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HYDROLOGY AND WELL AUGMENTATION IN THE SOUTH PLATTE RIVER BASIN 1) by Charles F. Leaf ABSTRACT A review of the scientific literature dating back to 1943, and Platte River Basin Water Balance Model@calibrated and validated simulations of the South Platte River reach between Kersey and Julesburg, CO during 1975 - 1994 have yielded the following results: 1. Negative alluvial aquifer storage changes relative to no pumping were assumed to represent encroach- ment by wells on senior surface water rights. These changes averaged -20,000 af/yr during the 1975- 1994 period, and 4 percent of total pumping. This compares with an overall-19 ,000 aflyr for the 1947- 1970 period derived by Hurr, et al. (1975). A frequency analysis of simulated year-to-year storage changes during 1975 - 1994 indicated that the -91,000 af storage change in 1977 - an especially dry year, was a less than 20-year recurrence interval event. This compares with -80,000 af in 2002, which also was a near 20-year recurrence interval event. These storage changes represent only one percent of the total alluvial aquifer storage of approximately 8,500,000 af in this reach of the river, and 20 percent of total pumping. 2. Current augmentation decrees would have required that wells in existence in 2002 have sufficient water in place to replace approximately 250,000 aflyr - each year. As a result, there have been massive well shutdowns in the valley. 3. According to this study, there was apparently no significant hydrologic impact of the wells during November - April during any year. Thus, no encroachment on senior reservoir rights. 4. Glover (1975) and this study which accurately simulated river behavior have shown that lag times in the Kersey to Julesburg reach of the river are short. Thus, it takes but a few years of lead time to establish a new hydrologic regimen. According to Glover (1975), at the end of this short period, "... a new regimen would have been established and what took place before will have minor importance... " Because what took place in the river (three or fewer) years earlier has minor significance today, it is gross error to cany forward, by mathematically complex uncalibrated and unvalidated lagging proce- dures, fictitious depletions from wells that began pumping more than 50 years ago. Key Words: Alluvial Aquifer, Hydrology, Simulation Models, South Platte Endangered Species Recovery Pro- gram, South Platte River Basin, Three States Agreement, Water Balance, Water Rights, Water Yield, Well Augmentation, Well Depletions. 1) A summary of this paper has been accepted for presentation at the American Water Resources Association 2007 Annual Conference, November 12-15, Albuquerque, New Mexico.