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zone of soil moisture. It is crisscrossed by roots, voids left by
decayed roots, and animal and worm burrows that enhance the
infiltration of rainwater into the soil. Soil water is used by plants in
life functions and transpiration. Some water also evaporates
directly back into the atmosphere. Water that is not held as soil
moisture will percolate downward until it reaches a zone where all
of the open spaces in sediment and rock are completely filled with
water. This is the zone of saturation. Water within it is called
groundwater. The upper limit of this zone is known as the water
table. The area above the water table where the soil, sediment, and
rock are not saturated is called the unsaturated zone. The pore
spaces in this zone 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 it clings too
tightly to rock and soil particles. By contrast, below the water
table, the water pressure is great enough to allow water to enter
wells, thus permitting groundwater to be withdrawn for use.
The water table, the upper limit of the zone of saturation, is
a very significant feature of the groundwater system. The water-
table level is important in predicting the productivity of wells,
explaining the changes in the flow of springs and streams, and
accounting for fluctuations in the levels of lakes.
Water soaks into the ground because bedrock, sediment,
and soil contain countless voids, or openings. These openings are
similar to those of a sponge and are often called pore spaces. The
quantity of groundwater that can be stored depends on the porosity
of the material, which is the percentage of the total volume of rock
or sediment that consists of pore spaces. Voids most often are
spaces between sedimentary particles, but also common are joints,
faults, cavities formed by the dissolving of soluble rocks such as
limestone, and vesicles (voids left by gases escaping from lava).
Porosity alone cannot measure a material’s capacity to
yield groundwater. Rock or sediment might be very porous yet still
not allow water to move through it. The pores must be connected

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