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I I I I I I I I I I I I I I I I I I I Chapter 1: South Platte Basin Inventory March 21,1998 Page 1-9 future wastewater flows from Cherry Creek, Plum Creek and Clear Creek; and 3) exercise of certain junior water rights on the South Platte between Metro and Strontia Springs. Each of these factors were considered in the context of Denver's Near Term model scenario. Runoff From Impervious Surfaces and Lawn lrriaation Return Flows. As a region urbanizes, much of the land is covered with impervious surfaces (streets, buildings, parking lots, etc.), which increase the amount of precipitation that runs off to surface streams, Also, as previously dry land becomes urbanized and regularly irrigated by municipal water supplies, return flow from those irrigated areas contributes to stream flows. This is the primary focus of Chapter 4 of this report. Denver Water, as a part of its PACSM modeling studies, developed estimates of increases in stream flows from 1947 through 1991 through the Denver metropolitan area attributable to runoff from impervious surfaces (RIS) and lawn irrigation return flows (L1RF), as discussed in Chapter 4, Denver continues to explore this issue and expects to refine its estimates of RIS and L1RF in the near future. Denver increased the RIS-related inflows in its model beyond the historical 1947- 1991 levels to represent growth of RIS commensurate with what was observed at the end of its modeled period (1991). Denver also increased the L1RF-related inflows in its model to be commensurate with the water demands explicitly included in its model. However, future RIS-related inflows will be greater than 1991 conditions. Future L1RF- related inflows will also be greater because of growth in water demands not explicitly included in Denver's model, such as Douglas County water demands. Thus while Denver's Near Term scenario simulates the operation of water supply systems serving a constant aggregate water demand associated with approximately 3,300,000 people, that model scenario includes within it RIS- and L1RF-related inflows resulting from a population of approximately 1,900,000 people. This factor was judged to be significant. Based on the results of Chapter 4, it is estimated that inflows in Denver's Near Term model representing RIS and L1RF from the Metro Denver area tributary to the Chatfield, Denver and Henderson gages are underestimated by approximately 1,400 acre-feet per year, 23,000 acre-feet per year, and 35 acre-feet per year, respectively. Denver's Near Term flow data for the Chatfield, Denver and Henderson gages were adjusted upward accordingly using the average monthly distribution of RIS and L1RF as shown in Figure 4-3 of Chapter 4. While it is likely that increased urban return flows of this magnitude would result in increased yield to the Burlington Ditch and to Hydrosphere Resource Consu~.nts, 1002 Walnut Su~e 200, Boulder, CO 80302