Groundwater recharge

The accumulation of groundwater requires that an aquifer first be filled with water (charged), then refilled (recharged) to replace water lost. Recharge is a difficult parameter to determine; it cannot be measured directly but must be estimated from other measurements and determinations. Recharge is the amount of water that infiltrates the ground surface, percolates through the unsaturated zone, and reaches the water table – or leaks from adjacent aquifers or confining units. While recharge by leakage from or through confining units is very slow, it does occur if there is permeability and a pressure gradient into the receiving aquifer. Recharge from precipitation is generally expressed as the fraction (or percentage) of precipitation (in inches per year) that infiltrates the ground surface and reaches the water table. Recharge volume is estimated by multiplying estimated inches of recharge by the outcrop area of an aquifer. The local rate of infiltration varies on the basis of:

  • Composition and hydraulic properties of the surficial materials (soil and underlying bedrock)
  • Depth and degree of bedrock weathering
  • Soil moisture
  • Vegetation (type, abundance)
  • Animal burrows, root zones
  • Type, timing, rate, and duration of precipitation
  • The difference between precipitation and evapotranspiration (ET) rates
  • Slope and aspect of the land surface
  • Depth and degree of bedrock fracturing
  • Natural and man-made large-scale openings or depressions (caves, mines, pits)
  • Opportunity for recharge from surface waters
  • Local land use (irrigation, soil stripping, paved areas)
  • Leakage from adjacent aquifers and/or confining units

Prior to infiltrating the subsurface, precipitation wets vegetation. Initially infiltrating water replaces any deficiency in soil moisture; then, available water percolates down through the unsaturated zone to the water table, below which saturated flow conditions prevail. The characteristics of the unsaturated zone affect the amount of water that reaches the water table. When precipitation exceeds the rate of infiltration, overland flow occurs; and additional recharge from surface flows may occur downstream from the site of precipitation. A common, very general assumption is that approximately 10 percent of precipitation recharges groundwater.

Conceptual cross section of groundwater features
Conceptual cross section of
groundwater features
Estimated net annual aquifer recharge
Estimated net annual
aquifer recharge

Artificial avenues of recharge include reservoirs, irrigation canals, injection wells and fields, infiltration galleries, unlined pits, and other surface water diversion projects, as well as flow between aquifers in poorly completed wells. (The extent of artificial recharge in the WBRB cannot be evaluated on a regional basis, but it might be determinable for a local area.) In the fairly dry climate of Wyoming, most natural recharge (directly from precipitation) occurs in the mountain ranges surrounding the basins, on thick alluvial materials that border the mountain ranges, and along stream channels within and underlain by thick, permeable alluvial deposits. Most recharge in Wyoming occurs during late fall, winter, and early spring: vegetation is dormant, evapotranspiration is minimal, precipitation is generally higher, and snowpack functions as a reservoir maximizing contact with the ground surface and enhancing infiltration. In the central basin areas where the ET rate generally exceeds the precipitation rate, the opportunity for widespread recharge is limited to infrequent high precipitation and thick snowpack melting events. Recharge to bedrock within interior basin areas is also hindered by the generally flat-lying strata of the Tertiary hydrogeologic units, because permeability is generally less efficient across stratification.

Reference View complete Wind/Bighorn Basin Water Plan