Pick Our Brain - April 2012

How does groundwater recharge work?

Groundwater is water that occurs beneath the earth’s surface and is an important component of the hydrologic (water) cycle. Recharge is a term hydrologists use to describe additions to groundwater. The main sources of natural groundwater recharge are from precipitation and seepage from streams, surface-water bodies (ponds and lakes) and excess irrigation water. Discharge, on the other hand, is the term used to describe removal of groundwater. This can occur naturally in the form of springs, or as flow into streams and other surface-water bodies, or when groundwater is pumped from wells.

Not all precipitation becomes groundwater. Some of it travels over the land surface into streams and other water bodies, some is used by plants, and some is evaporated back into the atmosphere. Water that moves past the plant root zone continues to move or percolate downward through spaces and cracks in soil and rocks until it reaches the location of groundwater called the saturated zone (a layer where all available air spaces are filled with water). The top of the saturated zone is called the water table. If you have ever dug a hole in the sand next to a stream or other water body and watched it fill with water, you have seen groundwater flowing from the saturated zone. Depending on the amount of water and the type of rock, sediment and soil through which the water passes, it can take the water anywhere from days to centuries or longer to reach different groundwater layers or aquifers.

The amount and source of water recharged to groundwater can vary greatly from year to year depending on the local geology of the area and the amount of precipitation. For instance, groundwater recharge in the Sacramento Valley is mainly from precipitation, whereas recharge in the San Joaquin Valley is mainly from excess irrigation water. Another means of recharging groundwater is known as “water banking”. In water banking, surplus surface water is used to recharge the groundwater to improve water supply. One common method of water banking in parts of the Central Valley is through the use of surface recharge basins (ponds). These ponds can be quickly filled with water, which then slowly percolates into the ground, recharging the groundwater.