NorthPlatteBasinWaterSupplyandNeedsReport

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Section 7 Availability of Existing Water Supplies in the North Platte Basin 7.1 Methods and Tools Employed to Evaluate Surface Water Supply Availability The availability of surface water and groundwater supplies for each basin are summarized in this section. Physical availability of surface and groundwater resources must be carefully evaluated against the legal right to divert, pump, or consume these resources. Surface water supply availability was estimated at selected points in each major river basin in Colorado. Colorado's DSS surface water allocation model, StateMod, and supporting datasets, were the primary tools used for this analysis when available. State Mod simulates daily or monthly hydrologic water availability in a river basin based on a stream's water rights, structures, and operating rules (http://cdss.state.co.us). For those basins without State Mod datasets, alternative sources and studies were used to summarize available water to the extent possible. 7.2 Overview of Groundwater Supplies and Availability Groundwater is present throughout the state. It is found in a variety of aquifers, from unconsolidated sand and gravel in the floodplains of the major rivers to bedrock deposits buried deep below the surface. The key aquifers in the state are located primarily in the unconsolidated deposits. These include the alluvial aquifer systems of the Arkansas, South Platte, Gunnison, Colorado, and North Platte Rivers. In addition, there is a significant aquifer located in unconsolidated deposits in the San Luis Valley in south central Colorado within the Rio Grande Basin. Of the many aquifer systems located in bedrock deposits, the most significant of these are the aquifers of the Denver Basin, located east of the Front Range, and the Ogallala (High Plains) aquifer located in eastern Colorado. S:\1177\BASIN REPORTS\NORTH PLATTElS7 _NORTH PLATTE. DOC 7.2.1 Definition of Groundwater Resources Groundwater is administered by the State DWR to regulate and manage its use. Section 4 provides additional information on water rights as it affects groundwater resources. To reiterate, Colorado recognizes four types of groundwater and has separate sets of rules for each. These are based on interaction with surface water and/or on geographic location: . Tributary - groundwater that is hydrologically connected to a natural stream. . Non-tributary - groundwater located outside of a designated basin, the withdrawal of which will not, within 100 years, deplete the flow of a stream at an annual rate greater than one-tenth of 1 percent of the annual rate of withdrawal. . Designated Basin - groundwater in areas not adjacent to a continuously flowing stream or required to fulfill decreed surface water rights, and located within the boundaries of a designated basin as defined by the legislature. . Denver Basin - groundwater located outside of a designated basin and located within the boundaries of the Denver Basin aquifers as defined in 1985. Tributary and non-tributary groundwater supplies are located throughout the state, while Denver Basin and designated basin groundwater are located in specified areas in eastern Colorado. Tributary groundwater occurs in the shallow alluvial aquifers adjacent to streams. This type of groundwater is administered under the Prior Appropriation System of water rights as are surface water supplies. In most basins, groundwater use is junior to surface water and so its use is allowed only if augmentation plans have been filed with the State Engineer that describe how the predicted depletions of stream flow due to the groundwater usage are offset CONI 7-1 COLORADO ~ DEPARTMENT OF NATURAL RESOURCES