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

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total range in PCC's determined during 1998 is shown in figure 5B. The equation used to determine percent difference shown in figure 5B is: Percent difference = 100 x (maximum PCC — minimum PCC) /site average PCC. (6) Percent differences in PCC data at wells ranged from less than 1 percent (sites 21 and 88) to more than 150 percent (site 27). The data indicated that 58 percent of the site comparisons had less than a 10- percent change and 86 percent of the site compari- sons had less than a 20- percent change in PCC's throughout the 1998 irrigation season. The PCC measurements made during the 1998 irrigation season were evaluated for systematic seasonal variations. Figure 6 shows that for the majority of instances, there are no evident seasonal patterns in the PCC measurements made during 1998. Comparisons of PCC's to depth to ground water did not reveal any systematic relation between changes in PCC's and depth to water. The PCC measurements made at 41 network sites during 1997 were compared to PCC measure- ments made during 1998 at the same 41 sites (fig. 7) to evaluate temporal variations during two irrigation seasons. Thirty -seven sites (90 percent) had at least one PCC measurement made in 1997 that was less than the range of PCC's made in 1998 (fig. 7); 16 of the sites (39 percent) had all 1997 PCC's less than the range of PCC's made during 1998. Only sites 83 and 87 had a large difference between the 2 years of data. Overall, the 2 years of data indicate that the PCC measurements were similar between 1997 and 1998. Long -Term Variations in Power Conversion Coefficients State - approved PCC measurements collected at the network sites during 1994 to 1997 for compliance with State rules (Office of the State Engineer, 1994 and 1996) were used to evaluate temporal variability that occurred in PCC's during the 4 -year period. The long -term variability between PCC's for wells in the 1998 network and corresponding State - approved PCC's during 1994 -97 is shown in figure 8A. Implicit in this comparison is the assumption that the State - approved PCC's determined during 1994 -97 are of the same quality as the PCC's determined during this study, including the removal of the cases where the PCC's change under Rule 3.5 (Office of the State Engineer, 1996) due to a change in pump or motor. The equation used to compute the percent differences shown in figure 8B is: Percent difference = 100 x (State- approved PCC — site average PCC) /site average PCC, (7) where site average PCC = the arithmetic mean of all PCC's determined at each site in 1998. Fifty comparisons of 103 PCC measure- ments (about 48 percent) had less than 10- percent difference between the State - approved PCC's and the site average PCC measured during 1998 and about 67 percent of the State - approved PCC measure- ments were less than 20 percent of the PCC's during 1998 (table 5). Twenty -one of the 103 site compari- sons indicated a positive percent difference of more than 20 percent in PCC's, and 13 site comparisons indicated a negative percent difference of more than 20 percent. A positive percent difference indicated that the 1994 -97 State - approved PCC was greater than the site average PCC in 1998. The percent difference between the State - approved 1994 -97 PCC's and the average 1998 PCC ranged from about —57 to 211 (table 5). The largest range in percent difference was between the State - approved PCC's measured in 1995 and the 1998 PCC's. A comparison of the percent differences computed using the State - approved PCC's from 1997 to the average 1998 PCC's indicated that 78 percent of the sites were within 10 percent and 89 percent of the sites were within 20 percent. During well operation, the PCC is generally constant for a specific discharge pressure and a stable pumping water level. Because the water level in a well often declines rapidly during the initial period of pumping, the PCC also changes rapidly until the pumping water level stabilizes. A potentially important change made in the Colorado amended rules in 1996 (Office of the State Engineer, 1996) required PCC measurements be made only after the pumping water level had not changed more than 10 percent in the hour prior to making the PCC measurement. TEMPORAL VARIATIONS IN POWER CONVERSION COEFFICIENTS 21