Scientific Investigations Report 2007–5007

Scientific Investigations Report 2007–5007

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Estimates of Ground-Water Recharge

The estimated mean annual recharge for predevelopment conditions (fig. 9) clearly shows large spatial variations in recharge, from more than 50 in. in the wet uplands to zero in the arid lowlands. The average predevelopment recharge for the entire 6,207 mi2 that was modeled for the Yakima River Basin aquifer system was estimated to be about 11.9 in. or 5,450 ft3/s (about 3.9 million acre-ft), which is about 44 percent of the total precipitation. About 97 percent of the recharge occurred in the upland parts of the 3,667 mi2 area included in the PRMS models, and nearly 90 percent of the total was in the upper Yakima and Naches modeled areas, herein called the ‘humid’ areas (fig. 7, tables 1 and 2). Most recharge in the humid areas discharges as shallow subsurface flow to support streamflow (Mastin and Vaccaro, 2002b) rather than entering the bedrock hydrogeologic units in the uplands. For example, PRMS calculated mean annual inflow to the ground-water reservoirs in the 1,127 mi2 upper Yakima area to be about 960 ft3/s (about 34 percent of the calculated 2,821 ft3/s of deep percolation), most of which provides baseflow to streams above Keechelus, Kachess, and Cle Elum Lakes. Inflow to the bedrock units (recharge to the aquifers) in the upland areas is limited because the units generally have low permeability (Molenaar and others, 1980) that is much less than the overlying soils and (or) the unconsolidated deposits are generally thin or missing in these areas (Jones and others, 2006). Estimated mean annual predevelopment recharge was only about 1.0 in., or 187 ft3/s (about 0.14 million acre-ft) for the 2,540  mi2 in the 14 areas modeled with DPM. Recharge in these areas, which is about 3 percent of the total basin recharge, was about 10 percent of the precipitation.

The mean annual predevelopment recharge estimates for modeled areas (table 2) vary widely, as do the ratios of recharge to precipitation (R/P). The large variations in mean annual recharge (0.08 to 33.9 in.) for the areas further highlight the spatial variations shown on figure 9. For example, recharge in the humid areas was 1 to 2 orders of magnitude larger than that in other areas, and R/P was above 0.60. In contrast, R/P ranged from 0.01 to 0.19 for the 14 areas modeled with DPM, of which 11 areas had R/P values of less than 0.10. One of the smallest recharge estimates, 0.11 in., was for the Prosser area (fig. 7, table 2), where recharge is less than 1 ft3/s (about 585 acre-ft) and R/P is 0.01. Intermediate amounts of recharge, 2.16 to 4.00 in., occur in the Ellensburg, ToppSatus, Yak Canyon, lower Naches, and Ahtanum areas, where precipitation was greater than 14 in. (fig. 7 and table 2), and the R/P values were between 0.18 and 0.22.

Except for the humid areas, the ratio of predevelopment recharge to model-calculated actual evapotranspiration (AET) approximates the R/P values (table 2) because AET accounts for most of the incident precipitation. The model-calculated potential evapotranspiration for the semiarid to arid areas also is much greater than precipitation.

Estimated current recharge increased as a result of human activities (fig. 10). Excluding septic-system recharge (discussed separately below), mean annual current condition recharge was estimated to be 15.6 in., or 7,132 ft3/s (5.2 million acre-ft)—an increase of 3.7 in. or 1,682 ft3/s (1.2 million acre-ft) from predevelopment conditions. The largest increases in recharge were in areas with the most surface-water irrigation; recharge in some irrigated arid areas was estimated to be similar to the recharge in the humid areas. For example, the Prosser area (predevelopment recharge of 0.11 in.) was estimated to receive about 22.5 in., or 165.6 ft3/s (about 0.12 million acre-ft) of recharge under current conditions (table 2), which is similar to the 25.3 in. of recharge for the humid Naches area. Therefore, under current LULC conditions, recharge is derived primarily from precipitation in the uplands and applied irrigation water in the lowlands.

Excluding recharge from septic-system drainfields, the 16 DPM-modeled areas with extensive human activities (fig. 8) produce about 1,955 ft3/s of recharge (an increase of about 1,769 ft3/s from predevelopment conditions for the DPM areas [table 2]). A large part of this quantity, however, is expressed as streamflow in drains and wasteways that ultimately becomes return flow to the streams. These return flows are relied on to meet downstream demands for irrigation and instream flows. Septic systems produce only about 17 ft3/s (about 12,000 acre-ft) of recharge in these 16 areas, and the basin-wide mean annual septic recharge is about 0.04 in. (about 0.2 percent of the total basin average current recharge). Locally, the septic-system recharge can be much greater than precipitation-derived recharge, especially in areas with concentrated population and or with low annual precipitation quantities. For example, a 4-acre census block in the lower Naches area had a population of 293 and a resulting large quantity of septic recharge. Also compare predevelopment recharge to septic recharge for the Selah-Wenas and Ahtanum areas (table 2).

The change in recharge between predevelopment and current conditions is most pronounced for the modeled areas with the most irrigation, but it is also pronounced for areas that are only partly irrigated. For example, LULC for the Ahtanum area (fig. 7, table 2) was estimated to be about 48 percent native sagebrush and grasslands, about 37 percent irrigated (surface water and ground water) agriculture, and about 12 percent high-density development (urban areas, residential, and commercial), with the remaining 3 percent of the area containing several other LULCs. Its average annual water-application rate, 15.9 in. (about 183 ft3/s), was about the median for the 16 areas. The spatial distribution of recharge for both LULC conditions (figs. 11–12) indicates the magnitude of the effects of human activities. Precipitation was about 14.4 in. and, with the 15.9 in. application rate, recharge increased by about 9 in. (103 ft3/s). Correspondingly, R/T (where T is the total water input—the sum of P and the application rate) doubled from 0.19 to 0.38 (table 2). The relatively uniform distribution of predevelopment recharge (fig. 11) changed dramatically under current conditions (fig. 12), and there are both large increases and some decreases in the areas with high-density development.

On an annual basis, recharge, and the difference between predevelopment and current recharge, vary widely. The annual values for the lower Naches area (fig. 13; location shown on fig. 8) clearly show large interannual variations (more than 5 in.) and the effects of irrigation on model-calculated recharge and AET.

The estimated current recharge in the built-up areas was zero due to the assumption that these areas are impervious LULC. Although this results in areas of no recharge to the aquifer system, most of these areas are surrounded by extensive irrigated lands, and the ground-water levels in the shallow hydrogeologic units would not be representative of areas with zero recharge.

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