Scientific Investigations Report 2006–5205

**U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5205**

Reliability of ground-water pumpage estimates establishes potential bounds on estimated pumpage. However, the most reliable estimates are those reported in the previous sections. Many factors affecting pumpage estimates make it difficult to assign a measure such as a percent error or a root mean square error. Therefore, a potential range in values is estimated for the pumpage categories. The range delineates an estimated minimum or maximum pumpage value for a category. In addition, the values probably have an increasing error as the estimates are extrapolated further back in time. Therefore, only the reliability of the 2000 estimates is discussed and it is assumed that estimates extrapolated back in time have an increasing bound, as a percent error, that is a function of the estimated quantity of pumpage for any particular category and year. Although standby/reserve pumpage would have been zero in 2000, the reliability of standby/reserve pumpage estimates is assessed in this section. The reliability is described for the categories in the same order they were presented in the previous sections. Frost protection category reliability is not discussed because pumpage was assumed to be negligible and was set equal to zero. Last, excluding the provided data, the methods used to estimate pumpage are invariant over time. For most water users, the amount of pumpage in any year varies, and these decreases or increases in pumpage are not captured in the estimates. Therefore, there is some, but an unknown amount, of error in the annual values.

Three classes of PWS estimates were: (1) municipal and large Group A systems with data available, (2) 62 Group A and B systems that provided some data, and (3) remaining systems with no data. In 2000, the first class served about 203,000 people, the second class served about 10,100 people, and the third class about 27,000 people. This total population of about 240,000 accounted for about 71 percent of the population in the basin in 2000. In addition, pumpage for the PWS category, which totals about 46,385 acre-ft, is the second largest pumpage category.

The pumpage estimate for the first two classes (about 41,496 acre-ft, table 10) can be considered the most reliable of the estimates because it is based on provided data. Errors inherent in metering, compilation of data to digital form from paper copies, and other factors are minor compared to both the total pumpage for this category and to total pumpage. The reliability of estimates for these two classes was assumed to be good with a potential error of about 5 percent or about 2,075 acre-ft, with most of that quantity (1,864 acre-ft) accounted for by the municipal systems. The lower and upper bounds for these two classes are 35,409 and 4,012 acre-ft and 39,137 and 4,434 acre-ft, respectively (table 10).

The third class of PWS pumpage has a larger potential error. This class totaled 4,890 acre-ft in 2000. Based on available databases and contacts with water-system operators, the population served by this class was the best estimate available. Therefore, the reliability of these estimates is a function of the basin-wide average per capita rates (251 and 109 gal/d) used to derive the estimates. The 251 gal/d rate was applied to systems outside irrigation districts that served about 10,000 people, and the 109 gal/d rate was applied to systems in irrigation districts that served about 17,000 people. When combined, these values yield an effective population weighted rate of 162 gal/d.

The domestic pumpage value of 251 gal/d is larger than the statewide average, but it is known that the per capita value in eastern Washington is larger than in western Washington due to a higher water demand resulting from less precipitation and warmer summers (Dion and Lum, 1977; Lane, 2004; Washington State Department of Health, 2001). This per capita rate, which was derived from provided data, also is larger than many municipal rates. However, Dion and Lum (1977) indicated that the per capita rate for the three counties in the study area ranged from about 227 to 344 gal/d, which are higher than Lane’s (2004) range of 115 to 230 gal/d. In addition, DOH’s average water-system design rate for the three-county area is on the order of 280 to 300 gal/d.

Average per capita rate for all systems that provided data and that do not use surface water as a drinking water source is 118 gal/d. The 118 gal/d value can be considered a lower bound on an effective basin-wide average, especially because this value includes municipal systems whose pumpage includes water for commercial and industrial uses. Using this rate divided by the effective population weighted rate of 162 gal/d and multiplying it by the pumpage estimate for Group A and B systems without data yields 3,562 acre-ft of pumpage as an estimate of a lower bound (table 10) with a difference of 1,329 acre-ft. Assuming that DOH’s rate of 300 gal/d is the upper bound for a basin-wide average, yields 9,056 acre-ft as an upper bound (table 10), a potential increase of 4,166 acre-ft. Note that 80 percent of per capita values calculated from the provided data were less than 300 gal/d.

