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Sustainability of Ground-Water Resources--Circular 1186


The following review of some basic facts and concepts about ground water serves as background for the discussion of ground-water sustainability.


Figure 4. The unsaturated zone, capillary fringe, water table, and saturated zone.

Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone. In the unsaturated zone, the spaces between particle grains and the cracks in rocks contain both air and water. Although a considerable amount of water can be present in the unsaturated zone, this water cannot be pumped by wells because capillary forces hold it too tightly.

In contrast to the unsaturated zone, the voids in the saturated zone are completely filled with water. The approximate upper surface of the saturated zone is referred to as the water table. Water in the saturated zone below the water table is referred to as ground water. Below the water table, the water pressure is high enough to allow water to enter a well as the water level in the well is lowered by pumping, thus permitting ground water to be withdrawn for use.

Between the unsaturated zone and the water table is a transition zone, the capillary fringe. In this zone, the voids are saturated or almost saturated with water that is held in place by capillary forces.


Figure 5.  A local scale ground-water-flow system.
In this local scale ground-water-flow system,inflow of water from areal recharge occurs at the water table. Outflow of water occurs as (1) discharge to the atmosphere as ground-water evapotranspiration (transpiration by vegetation rooted at or near the water table or direct evaporation from the water table when it is at or close to the land surface) and (2) discharge of ground water directly through the streambed. Short, shallow flow paths originate at the water table near the stream. As distance from the stream increases, flow paths to the stream are longer and deeper. For long-term average conditions, inflow to this natural ground-water system must equal outflow.


Figure 6. A regional ground-water-flow system that comprises subsystems at different scales and a complex hydrogeologic framework. (Modified from Sun, 1986.)
Significant features of this depiction of part of a regional ground-water-flow system include (1) local ground-water subsystems in the upper water-table aquifer that discharge to the nearest surface-water bodies (lakes or streams) and are separated by ground-water divides beneath topographically high areas; (2) a subregional ground-water subsystem in the water-table aquifer in which flow paths originating at the water table do not discharge into the nearest surface-water body but into a more distant one; and (3) a deep, regional ground-water-flow subsystem that lies beneath the water-table subsystems and is hydraulically connected to them. The hydrogeologic framework of the flow system exhibits a complicated spatial arrangement of high hydraulic-conductivity aquifer units and low hydraulic-conductivity confining units. The horizontal scale of the figure could range from tens to hundreds of miles.


Figure 7. The concept of "hydraulic head" or "head" at a point in an aquifer.
Consider the elevations above sea level at points A and B in an unconfined aquifer and C in a confined aquifer. Now consider the addition of wells with short screened intervals at these three points. The vertical distance from the water level in each well to sea level is a measure of hydraulic head or head, referenced to a common datum at each point A, B, and C, respectively. Thus, head at a point in an aquifer is the sum of (a) the elevation of the point above a common datum, usually sea level, and (b) the height above the point of a column of static water in a well that is screened at the point. When we discuss declines or rises in ground-water levels in a particular aquifer in this report, we are referring to changes in head or water levels in wells that are screened or have an open interval in that aquifer.

(Box A)



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