NorthPlatteBasinWaterSupplyandNeedsReport

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Section 7 Availability of Existing Water Supplies in the North Platte Basin 7.3.1 Surface Water Supplies State Mod datasets are not available for the North Platte Basin. There are, however, a number of USGS flow gages, with extensive periods of record, located throughout the basin. Two of these gages, shown in Figure 7-2, were used to characterize historical physically available flow in the basin. These flows are measured and correspond to actual historical, rather than current diversions and demands. The period of record varies by gage, spanning the time period 1915 to 2001 (full calendar years). The selected gage locations are: . North Platte River near Northgate (1916-2001) . Laramie River near Glendevey (1915-1981) Minimum, median, and maximum annual measured flows are summarized for each location in Figure 7-3. To better represent the effects of seasonal and year to year hydrologic variation, annual time series, and monthly summaries of historical physical flows are shown in Figures 7-4 through 7-7. Median annual flows and 3-year running averages are also included on the annual time series plots. The monthly analyses highlight the fact that physical flows vary greatly with season, with the greatest amounts of water present in the summer months and a sharp decline in flows in the autumn and winter. The annual time series plots also show large variation with a notable extended drought periods in the mid-1950s and the early 1990s. Extended wet periods appear to have occurred in the mid-1980s and mid-1990s. The interpretation above is in general agreement with the CWCB Drought Study (HDR 2003), which summarized the history of drought in Colorado and identified significant drought periods in the last 100 years. The Drought Study states that the most recent drought analyzed for years 2000 to 2003 exceeds many of the drought records established during the 20th century. The North Platte River Basin Decree is a Supreme Court decree that limits the total number of acres that can be S:\1177\BASIN REPORTS\NORTH PLATTElS7 _NORTH PLATTE. DOC irrigated in the North Platte Basin. The decree also limits the amount of water that can be stored for irrigation and the amount of water than can be exported out of the basin. This decree is described in Section 4. Currently, Colorado is not maximizing its allocation of water rights available under the decree. Estimates indicate that there is the potential to irrigate additional acres based on hydrologic conditions from 1975 to 2002. T ransbasin diversions have also not been maximized per the Decree over the period of record. Transbasin diversion limits are limited on a running ten year total to 60, 000 AF. Recent diversions have averaged 44,600 AF for the most recent 10-year period (Leonard Rice Consulting Water Engineers 2004). Another factor to be considered when assessing supply availability in the basin is the need and/or desire to maintain or enhance environmental flows. Environmental considerations are further developed in Sections 6 and 9 of this report 7.4 Availability for Water Supply Development under Interstate Compacts and Decrees Colorado has entered into and is affected by nine interstate compacts, two equitable apportionment decrees, and one international treaty. These agreements establish how water is apportioned between states and the Country of Mexico and have a significant effect on how Colorado can develop our future water supply as shown in Table 7-2. There are no reliable additional water supplies that can be developed in the Arkansas and Rio Grande Basins, though water may be available in very wet years. The North Platte has the ability to increase irrigated acres consistent with the North Platte Decrees, but during the Basin Roundtable process, this was not proposed due the inability of agricultural users to pay for the infrastructure. CONI 7-5