Reliability of domestic (primarily exempt wells) estimates can be calculated as above using an effective rate of 180 gal/d based on the distribution of exempt population residing in and out of irrigation districts. Ratios of the low and high per capita values to the effective rate multiplied by the estimated pumpage yields a lower bound of 13,135 acre-ft and an upper bound of 33,393 acre-ft (table 10). In addition, about 322 acre-ft of pumpage was estimated for domestic use included in other categories, primarily the irrigation category. If it is assumed that the census numbers are exact and all domestic use was accounted for, the exempt well pumpage could be 322 acre-ft too large. However, this small value is much less than potential errors due to the estimates of the average daily per capita rate. For example, increasing or decreasing the effective basin-average daily rate by only 5 gal, increases or decreases the total pumpage by about 500 acre-ft.

Several factors help to define the reliability of irrigation estimates; however, reliability of irrigation pumpage estimates is more difficult to assess because no reliable source is available for comparison. The reliability discussed is for the estimated pumpage for primary irrigation rights, which was 188,230 acre-ft. Reliability of pumpage estimates for standby/reserve rights, which was 82,812 acre-ft, is addressed after the discussion of the primary rights.

For areas defined by zip codes in the part of the basin where power data was available, we estimated 79,051 acre-ft of pumpage outside of the irrigation districts and within boundaries of irrigated lands as defined by the spatial distribution of land use developed for the study. Based on power consumption data for the zip code areas, pumpage outside the irrigation districts was 27,788 acre-ft for the basalt well equation 1, 61,636 acre-ft for the overburden well equation 2, and 61,851 to 149,988 acre-ft for the PCC equation. The basalt equation produces an unreasonable value because the average crop-water use would need to be less than 1 acre-ft/acre, which could not support most crop types such as orchards, hops, alfalfa, and row crops. The large pumpage value calculated from the median of the measured PCC values yields about 5.5 acre‑ft/acre, which also is unreasonable because this value is 2 to 3 ft more than the needs of most crop types grown in the zip code areas. Therefore, a range of about 62,000 to 100,000 acre-ft is more reasonable, suggesting that the pumpage estimates may be on the order of 22 percent too large to 21 percent too small. This coarse comparison assumes that the allocation of power consumption data based on using irrigated lands and boundaries of irrigation districts is reasonable, and that power consumption for surface- and ground-water uses are approximately the same.

Comparison of DOA acreage of surveyed fields and allowable acreage for irrigation rights for selected TRSs outside of irrigation districts indicated that allowable acreage generally was greater than actual acreage, thus the acreage of irrigated croplands used to calculate pumpage estimates may be high. However, given the aggregation of the DOA field data to the section level, the varying number of fields in each section, and that the allowable acreage for a right may extend across several sections, it is not possible to determine the actual versus allowable acreage for most rights. The acres used for the calculations generally could be on the order of 5 percent too large, which in turn, could result in estimates on the order of 10,000 acre-ft too large.

For the cases where pumpage data were provided, the difference between estimated and provided values was only 1 percent for a large orchard, but was 1.25 to 2.14 times larger for two large vineyards. The information provided indicates the highly efficient use of water (1.0 to 1.7 acre-ft) in the vineyards. The total irrigation pumpage estimate may be as much as about 15,000 acre-ft too high if all vineyards are as efficient (use of drip irrigation).

