Managing Semi-Arid Watersheds: Watershed Basics - Surface Water and Ground Water

Water that occurs in saturated zones beneath the soil surface is ground water. In contrast to the more visible surface water in streams, rivers, ponds, lakes, and reservoirs, ground water comprises over 97% of all fresh, liquid water on the earth. About 30% of all streamflow in the United States is contributed by ground water. Furthermore, nearly 50% of all drinking water used in the United States comes from ground water by the use of wells. Although ground water is an important source of fresh, liquid water, it does not always occur where it is most needed and sometimes it is difficult to extract. Without proper land management, large quantities of this valuable resource can become unusable because of contamination (pollution) or by deep pumping to the point that further extraction is not economically feasible.

Ground water is perceived by some to occur as vast underground lakes and rivers, but generally, ground water occurs in voids between soil or rock particles in an area called the zone of saturation. Ground water occurs in many different types of soil and rock strata. The amount of ground water that is stored and released from water-bearing strata depends on the porosity, size of pore spaces, and continuity of pores. Water-bearing porous soil or rock strata that yield significant amounts of water to wells are called aquifers.

An aquifer can be an underground lens of sand or gravel, a layer of sandstone, a zone of highly fractured rock (even granite), or a layer of cavernous limestone. An aquifer can range from a few feet to hundreds of feet thick and can underlie a few acres or thousands of acres. For example, the Ogallala aquifer extends from Texas to South Dakota in the United States.

Water flowing through a soil or rock material follows the path of least resistance. It will move through permeable materials and around impermeable ones. As the complexity of the geology of an area increases (amount of folding, uplifting, and fracturing of strata increases), the pathways of water flow likewise become complex. Ground water can flow slowly for hundreds of miles before emerging as a natural spring, seeping into a stream, being tapped by a well, or emerging into the ocean. Just as discharge can be varied, recharge (the addition of water to an aquifer) into an aquifer also can occur in many areas that are dispersed spatially over a large area. Recharge areas may occur at large distances from the discharge areas of an aquifer. Because water moves through aquifers under the influence of gravity, recharge zones are always higher in elevation than areas of discharge.

Recharge usually takes place in areas where permeable soil and rock materials are relatively close to the land surface and where there is an excess of water from precipitation. The rate of recharge and the area over which recharge takes place are important considerations when ground water pumping is being considered. When more ground water is removed by pumping than is being recharged, the aquifer is being mined.