Overview of the South Platte Basin within SPRAT

Located in the northeastern corner of Colorado, the South Platte River basin is unique in that it serves both the most populous section of the State and includes the nation’s third largest irrigation system (Platt 2000). Within SPRAT the South Platte is divided into four sub-regions. Each sub-region was identified based on its water demand and supply characteristics relative to the rest of the South Platte and is outlined in the figure below.

The rapidly growing Southern sub-region includes parts of Arapahoe, Douglas and Elbert counties and is, for the most part, devoid of any agricultural demands. Moreover, it is unique in that its primary source of supply is non-tributary groundwater.

The Central sub-region (shaded in green) includes the counties of Gilpin, Jefferson, Adams, Clear Creek, Denver and Park which comprise more than 65 percent of current total municipal and industrial (M&I) water demand in the basin. Agricultural (Ag) demands of limited magnitude (relative to other sub-regions) exist downstream of the main population centers. In addition to having extensive native flow rights, cities in this sub-region also receive several hundred thousand acre feet of water imported from neighboring basins.

The Northern sub-region consists of Larimer, Boulder, Weld, Morgan, Washington, and parts of Broomfield County. This sub-region reflects the greatest mix of user types, including, both, significant M&I and agricultural demands. Agricultural demands exist, and are modeled, both at upstream locations parallel to M&I demands and downstream of M&I demand centers. The spatial variability of agricultural users relative to M&I demands within this sub-region is unique to this region and is important when interpreting model output that is presented in Section five. The counties that comprise the Northern sub-region are all within the boundaries of the Northern Colorado Water Conservancy District (NCWCD). Thus, they all have access to the highly reliable trans-basin supplies available through the federal Colorado-Big Thompson project.

The Downstream sub-region, as the name implies, is located downstream of the intersections of the major tributaries and the main-stem of the South Platte. It includes only agricultural users who have access to both streamflow and groundwater. Agricultural demands in Logan, Morgan, Sedgwick, Washington, and part of Weld County are included in this sub-region