Ground Water

(definition -- ground water--(1) water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper surface of the saturated zone is called the water table. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the Earth's crust.)

The term groundwater literally means water in the ground, but when used by hydrologists (people who study water) is usually referring only to water below the soil layer where water fills all the the space between and within the rocks. Groundwater makes up about 30% of California’s water supply in an average year and about 40% in dry years (Department of Water Resources 2003). Groundwater is a slow-moving and slowly changing part of the hydrologic cycle, only interacting with the surface as surface water percolates down, or as ground-water is pumped up by humans, or as it feeds a surface waterbody . The role of groundwater in your watershed’s hydrology and water management should be evaluated in watershed assessment and planning.

Groundwater exists in geologic formations called aquifers that contain and move "significant" quantities of water. An aquifer can be composed of hard rock or of loose materials, such as sand and gravel. Aquifers vary widely in the total amount of pore space filled with water and the how much connection allows water to flow between the spaces. Aquifers can be unconfined, where the water level is free to rise and fall with changes in water volume in the pore space. The top of water in the saturated zone of the aquifer is called a water table, and unconfined aquifers are also known as water-table aquifers. Other aquifers can be confined by an impervious layer that overlays the aquifer. Water in confined aquifers tends to be under pressure as a result of the confining layer acting as a cap, and the water will rise in a well that penetrates the confining layer.

Groundwater in unconfined aquifers flows from areas of higher elevation to areas of lower elevations or in the downslope direction of a sloping water table. In confined aquifers, the pressure is combined with the elevation to determine the direction and driving force of groundwater movement. Groundwater flow rates are also controlled by how permeable the aquifer is (its capability to move water).

A water balance for a groundwater basin or an aquifer can aid in understanding the inflows and outflows of groundwater and the resulting changes in storage in a manner similar to a water balance for a watershed.

Ground Water Management

Groundwater storage remains fairly constant from decade to decade in relatively undisturbed watersheds that have little or no groundwater pumping. In smaller watersheds, groundwater storage may vary between seasons and between wet years and dry years. The subsurface water balance is readily affected by human activities both indirectly and through active use and management of groundwater resources. Often groundwater management consists only of pumping water out of the aquifers to meet urban or agricultural needs. In other cases, such as in the lower Tujunga Wash watersehd, groundwater management also includes recharging groundwater basins through enhanced percolation (e.g., in percolation basins or ponds).

Percolation and Recharge

These two terms refer to the passive movement of water from the surface to the sub-surface (percolation) in a way that partially or completely refills the aquifer or groundwater basin (recharge). Percolation is inhibited by the construction of impervious surfaces, typical of urban and urbanizing areas. These impervious surfaces encourage water to runoff rather than to penetrate the ground. One approach to increasing percolation opportunities is to design urban/developed settings so that stormwater is detained in neighborhood swales or basins until it has a chance to percolate down through the soil.

References/Resources

Department of Water Resources. 2003. California’s groundwater. DWR Bulletin 118 update 2003. Sacramento: Department of Water Resources.

Heath, R.C. 1983. Basic ground-water hydrology. Raleigh: North Carolina Department of Natural Resources.