Ground water recharge includes recharge as a natural part of the hydrologic cycle and human-induced recharge, either directly through spreading basins or injection wells, or as a consequence of human activities such as irrigation and waste disposal. Artificial recharge with excess surface water or reclaimed wastewater is increasing in many areas, thus becoming a more important component of the hydrologic cycle.
Natural recharge to the water table can be diffuse or localized. Diffuse recharge is the widespread movement of water from land surface to the water table as a result of precipitation over large areas infiltrating and percolating through the unsaturated zone. Localized recharge refers to the movement of water from surface water bodies to the ground water system and is less uniform in space than diffuse recharge. Most ground water systems receive both diffuse and localized recharge. In general, the importance of diffuse recharge decreases as the aridity of a region increases.
Typically, most water from precipitation that infiltrates does not become recharge. Instead, it is stored in the soil zone and eventually returned to the atmosphere by evaporation and plant transpiration. The percentage of precipitation that becomes diffuse recharge is highly variable, being influenced by factors such as weather patterns, properties of surface soils, vegetation, local topography, depth to the water table, and the time and space scales over which calculations are made. Recharge to the water table can occur in response to individual precipitation events in regions having shallow water tables. In contrast, unsaturated zone water in some desert regions is estimated to have infiltrated the soil surface as long as tens of thousands of years ago.