Surface Water and Groundwater Interactions
Water that makes it to the ground surface infiltrates into the soils. If the hydraulic conductivity of the soils are high then most snow melt or rain infiltrates and becomes part of the soil water. At the other extreme, low hydraulic conductivity soils such as silts and clays will only allow slow infiltration of surface water, especially after the surface soils are very wet or saturated. If the soils are coarse sands and gravels the infiltrating water migrates through the soils relatively rapidly and ultimately reaches the water table or saturated zone. If the soils are silty or loamy and are thick, then the fate of infiltrating water becomes decidedly more complex.
Most infiltrating surface water is stored in soils where it is transpired by plants. As water in the root zone is transpired, the water flows or moves in the soils as unsaturated flow, migrating slowly from areas where moisture content is higher to areas where it is lower. In many situations there little if any recharge of an underlying watertable.
When sufficient water infiltrates and fully saturate soils, the soil water can migrated deeper past the root zone. Once past the root zone the water will reach the water table only if there is enough water present so that the flow of water continues. As a result most groundwater recharge is episodic or periodic and there may be significant time lags between an infiltration event and when recharge reaches the watertable.
If the watertable is shallow enough the site may be subirrigated with surface water infiltrating down into the groundwater, but then as the soils are dried out by evapotranspiration, the groundwater begins moving up into overlying soils as capillary forces pull water up into soils.
The key to developing solutions for many groundwater problems including hazardous waste, nutrients, and water rights, begins with understanding and quantifying the fate of water in the soils and bedrock overlying groundwater.