Average annual recharge
As shown in the preceding section, although the saturated geologic units in the groundwater basins function as reservoirs that store enormous volumes of groundwater, with the exception of unconfined aquifers (primarily Quaternary strata) only a small fraction of the groundwater in storage can be withdrawn for beneficial use. If only the volume of potentially producible water in storage were considered, it would clearly be a nonrenewable resource. The amount of groundwater withdrawal that can be sustained from a typical aquifer system is controlled by recharge, most strongly as withdrawal approaches or exceeds recharge.
The availability of average annual recharge from Hamerlinck and Arneson (1998) and our mapping of the outcrop areas of the hydrogeologic units in the WBRB (Figure 5-1 and Plate IV) provided the opportunity to evaluate recharge on a regional scale that has not been attempted previously. This section describes how the volume of average annual recharge within the WBRB groundwater basin was estimated and evaluated for this study.
Aquifer-specific average annual recharge less estimated present annual discharge (both natural and by pumping) establishes an estimate of how much groundwater development can be sustained without unacceptably drawing down storage or causing permanent structural damage (irreversible compression of a confined aquifer). Recharge is estimated in this study; however, discharge is difficult to estimate, especially on an aquifer-specific basis. BRS Inc. (2003e) and MWH et al. (2010) developed estimates of annual groundwater withdrawals and consumptive uses that are used in this study (Chapter 8). MWH et al. also developed a water-balance for the WBRB drainage basin. Comparison of recharge with stored and available groundwater volumes (this section); analysis of recharge as percentages of precipitation and of other basin-wide water balance statistics; and current groundwater consumptive use and future groundwater requirements as a percentage of recharge (Chapter 8) also contribute to this inventory of WBRB groundwater resources.
Estimated average annual recharge in the WBRB ranges from less than 1 inch per year in interior areas of the basins to more than 55 inches per year in the surrounding mountains (Figure 5-1). The mountain and foothill areas are characterized by higher recharge than the lowlands in the basins because of:
- greater precipitation and more persistent snow pack
- more abundant vegetation
- soil and vegetation combinations more favorable to infiltration
- less evapotranspiration
- better exposure of the upturned and eroded edges of hydrogeologic units and associated greater permeability parallel to bedding
- structural features that enhance recharge: faults, fractures, fault/fracture-controlled surface drainage
Figure 6-7, a map of recharge efficiency (recharge as a percentage of precipitation), illustrates more efficient recharge in the highlands surrounding the basins. Figure 6-7 was compiled to show how the recharge efficiency varies over the WBRB and to inform speculation on the factors that control recharge. The figure shows areas of recharge efficiency, grouped in ranges, each area containing the set of 4,000-meter grid cells with recharge efficiencies within a given range. The recharge efficiency in each grid cell was calculated by dividing the average annual recharge to the cell (see Figure 5-1) by the average annual precipitation to the cell (see Figure 3-3). Figure 5-1, average annual recharge, is based on percolation percentages for different soil/vegetation combinations multiplied by precipitation to give average annual recharge. Although this does not take all the factors that affect recharge into consideration, initial infiltration and precipitation are probably the most important in a regional sense, and the other factors listed above and in Section 18.104.22.168 should confirm the general pattern of recharge efficiency displayed in Figure 6-7. As discussed previously (Sections 22.214.171.124 and 5.4), local recharge may be dominated by site-specific hydrogeologic conditions, such as solutionenhanced fracture permeability.
In adapting Figure 5-1 from Hamerlinck and Arneson, 1998, and developing Figure 6-7, all recharge values less than or equal to zero were changed to 0.5 inches prior to performing the calculation. This change was made for most of the cells within the interior basins areas, and therefore has a strong influence on the appearance of both maps in these areas. The resulting large area of constant recharge in the interior basin areas is clearly shown on Figure 5-1. The higher range of recharge/ precipitation (R/P) in the central, driest areas of the basin than around the basin perimeter shown on Figure 6-7 is the result of a constant recharge value divided by lower precipitation values in the interior basin areas. The original data from Hamerlinck and Arneson (1998) that showed most of the interior basin areas receiving no recharge (including some outlier negative values) was changed for this report because it is well known that shallow groundwater occurs in both the alluvial and bedrock aquifers throughout the interior lowlands of both the Wind River and Bighorn basins. If no recharge were occurring in these areas, shallow groundwater could not be sustained.
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