The natural discharge of groundwater in Wyoming basins includes leakage between geologic units; flow from springs; seepage into streams, wetlands, lakes and other surface waters, especially within thick alluvial deposits; and direct evaporation where the water table is shallow enough that capillarity or plant transpiration brings groundwater to the surface (evapotranspiration). Like recharge, natural discharge is difficult to determine, especially on a basin-wide basis, and cannot be measured directly but must be estimated from other measurements and determinations. Leakage between geologic units, which can be a significant part of natural discharge, occurs in the subsurface, and evapotranspiration and groundwater discharge to surface waters generally occur over large areas – these discharges cannot be measured and must be estimated.
In addition to wells, artificial avenues of groundwater discharge include seepage into mines, other excavations, irrigation canals, and drainage canals as well as flow between aquifers in poorly completed wells. In some areas of the WBRB, drainage tiles that discharge shallow groundwater directly to surface waters are installed in irrigated lands to lower the water table and prevent waterlogging and salt deposition.
Groundwater discharge, buffered by the storage function of an aquifer, is generally a more efficient process than recharge and occurs over a smaller area. While recharge occurs intermittently by water percolating through unsaturated material both across and parallel to stratification, discharge is a more continuous process that occurs under more efficient saturated flow conditions in the direction of highest hydraulic conductivity, generally parallel to stratification. Under natural conditions (no artificial discharge of groundwater) and over a time period that depends on the precipitation, hydrogeology, and hydrologic cycle in a groundwater basin, recharge and discharge are balanced. Reasonably accurate estimates of both recharge and discharge are necessary to evaluate safe/sustainable yield.
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