water, canal leakage, stream losses, and decrease <br />in storage). <br />The Republican River Basin model spans 19 million acres with average precipitation ranging <br />from approximately 16 inches per year in the western part of the model area (eastern Colorado) <br />to approximately 27 inches per year in the eastern portion. While these numbers represent the <br />averages, certain portions of the study area receive up to approximately 50 inches precipitation <br />per year. The amount of precipitation recharge increases with the amount of total precipitation. <br />There is also a wide range in soil types throughout the model area. Given the same amount of <br />precipitation, more recharge occurs in course versus fine soil types. <br />Multiple precipitation recharge curves (recharge versus total precipitation) were developed to <br />represent the variability in total precipitation and soil type throughout the model area (Figure 3). <br />STATSGO soil maps were used to delineate course, medium, and fine soil classes (i.e. sand, <br />loam, and clay). Two curves were developed for each soil class -one for irrigated lands and the <br />other for non-irrigated lands. To further characterize differences throughout the basin, a unique <br />set of curves were developed for tributary versus Republican River mainstem alluvium. <br />Hydraulic conductivity was the principal terms used in calibrating the ground water model. <br />Minor adjustments were also made to the precipitation recharge curves during the calibration <br />process, therefore it was also considered a calibration term. <br />Task 64.doc 4 of 13 <br />