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I I I I I I I I I I I I I I I I I I I The model of the Denver Basin aquifer system supported by the Colorado Division of Water Resources is an important tool in the administration of State water law within the Denver Basin. The accuracy of the model is directly related to the availability and accuracy of hydrologic data used to construct the model. Most readily available hydrologic data already have been used in the construction of the model, but much of the data is either sparse, or of poor quality. The uncertainty in the data leads to uncertainty in the model results. Some of the model results have large economic and political ramifications for water users in the rapidly developing Denver Basin. The Technical Review Committee for the Senate Bill 96-074 report "Denver Basin and South Platte River Basin Technical Study" was given the task of preparing a list of operational charges and hydrologic studies that could, if implemented, provide additional hydrologic definition of the Basin. These new data would enable the refinement of the Denver Basin model, and would allow more direct comparison of model parameters and model results to real measurements of the hydrologic conditions in the basin. To this end an informal working group met on February 18, 1998, and formulated the following list of model parameters in need of refinement, and possible procedures that could be used to better define the parameters. Costs and time shown are rough estimates only intended to indicate the general magnitude of effort required. 1. A streambed conductance factor is used in the model to simulate the cumulative effect of reduced vertical hydraulic conductivity in the geologic materials between the streambed and the center of the uppermost node in the underlying bedrock aquifer. In layman terms, this conductance factor is a restriction to water movement between the stream and the underlying bedrock aquifer that is used to account for the fact that changes in streamflow do not have an immediate effect on the water level in the underlying bedrock aquifer. Few actual data are available to define the magnitude of the streambed conductance. As a result the conductance generally has been adjusted within the model to a value that is compatible with other model parameters. The streambed conductance is a particularly important model parameter because it readily affects the rate of depletion in streamflow caused by water-level declines from pumping in the bedrock aquifer. If efforts were made to better define the streambed conductance factor, the new data could improve the simulation results of the model, and would provide a real-world measurement of this critical model parameter. One technique for measuring the streambed conductance involves running an aquifer test at a carefully chosen site where the bedrock aquifer could be pumped and water-level changes monitored in nested piezometers completed at various depths in the bedrock and overlying alluvial aquifer near a stream. Core samples could be collected during drilling of the pumped bedrock well and analyzed for lateral and vertical hydraulic conductivity. Borehole-geophysical logs would be run in the well and used with core analyses and aquifer test results to relate streambed conductance results to other sites near a stream with a suitable bedrock well and geophysical logs. As an initial effort, one test could be performed in each of the six bedrock aquifers simulated in the Denver Basin Model. Such testing might require $300,000-400,000 during a 2-year effort. 2. As discussed in item 1, the model calculates the change in bedrock-aquifer discharge to stream valleys caused by water-level changes in the bedrock aquifer. It may be possible to locate a few sites in the basin where this change in bedrock discharge can be directly 1