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

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there was inadequate upstream or dowrx- dream distances available to correctly install a FM. (32 wells rejected) 3. The well owner declined to participate in _-- ____J�e TFM/PCC study. (19 wells rejected) 4. The well had less than 10 acre -ft of pump ge reported the previous year. (12 wells rej cted) 5. The well appeared to be inactive, and the wner indicated it was not used. (9 wells reject d) 6. The discharge pipe was not a correct size for instal- lation of a Signet TFM (one of the bran JNN of TFM used in the study). Pipe was small � than 8 -inch diameter during 1997, or smaller —khan 6 -inch diameter during 1998, or was lar than 12 -inch diameter during either year. (24 wells rejected) In 1997, permission to measure dischar -- e at 46 wells was obtained, including 11 wells the —� had pre- existing TFM's and 35 wells where new M's were planned to be installed during the 1997 i igation season. During 1997, discharge measurement of installed TFM's were made at 43 of the 46 we - - --=:Is in the monitoring network. One new TFM was not i stalled until the end of the 1997 irrigation season, an two of the new TFM's were returned to the factory f A cali- bration and were not reinstalled until after the 1997 irrigation season. One pre- existing TFM well as reconfigured to a complex system after the 19 17 im- gation season and was dropped from the study During 1998, permission to install TFM's and measur -- discharge at 60 additional wells was obtained_ The changes resulted in a final monitoring networ of 105 wells having TFM's. However, upon eval a ation of the data, an electric power meter at one site was found to be malfunctioning, resulting in 104 ells being used for analysis of variations in PCC's and a TFM was found to be malfunctioning at anotlh r site, resulting in 103 wells being used to compare round - water pumpage. Each well in the network was visited to dentify discharge system characteristics and to confi_r that the PCC approach could be properly applied a the well in accordance with the amended rules (O ce of the State Engineer, 1996). When possible, ell owners and operators were interviewed and ire _ orma- tion was collected about normal operating con ----------------- litions, flow ranges and pressures, and number of disc -1 large distribution outlet locations. Well- identifying data were recorded from the motor, pump, and electrical meter nameplates during the visit. The CDWR made an onsite identification of the type of discharge distribution system at each of the wells in the network, based on a visual observation of the discharge plumbing during the initial visit, which was confirmed before making subsequent field measurements. For this study, four major types of discharge distribution systems were identified. The well network included 65 open- discharge, 18 low- pressure, 10 sprinkler, and 12 complex discharge distribution systems. Hereinafter, the open- discharge distribution system type is referred to as type O, the low- pressure discharge distribution system type is referred to as type L, the sprinkler discharge distribu- tion system type is referred to as type S, and the complex discharge distribution system type is referred to as type C. According to the CDWR, well sites that are classified as complex systems will vary the total dynamic head (TDH) at the pump during the irrigation season. The change in TDH may result from wells that discharge into a pipeline with multiple outlet loca- tions, multiple wells that discharge into one common pipeline, or wells where the method of water delivery changes between different types of distribution systems, such as open- discharge and sprinkler systems. The complex discharge sites that were included in the study network were sites where the wells discharged into a pipeline with more than one point of discharge (multiple outlet locations). As such, these sites qualified for use of the PCC approach pursuant to Rule 3.6 of the amended rules (Office of the State Engineer, 1996). For such sites, a PCC measurement was determined under the high TDH discharge point and a second PCC measurement deter- mined under the low TDH discharge point; and a system PCC was calculated that was weighted on the basis of the PCC's at the discharge points and the expected crop water demand at each discharge point. Totalizing Flowmeter Measurements The accuracy of many factory- calibrated TFM's is reportedly 2 to 3 percent of discharge (M.H. Noffke, Great Plains Meter, Inc., written commun., 1998). To obtain an accuracy of 2 to 3 percent of discharge, a METHODS OF INVESTIGATION 7