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

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because the well owner had reconfigured the discharge distribution system and combined the plumbing of two wells together. This activity created a complex well that was not suitable under the amended rules for Rule 3.6 analyses (Office of the State Engineer, 1996), making the well unacceptable for the continued appli- cation of a PCC to determine ground -water pumpage. During the study, PCC's were calculated each time a portable flowmeter measurement of the instan- taneous discharge and power demand were made at a well. At 104 of the wells, PCC's determined during the 1998 irrigation season were applied to the total elec- trical power consumption recorded between the initial and final readings at the site in 1998 to estimate total ground -water pumpage for the period. The total pumpage estimate derived using the PCC calculation then was compared to the total pumpage measured using the TFM at 104 wells. However, pumpage data from one well were omitted because it was determined that the existing TFM (make R) was not working prop- erly, which resulted in 103 wells that were used for comparison of ground -water pumpage. Acknowledgments The authors wish to thank the CDWR personnel, specifically, Keith Kepler P.E., G.R. Barta P.E., D.G. (Dan) DiRezza, W.A. (Bill) Richie, W.W. (Bill) Tyner, and R.L. (Bob) Plese, Jr., who obtained well -owner permissions, arranged the purchase and installation of TFM's, measured instantaneous discharges, provided electrical power consumption records, recorded field measurements, and provided the USGS with electronic data that were used for this study. Without their dedi- cated work, the study could not have been completed. The authors acknowledge the support of the Colorado Water Conservation Board, and in particular Steve Miller. The authors also appreciate the cooperation of many private landowners in the basin who allowed their irrigation wells to be used in this study. METHODS OF INVESTIGATION Data collection and analysis consisted of several phases: (1) Identification of potential sites; (2) selec- tion of sites for TFM/PCC comparisons; (3) installa- tion of the TFM's; (4) measurement of instantaneous discharges; (5) determination of PCC's; (6) computa- tion of ground -water pumpage using TFM and PCC approaches; and (7) analysis of data. Initially, the CDWR identified more than 1,300 large- capacity irrigation wells (wells that discharge more than 50 gal/min) in the Arkansas River Valley between Pueblo, Colorado, and the Colorado - Kansas State line for which the PCC approach might be used to determine ground -water pumpage under the amended rules established by the Office of the State Engineer (1996). This initial list of wells was decreased to about 800 potential sites for TFM/PCC consideration based on the following criteria: 1. The well was reported as active and was connected to a power source. 2. The well used an electric motor, as opposed to an internal combustion engine. 3. The well had at least 10 acre -ft of reported annual pumpage at least once since 1994. A computer program (Scott, 1990) was used to randomly select one primary and four alternative sites for each potential well in the TFM/PCC network. Each primary site was, evaluated by CDWR and inventoried to determine its suitability for inclusion in the TFM/PCC study. If a primary site was rejected, a randomly selected alternative site was evaluated and so on down the list of alternatives until a suitable site was found. During 1997, CDWR evaluated 107 wells for potential TFM installation; in 1998, CDWR evalu- ated 122 wells for additional TFM installations. The most common reasons for rejection and the total number of well sites rejected during 1997 -98 were as follows: The site was determined to be a complex system and was found unsuitable for Rule 3.6 analyses, or the site was determined to be a compound system, or the owner indicated future modifica- tions were planned that would make the site unsuitable for continued application of the PCC approach. Compound system means that more than one electrical device is being operated from the same electrical power meter. (38 wells rejected) 2. The discharge pipe was in poor physical condition, the pump surged or was unable to maintain a full pipeline of flow at a measurement section, or Comparison of Two Approaches for Determining Ground -Water Discharge and Pumpage in the Lower Arkansas River Basin, Colorado, 1997 -98