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BASIN LAG TIMES Basin depletions and returns were lagged to the river according to the time distribution shown in Table 1. This distribution was derived by Glover (1975). Month-to- month simulations of key elements in the water balance for the average year are plotted in Figure 5. The simu- lated hydro graph in Figure 5 was obtained by merely adding and subtracting ordinates. Table 2 presents the simulated water balance shown in Figure 5. The reasonable agreement between the observed and simulated average annual hydro graph at Julesburg is ob- vious (see Table 2 (A), Appendix I). The simulation in Figure 5 shows that by and large, it takes but a few years oflead time to establish a new regimen in the river. It is interesting that Glover (1975) also concluded that at the end of this short period of time, "... a new regimen will have been established and what took place before will have minor importance..." on river behavior. INJURY While not an explicit quantification of the hydrologic impact of wells, it is entirely reasonable to assume that the change in storage ~s, is a reliable indication of this impact. Consistent with Hurr, et al. (1975), the simu- lated average annual change in storage is approximately -20,000 af/yr. As seen in Figure 5, aquifer storage re- ductions take place during the months of May through October during the average year. The largest changes in storage occur during July through September, pre- cisely when single-source and supplemental water sup- plies from groundwater are most needed. It has been argued that the negative changes in storage have en- croached upon senior surface water rights, thus causing injury to these rights. The annual occurence and magnitude of these changes will be discussed next. YEAR-To-YEAR CHANGES IN AQUIFER STORAGE DUE To PUMPING Glover (1975) emphasized that Colorado Water Law requires that junior pumpers restore the river to what it would have been had there been no pumping. This hy- drologic impact can be obtained by: (a) simulating the water balance without the wells (Qgd in equation [1]), (b) simulating the water balance with the wells, and cal- culating the difference only during those months when simulated flows at Julesburg are negative which indi- cates a reduction in storage. This technique has often been used to quantify hydrologic impacts (see Leaf, 1974, Forrester, 1961, and Wright Water Engineers and Leaf, 1986). The monthly change in storage, ~s, attributed to well pumping was calculated for each year in the 1975 - 1994 record period by equations [2] through [4] below. ~s = Q + Q w dnl dn2 [2] where Qdnl Qup - Qsdl - Qgdl + 0< (Qgdl + Qsdl) [3] + L (Qsi - Qse - Qcs - Qrs + Qm) and Qdn2 =Qup - Qsd2 - Qgd2 + 0< (Qgd2 + Qsd2) + L (Qsi - Qse - Qcs - Qrs + Qm) [4] In equation [2] above, ~sw = the change in groundwater storage attrib- uted to wells, and all other terms are as defined below in equation [1 ]. Table 3 summarizes the simulated monthly hydrologic impacts of the wells. The impacts are expressed as "Replacement Water Requirements" assuming that the quantities summarized actually encroached on senior surface water rights. The results in Table 3 are plotted in Figure 6. Also plotted for comparison are monthly estimates of lagged annual groundwater consumptive use from all of the wells which averaged some 248,000 aflyr. At least two significant results have emerged from Table 1 and the comparison shown in Figure 6 for the 1975 - 1994 record period: 1. The hydrologic impact of groundwater diver- sions in the entire reach of the South Platte River Page 5