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by Hurr, et al. (1975) and more recent data. SOUTH PLATTE RIVER BASIN HYDROLOGY Figure 2, developed by Hurr, et al. (1975) shows an av- erage-year water balance for the Henderson to Julesburg reach of the river based on the 1947 - 1970 period of record. The most significant result in this graphic is a -19,000 af/yr change in storage from a fully recharged alluvial aquifer. By some accounts, total aquifer stor- age in this reach is approximately 8,850,000 af(Robson, 1989). Hence, the average annual depletion was only 0.2 percent between 1947- 1970 in spite of the fact that groundwater pumping during that peroid averaged 420,000 af/yr (Figure 2). It should be noted that water yields have increased in the reach between Denver and Julesburg since the mid- fifties primarily as the result of transbasin imports and increased return flows (Leaf 1998 and Stenzel, 2006). Accordingly, the water balance and particularly the change in alluvial storage shown in Figure 2 was not changed significantly during the 1975 - 1994 record period. Since 1995, however, (1) accelerated changes in irrigation practices (flood irrigation to sprinklers), (2) expansion of the riparian zone, (3) the Cooperative Agreement for Endangered Species Recovery (Dept. of the Interior, 2006), (4) Front Range development, (5) well augmentation, (6) massive well shutdowns, and (7) evolving water rights administration are disrupting the water balance shown in Figure 2. PLATTE RIVER BASIN WATER BALANCE MODEL @ The extensive work by Hurr, et al. (1975) has provided "the foundation upon which an analysis of the cause- and-effect relationships for proposed changes in water resource management can be made. " The Platte River Basin Water Balance Model @ developed by Leaf (1999) builds on the foundation provided by Hurr, et al. (1975). Figure 3 shows a typical reach of the river in the Platte River Basin. On this figure are the important hydro- logic variables necessary to quantify a water balance. The mathematics used in the PRBWBM for computing both surface and subsurface components of the water balance is given by the equation: Qdn = Qnp + Qsi - Qsd + c< (Qgd + Qs) - Qgd - Qse + Qcs + Qrs + Qnr i: ~s [1] where Qdn = water yield at downstream node, Qup = water yield at upstream node, Qsi = surface inflow between upstream and downstream nodes, Qsd = surface water diversions, c< = irrigation return flow coefficient, Qgd = groundwater diversions, Qse = evaporation from surface water sources, Qcs = canal seepage, Qrs = seepage from reservoirs and natural bod- ies of water, Qnr = natural recharge from precipitation and ~s = change in storage The water balance can be computed on a monthly, sea- sonal, or annual basis. In most cases, explicit quantification of all terms in equa- tion [1] is not possible. Accordingly, several terms must be combined and detennined as a residual in the com- putation of a reach water balance. For the reach of the South Platte River between Kersey and Julesburg (Fig- ure 1), L(Qcs + Qrs + Qsi + Qnr - Qse) are included as one term in the water balance calculations. The irrigation return flow coefficient, c< is assumed to be 0.45 after Hurr et al. (1975). Leaf(1999) calibrated the PRBWBM to the Henderson- Julesburg reach using the 1947 - 1970 data base. Vali- dation was done for 1975 - 1994. Simulations of the year-to-year water balance for the Kersey to Julesburg reach are plotted in Figure 4. Goodness of fit analyses between observed and simulated water yields in Figure 4 are presented in Table 1 (A) Appendix I for quantifi- cation of the variables in equation [1]. Data sources for quantification of the variables in Equation [1] and plotted and Figure 4 include published data by the Colo- rado State Engineer, U.S. Geological Survey, Natural Resource Conservation Service, Dille (1960), and Glover (1975). Page 2