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

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proportional to the water velocity, and mean water velocity multiplied by the cross - sectional area of the pipe is the instantaneous discharge. The second type of portable flowmeter was an ultrasonic flowmeter. Typical accuracy of an ultra- sonic flowmeter is reportedly 1 to 5 percent (Omega Engineering Inc., 1992). An ultrasonic flowmeter manufactured by Polysonic, hereinafter referred to as method "P ", was used in this study and uses the transit -time method for flow measurement. Two trans- ducers were mounted on the outside of the discharge pipe and functioned alternately as a transmitter and a receiver of ultrasonic signals sent upstream and down- stream through the pipe. The time difference between the signals, averaged in the upstream and downstream directions, is proportional to the velocity of water flow. The flowmeter was programmed to process the infor- mation and output a discharge value every minute. Generally, 10 or more of the discharge readings were averaged to obtain the instantaneous discharge. Diag- nostic menus were used to determine the acceptability during each test. Diagnostic parameters such as signal strength and a difference count were supplied by the equipment and had to be within specified limits to be a valid well discharge measurement. The third portable flowmeter was a typical propeller -type flowmeter manufactured by McCrometer, hereinafter referred to as method "M ". The propeller -type flowmeter was mounted to the end of a section of plastic pipe with sufficient upstream length and attached with a rubber coupler to the open end of the discharge pipe to make a discharge measurement. During each site visit, well discharge measurements were made with a method M portable flowmeter by reading registers dials of the TFM and timing the index wheel for one complete revolution, and dividing the indicated volume by the elapsed time. Generally, 10 readings were made at each site and the recorded discharge was the average of the 10 values. Power Conversion Calculations and Computations of Pumpage The PCC is defined as the number of kilo - watthours required to pump 1 acre -ft of water. Elec- trical power meters contain a disk that revolves as electricity passes through the meter. During a site visit, the meter disk was timed with a stopwatch for 10 complete disk revolutions to measure the rate per revolution. This rate measurement was repeated three times and used to determine the average rate of a disk revolution. Power demand, in kilowatts, was calcu- lated from the equation: power demand = (rate) x (3.6) x (Kh factor), (1) where rate = average time of disk revolution, in revolutions per second, 3.6 = conversion factor (kilowatt seconds per watthour), and Kh factor = watthours per revolution (imprinted on the front of power meter). Determining the PCC combines a concurrent measure- ment of well discharge (in gallons per minute) with the power demand of the pump (in kilowatts). The PCC, in kilowatthours per acre -foot, is then calculated from the equation: PCC = (power demand) x (5433)/(well discharge), (2) where 5433 = conversion factor (in gallon hours per acre -foot minutes), and well discharge = instantaneous ground -water discharge, in gallons per minute. A PCC was computed for every instantaneous discharge measurement that was made at a well. The PCC's derived in 1997 and in 1998 were used to eval- uate temporal variations in the PCC data. However, because the majority of PLC's were measured late in the 1997 irrigation season, only the PCC's determined from the 1998 measurements were used to compute ground -water pumpage estimates for each well and to compare differences in total pumpage between the TFM and PCC approaches. Pumpage estimates were calculated using every PCC measurement made at a well during 1998. This was done by dividing the total 1998 power consumed, in kilowatthours, by each unique PCC measurement made at the well during 1998. The number of kilo - watthours used between onsite visits was determined by reading the electric meter at the beginning of a site visit. The total electrical power used was determined from readings of the electrical meter at the beginning METHODS OF INVESTIGATION