Cline and Knadle (1990) and Van Metre and Seevers (1991) indicate that the estimated pumpage for an individual well may vary from the actual value but that the total estimate for many wells is reasonable; Van Metre and Seevers (1991) identify the total error as 4 percent. However, this error generally is related to some particular year. Based on local climatic conditions, soil moisture, and soil properties, pumpage for any particular field with the same crop varies on a year to year basis. For example, one farm provided irrigation pumpage data for several years that averaged 860 acre-ft, but the annual values ranged from about 600 to 1,100 acre-ft. For the crop types in the basin, the long-term average crop-water use value used for estimating pumpage had interannual variations over the 50-year calculation period of as much as 3-4 in. due to the combination of temporally varying climate conditions and soil properties. It is assumed that by using an effective long-term crop-water use value the pumpage estimate should represent a reasonable average, especially considering the continually changing crop patterns.

Kinnison and Sceva (1963) estimated about 24,000 acre-ft of irrigation pumpage (about 50 percent of the annual appropriated quantity) for 1953 compared to this study’s estimate of about 36,000 acre-ft (about 77 percent of the annual appropriated quantity). Laird and Walters (1967) estimated about 79,000 acre-ft of irrigation pumpage (about 97 percent of the annual appropriated quantity) for 1965, compared to this study’s estimate of about 56,000 acre-ft (about 73 percent of the appropriated quantity). Last, Parker (1971) estimated irrigation pumpage at about 81,000 acre-ft for 1970 compared to this study’s estimate of about 75,000 acre-ft; both estimates have a similar percentage of the appropriated quantity (73 and 78 percent, respectively). Note that the annual appropriated quantity increased by about 25,000 acre-ft from 1965 to 1970, and either the previous 1965 estimate is too high or the 1970 estimate is too low. In addition, previous estimates for 1965 and 1970 were for the three-county area, which is larger than the study area and contains additional irrigated croplands in the part of Benton County outside the basin.

The TCWRA (Tri-County Water Resource Agency, 2003) used the appropriated right for both primary and standby/reserve rights as the estimate of irrigation pumpage. Using such a value would increase the 2000 estimate of 271,042 acre-ft by about 100,000 acre-ft. Based on values measured during this study, pumpage averaged 89 percent (median of 71 percent) of the appropriated value and excluding the largest percent, the average percentage of appropriated water pumped was 73 percent (median of 67 percent). These percentages suggest that the 2000 pumpage estimate would not be 100,000 acre-ft too small.

As can be seen from above, the irrigation estimate for primary rights can possibly be either too low or too high. Thus, the potential range is estimated to be on the order of 20 percent of the annual value (about 38,000 acre-ft). The latter value yields 150,230 and 226,230 acre-ft as the lower and upper bounds (table 10).

The possible variations in standby/reserve pumpage were described previously in the ‘Irrigation’ section of the pumpage estimates; the section concluded that the pumpage would be zero in years without prorating. Therefore, the lower bound would be 0 acre-ft for standby/reserve pumpage (table 10), for example, in 1996 or 2000, years without prorating, there would have been no standby/reserve pumpage. The upper bound would be at most equal to the estimated value of 82,812 acre-ft (table 10), especially considering that the 207 standby/reserve rights with a smaller water duty of 0.1 to 2.0 acre-ft/acre were estimated to be fully used.

The pumpage estimate for livestock was based on the full appropriated right. Information was available for only one livestock right that also had PRUs of DM and IR, but had no allowable acreage for irrigation. The provided information was only 15 percent of the right but this appeared to be a unique situation. Therefore, this value was not directly used for estimating a potential range in estimates.

