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Glenn Graham and George Vanslyke elevations of the tops of each aquifer and the aquifer thickness were obtained for the center of each section. Val- ues for hydraulic conductivity, or intrinsic permeability, and potentiometric surface elevation were adopted from the Hydrologic Atlas Series HA -659, HA -647, and HA -646, pub- lished by the USGS in 1981. Data for the potentiometric sur- face of the Dawson aquifer were modified based on actual water level measurements in the records of the CDWR. Seventeen separate digital models were constructed to compute the depletion to specific stream systems as a result of pumping in any or all of the six aquifer intervals. When the distance between the location of a modeled pumping well and a stream system became large enough that the depletion to the stream was less than one -tenth of one percent, the statutory definition of nontributary groundwater, that location became a point that was used to define a line which identified the area which overlays nontributary groundwater. Over 3,000 model runs were made to define the one -tenth of one percent stream deple- tion line to the nearest section line. These model runs resulted in a series of maps for each aquifer that identified the location of nontributary groundwater. Groundwater located between the nontributary line and the stream sys- tem is not - nontributary groundwater. Use of this water requires court approval of a plan to provide replacement water to the stream system to mitigate depletions caused by pumping of the not nontributary groundwater. Rather than attempting to model every pumping scenario for depletions, the Denver Basin Rules provide for two levels of replacement in the not - nontributary area. Based on the maps of the alluvium of streams in contact with the out- crops of the bedrock aquifers, the developer of the not - nontributary groundwater must provide replacement water to the affected stream under one of two scenarios. If the well is located within one mile of the contact of the stream alluvium with the outcrop of the aquifer, then actual deple- tions must be calculated and that amount of water must be replaced in the stream. If the well is located more than one mile from the contact of the stream alluvium with the out- crop of the bedrock aquifer, then 4% of the amount of water pumped on an annual basis must be replaced. The one exception to this provision is in the Dawson aquifer, which has no 4% replacement requirement. Groundwater in the Dawson is considered to be in an actual effects area, therefore actual depletions must be calculated and that amount of water replaced to the affected streams. In the case of pumping of nontributary groundwater, at least 2% of the amount of water pumped must be relin- quished to the stream. The CDWR generally assumes that this requirement is met by return flows through wastewater disposal systems, or percolation of irrigation water. Because of these provisions, the key assumptions that went into crafting the Denver Basin rules and regulations were met: The Rocky Mountain Association of Geologists 158 • The definition of nontributary groundwater was clarified. • The 2% relinquishment requirement and the creation of not- nontributary groundwater category recognize that none of the Denver Basin bedrock groundwater is totally nontributary. • The 4% and actual effects areas of not - nontributary groundwater compensate the stream systems in a pre- dictable and orderly fashion. • Groundwater is administered on the basis of ownership of the land surface and the amount of drainable water available. • The use of the Denver Basin Atlases available to every- one did make the administrative processes for permit- ting and adjudication more predictable. Whether this resulted in reducing the expense and complication of the water rights claim process is debatable. CURRENT STUDIES In the early 1990s, the CDWR initiated a program to develop a tool to help administer the waters of the State. This tool was named the "Colorado Decision Support Sys- tem", or CDSS. CDSS is an integrated network of a rela- tional database; planning models to simulate surface water flow, groundwater, and crop consumptive use; administra- tive tools (real time stream flow data), historic stream flow data and groundwater level data; and several data manage- ment interfaces to allow quick access to data sets for view- ing and manipulation. The CDSS is intended to be accessible to the public over the World Wide Web. The first application of this technology was used to support administration of the Colorado River. This tool became known as the CRDSS, or the Colorado River Decision Sup- port System. Additional support systems are being devel- oped to facilitate administration of the Rio Grande, the Arkansas, and the South Platte Rivers. In 1996, legislation known as SB 96 -74 was imple- mented, providing funds for a study of the South Platte River system in anticipation of developing the SPDSS, or South Platte Decision Support System. Because of the level of development of both surface and groundwater in the South Platte River Basin, the interaction between the South Platte River and the bedrock aquifers of the Denver Basin is particularly important. SB 96 -74 calls for a review of existing data on water use in the basin. The study will also identify existing agricultural cropping patterns, identify consumptive use and evapotranspiration from agricultural water diversions, and quantify groundwater pumpage. The SPDSS will consist of five components: Management; Ground Water; Surface Water; Consumptive Use and Water Budget; and GIS and System Integration. More information cai ch, the wa cor spe anc clai Ruli any plai phy fron wat( watf