Aquifer recharge is water that moves from the land surface or unsaturated zone into the saturated zone. Quantitative estimation of recharge rate contributes to the understanding of large-scale hydrologic processes. It is important for evaluating the sustainability of groundwater supplies, though it does not equate with a sustainable rate of extraction. Where contamination of an aquifer is a concern, estimating the recharge rate is a first step toward predicting solute transport to the aquifer. Recharge may cause a short- or long-term rise of the water table. Artificial drainage, e.g., with horizontal porous pipes buried at a chosen depth, is sometimes used to maintain a minimal thickness of vadose zone for agricultural or other purposes.
Recharge rates vary considerably in time and space. Recharge often occurs episodically in response to storms and other short-term, high-intensity inputs. For a given amount of infiltration, temporal concentration enhances recharge because it entails shorter residence times for water in the portions of the soil from which evapotranspiration takes place. Similarly, a larger fraction will become recharge if it is concentrated in narrow channels such as fingers or macropores, not only because this tends to hasten its passage through the unsaturated zone, but also because the water then occupies less of the volume of soil from which evapotranspiration takes place.