The full appropriated right is a reasonable assumption for dairy use, which accounts for 23 percent of the total. However, the remaining livestock pumpage may be less. Including the probable dairy use as part of the dairy use reduces the remaining livestock estimate to about 4,900 acre-ft. About 88 percent of the latter is withdrawn by 4 large operations (more than 80 percent is accounted for by one operation). Interviews with personnel from a large operation indicate that in most years they withdraw the appropriated quantity, with highest demand in summer and lowest demand in winter. Therefore, about 4,350 acre-ft of the remaining pumpage estimate appears to be reasonable, leaving about 550 acre-ft of pumpage. Larger appropriated rights average about 180 acre-ft and range from 16.5 to 770 acre-ft, whereas the rights for the remaining 550 acre-ft of pumpage average about 19 acre-ft and range from 1 to 86 acre-ft with 70 percent less than 20 acre-ft. Most estimates associated with the 550 acre-ft are thus for small operations that may or may not use their full right. For estimating purposes, this value was assumed to be as small as 225 acre-ft. Given that the number of livestock fluctuates and their needs change due to various factors, the remaining 6,174 acre-ft of estimated livestock pumpage was assumed to potentially be 10 percent smaller. Therefore, pumpage estimates may be as much as 845 acre-ft (225 + 620) too large, yielding a lower bound of 5,881 acre-ft (table 10).

Percentages of appropriated rights used for estimating the pumpage associated with the PRUs of CI and HE were much smaller than would be expected, but were based on provided data. These percentages were 4 and 14 percent for rights with a single PRU of HE and the remaining rights had a percentage of 25. Excluding the largest percentage, the second largest percentage for the provided data was 39 percent. Previous industrial pumpage estimates ranged from slightly more than the appropriated quantity to 47 percent of the appropriations. The most current previous percentage for the study area was about 48 percent of appropriated water was being pumped (Dion and Lum, 1977), which is similar to the 47 percent estimated by Parker (1971). Assuming that the potential upper range in estimates is on the order of 48 percent, the estimate may be as much as 3,470 acre-ft too small. The upper bound then would be about 10,700 acre-ft (table 10). For a lower bound, the authors determined that less than 4 percent of the allocated water would be too small and chose to use a 4 percent value as the smallest value that should be applied to all rights. This percentage gives a lower bound of about 1,030 acre-ft (table 10). In comparison, the commercial and industrial estimates for irrigation rights were estimated to average about 25 percent of the allocated water, with most values between 15-29 percent. Provided data for commercial and industrial uses also indicated values on the order of 25 percent. The above suggests that the estimate of the lower bound using 4 percent of the appropriated water should be considered too low, but establishes the smallest possible value.

The estimated pumpage for fish and wildlife propagation (9,369 acre-ft) was primarily based on provided data that account for about 99 percent of the pumpage. Similar to the first two classes of PWS, the reliability was assumed to be good with a potential error of about 5 percent. The 5 percent value results in lower and upper bounds of 8,901 and 9,837 acre-ft, respectively (table 10).

Ground-water claims were the most problematic category of pumpage to derive estimates. Estimated pumpage for claims was 34,310 acre-ft and the claims in WRTS total about 270,000 acre-ft. The latter value could not be possible because the total ground-water rights in the basin prior to July 1945 totaled only about 39,000 acre-ft, and maximizing the estimates of drinking water supplies using a 300 gal/d per capita rate and the non-municipal ground-water supplied population from 1950 gives only 33,000 acre-ft. Together, the total appropriated quantity and drinking water supply would only account for 27 percent of the claimed annual pumpage. This indicates that at least an additional 49,000 acres would have to have been irrigated with ground water by claims or nearly 4 times the allowable amount of acres for the rights, which is unreasonable. Indeed, the claims included in the estimate already account for about 12,790 acres of croplands, which is more than the 9,682 allowed under the irrigation rights as of July 1945.

In addition, pumpage for claims for this report was an estimate of pumpage that was likely not accounted for in the other categories. The PWS and domestic pumpage are reasonably accounted for, except for potentially about 900-1,200 acre-ft of applied water for the 451 claims for self-supplied domestic use with 1 acre of allowable irrigation claimed. Commercial and industrial pumpage should account for most of the actual use with possibly only a few unaccounted for claims.

Based on the above, the pumpage estimate for ground-water claims probably accounts for most, if not more, of pumpage not included in the other categories. The reliability of the estimate is assumed to be about 20 percent (6,900 acre-ft) with the lower and upper bounds being 27,410 and 41,210 acre-ft, respectively (table 10).