Geoscience Reference
In-Depth Information
surface
water
water
table
unsaturated soil
ground water
(saturated soil)
Fig. 9.2
Groundwater table or simply the water table separating saturated soil from the upper
region of unsaturated soil. This boundary identifi es saturated and unsaturated fl ow regimes that
simplifi es approximate calculations of practical problems. Water fl ows across this boundary either
up at the evaporation or contributing to the supplying of plant roots by water. Or water fl ows down
when rain contributes to the rise of groundwater level
or megascale is essentially comparable with the slope of the water level in a river if
its riverbed is approximately straight, its length small, its fl ow rate small, and its
temperature together with its content of dissolved salts does not change along the
observed part of the river. In other words, when we observe and estimate the water
fl ow in a river, we consider only the gravitational potential (Fig.
9.2
).
Similar laws are valid for fl ow of water in soil, i.e., in porous media on a substan-
tially smaller scale that is commonly called the “pore scale.” Two principal water
fl ow regimes in soil are usually designated. When a soil is water saturated and has
all of its pores fi lled with water as typically occurs below a groundwater table, the
fl ow is described as saturated fl ow. When only a portion of the pores in a soil is fi lled
with water, we speak about unsaturated soil and unsaturated fl ow. Groundwater is
separated from the unsaturated soil by the groundwater table or simply water table.
9.2.1
Saturated Flow
The most frequent type of saturated fl ow is the fl ow that occurs in groundwater.
With the main driving force being the slope of the groundwater level, we measure it
using two wells separated by a horizontal distance across which we wish to calcu-
late the rate of groundwater fl ow. The wells are either dug out, or more frequently
drilled, and provided with a fl oater. Just after digging or drilling an observation
well, the water level inside of the well indicated by the fl oater rises owing to the
infl ow of water from the water-saturated region below the water table. After awhile
