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11.6 Groundwater Discharge 371
4. Ground cover. Vegetation retards runoff and increases surface evaporation as well
as retention and transpiration of soil water. Effects such as these are most marked
during the growing season.
5. Geology. Geologic structure has much to do with infiltration. Examples are (a)
lenses of impervious materials, which intercept incoming water and keep it from
reaching the groundwater table; and (b) confining layers of tight materials, which
direct water into closed-channel flow. Independent zones of saturation above
lenses of impervious materials store perched water; continuous zones of saturation
(aquifers) lying between impervious materials hold artesian water.
6. Surface slope. Steep slopes hasten surface runoff and reduce infiltration. Earth’s
crust is porous to depths of 2 to 8 miles (3.22 to 12.87 km). Beyond that, pressures
are so great that plastic flow closes all interstices.
As explained earlier, infiltration into various soil types supporting different kinds of
vegetation is measured in lysimeters. The variables are many, and groundwater flow and
yield can be related to rainfall information only in very simple geologic situations such as
isolated sand dunes. Nevertheless, it is reasonable, for comparative or bookkeeping pur-
poses, to express annual results in the same units as rainfall. Infiltration of about half the
rainfall is not unusual.

11.6 GROUNDWATER DISCHARGE

In nature, subsurface waters are discharged from the ground (a) to the surface through
springs and seepage outcrops (hydraulic discharge), and (b) to the atmosphere from the
soil or through vegetation (evaporative discharge).
Hydraulic discharge takes place wherever the groundwater table intersects the land
surface. Geologic and hydraulic conditions that combine to force the return of groundwa-
ter to Earth’s surface as springs include the following:

1. Outcroppings of impervious strata covered by pervious soils or other water-bearing
formations
2. Overflows of subterranean basins in limestone or lava
3. Leakage from artesian systems through faults that obstruct flow
4. Steep surface slopes that cut into the water table. In humid regions, groundwater
may seep into streams throughout their length.

Evaporative discharge from soil is commonly confined to the belt of soil water, but it
also affects aquifers passing within capillary distance from the land surface. Plants seek
moisture at whatever levels their roots can thrive, usually from the belt of soil water or the
capillary fringe. Trees and phreatophytes may draw water from as far down as the zone of
saturation, xerophytes only from the zone of aeration. Ways of natural discharge of subsur-
face waters are illustrated in Fig. 11.1. Evaporative discharge is normally confined to a few
feet in humid climates, and about 20 ft (6.1 m) in dry climates. The roots of phreatophytes
may reach downward as far as 50 ft (15.2 m).
Differences between rates of recharge and discharge are correlated with changes in
water stored in the saturation zone. During wet weather, the water table rises; during dry
weather, it falls. Because the dry-weather flow of most surface streams is supported by
groundwater discharge, correlation between low stream flows and groundwater levels is
good, too, and observed coefficients can be used to predict ground storage.