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Chapter 1: South Platte Basin Inventory March 21, 1998 Page 1-8 Natural flows estimates for the South Platte at Kersey gage location are presented in Table 1-5 and Figure 1-5. 1.1.3 Historical Flows Historical flows were compiled directly from USGS gage data for the 1950-1980 period of record. For the South Platte below Chatfield gage location, historical flows were estimated by summing the gage data for South Platte at Waterton and Plum Creek at Louviers. Historical flows for the South Platte, Chatfield, Denver, Henderson and Kersey gages are shown in Tables 1-6 through 1-10 and Figures 1-6 through 1-10. 1.1.4 Future Flows For the purposes of this study, future flows are defined as those flows which reflect "reasonably certain" future conditions with respect to municipal and industrial water supply system development and associated water demands within the South Platte River basin. 1,1.4.1 South Platte, Chatfield, Denver and Henderson Gages For the South Platte, Chatfield, Denver and Henderson gage locations we relied on output data from Denver Water's PACSM model reflecting Denver's Near Term modeling scenario, This scenario reflects the operation of Denver's water supply system at an annual raw water demand of 390,000 acre-feet. This is the demand level that Denver's system will be capable of reliably meeting with the addition of several near term future measures including additional effluent exchanges, water conservation programs, nonpotable reuse projects and other minor supply-side additions, Denver expects that its service area demands will take at least 30 years to reach this level. By comparison, Denver's current raw water demand is approximately 265,000 acre-feet per year. (Denver has recently revised its Near Term scenario to reflect a slightly larger annual raw water demand of 401,000 acre-feet. Due to time and budget constraints, the future flows in this report do not reflect this change in Denver's plan's, We believe that any changes to future flows would be very minor.) Denver's Near Term scenario also generally reflects future water demands and water supply system operations for Aurora, Thornton, Englewood and Centennial Water & Sanitation District. Increased levels of municipal return flows as well as diversions are reflected in this scenario. Denver's Near Term modeled stream flows do not reflect three potentially significant factors: 1) runoff from impervious surfaces and lawn return flows derived from increased urbanization in the metro Denver area beyond 1991 levels; 2) increased Hydrosphere Resource Consunants, 1002 Walnut Suite 200, Boulder, CO 60302 I I I I I I I I I I I I I I I I I I I