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Comparison of Two Approaches for Determining <br />Ground -Water Discharge and Pumpage in the <br />Lower Arkansas River Basin, Colorado, 1997 -98 <br />By Russell G. Dash, Brent M. Troutman, and Patrick Edelmann <br />Several sections of this report contain detailed mathematical derivations and statistics. To facilitate <br />reading and use of this report, the report is organized in a manner that presents the primary results first, <br />then the detailed mathematical derivations and statistics in the sections that follow titled "Details of <br />Analysis and Results ". For those readers who are interested only in the primary results, rather than <br />the derivations and details, they may wish to read the sections titled "Primary Results" and skip the <br />sections titled "Details of Analysis and Results ".' <br />EXECUTIVE SUMMARY <br />Introduction <br />In March 1994, the Colorado Division of Water Resources (CDWR) adopted "Rules Governing <br />the Measurement of Tributary Ground Water Diversions Located in the Arkansas River Basin" <br />(Office of the State Engineer, 1994); these initial rules were amended in February 1996 (Office of the <br />State Engineer, 1996). The amended rules require users of wells that divert tributary ground water to <br />annually report the water pumped monthly by each well. The rules allow a well owner to report the <br />pumpage measured by a totalizing flowmeter (TFM) or pumpage determined from electrical power data <br />and a power conversion coefficient (PCC) (Hurr and Litke, 1989). <br />Opinions by representatives of the State of Kansas, presented before the Special Master hearing <br />a court case [State of Kansas v. State of Colorado, No. 105 Original (1996)] concerning post- Compact <br />well pumping, stated that the PCC approach does not provide the same level of accuracy and reliability <br />as a TFM when used to determine purnpage. <br />In 1997, the U.S. Geological Survey (USGS), in cooperation with the CDWR, began a 2 -year <br />study to compare ground -water pumpage estimates made using the TFM and the PCC approaches. The <br />study area was along the Arkansas River between Pueblo, Colorado, and the Colorado- Kansas State line <br />(fig. 1). <br />The two approaches for estimating ground -water discharge and pumpage were compared for more <br />than 100 wells completed in the alluvial aquifer of the Arkansas River Basin. The TFM approach uses an <br />inline flowmeter to directly measure instantaneous discharge and the total volume of water pumped at a <br />well. The PCC approach uses electrical power consumption records and a power conversion coefficient <br />to estimate the pumpage at ground -water wells. <br />EXECUTIVE SUMMARY 1 <br />