Comparison of Two Approaces for Determining Ground-Water Discharge and Pumpage in the Lower Arkansas River Basin Colorado 1997-98

Home Browse Search

INTRODUCTION Irrigation is the largest use of water in south- eastern Colorado, and ground water is a supplemental source for irrigators in the Arkansas River Basin because surface -water supplies in the basin are inade- quate to meet irrigation demand. During the past 40 years, ground -water withdrawals were occasionally measured (Luckey, 1972) but were not routinely metered. Some estimates of ground -water withdrawals were reported ( Litke and Appel, 1989). However, the accuracy of the ground -water withdrawal estimates were not known. In March 1994, the Colorado Division of Water Resources (CDWR) adopted "Rules Governing the Measurement of Tributary Ground Water Diversions Located in the Arkansas River Basin" (Office of the State Engineer, 1994); these initial measurement rules were amended in February 1996 (Office of the State Engineer, 1996). The "Amendments to Rules Governing the Measurement of Tributary Ground Water Diversions Located in the Arkansas River Basin" were approved in June 1996 and require that about 1,600 wells that divert tributary ground water must annually report the water pumped monthly by each well. The rules allow a well owner the option of reporting pumpage measured by a totalizing flowmeter (TFM) or estimated using electrical power consump- tion data and a power conversion coefficient (PCC) (Hurr and Litke, 1989). The inline TFM and the PCC rating must be checked at least once every 4 years by a person approved by the State Engineer. A TFM is an inline flowmeter that directly measures the total volume of water pumped from the well. The PCC approach uses measurements of instantaneous ground- water discharge, hereinafter referred as instantaneous discharge, and instantaneous electrical power demand, hereinafter referred as power demand, to determine the number of kilowatthours of energy required to pump 1 acre -foot of water. Since 1994 when the rules became effective in the river basin, most well owners have chosen to use the PCC approach to determine ground -water pumpage from their irrigation wells. Opinions by representatives of the State of Kansas, presented before the Special Master of the U.S. Supreme Court hearing a case (State of Kansas v. State of Colorado, No. 105 Original (1996)) concerning well pumping after approval of the Arkansas River Compact of 1948, stated that the PCC approach does not provide the same level of accuracy and reliability as the TFM's when used to determine annual ground -water pumpage. Thereafter, the Colorado State Engineer proposed a study to deter- mine the comparability of estimates of ground -water pumpage using the TFM and PCC approaches. In 1997, the U.S. Geological Survey (USGS), in coopera- tion with the Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engineer (CDWR), began a 2 -year study to compare ground -water pumpage estimates made using the TFM and PCC approaches. The study area was the Arkansas River alluvial valley between Pueblo, Colorado, and the Colorado- Kansas State line (fig. 1). Purpose and Scope This report provides a comparison of two approaches for determining ground -water discharge and pumpage. Specifically, this report: 1. Evaluates differences in instantaneous discharge between TFM's and three portable flowmeters used with the PCC approach, and determines if differences in instantaneous discharge for the TFM and PCC approach are statistically significant; 2. Evaluates short- and long -term variations in PCC's, including whether seasonal variations in PCC's were evident; 3. Evaluates differences in ground -water pumpage estimated with the TFM and PCC approaches, and determines if differences in ground -water pumpage estimated with the TFM and PCC approaches are statistically significant; 4. Evaluates potential sources of discrepancy between pumpage estimates; and 5. Estimates differences in total network pumpage using the two approaches. One hundred and six irrigation wells that are powered by electric pumps were selected for this study from about 1,300 irrigation wells in the study area. The network of 106 irrigation wells consisted of 11 wells that had TFM's installed prior to the study and 95 randomly selected wells that had new TFM's installed during 1997 -98. During the irrigation season of 1997, instantaneous discharge was measured at 46 wells (43 of which had TFM's in 1997) and, during 1998, at 105 wells. One irrigation well was dropped from the network following the 1997 irrigation season INTRODUCTION