Geoscience Reference
In-Depth Information
many approximate procedures based on simple, frequently measured data were pro-
posed. Examples of two such procedures are the linear correlations between soil
texture and either capillary capacity or wilting point.
During the early empirical period of soil physics, the fl ow of soil water was char-
acterized by three phases. The soil fl ow rate was high and the soil rapidly drained
when the soil water content ranged between saturation and capillary capacity. When
the soil water content ranged between capillary capacity and wilting point, the fl ow
rate was low. And when the soil water content was below the wilting point, the fl ow
rate was so extremely low that soil scientists assumed that soil water was not fl ow-
ing. Nowadays, scientifi c segments of classical Newtonian physics, hydraulics, and
mechanics are combined to understand and predict the basic dynamic processes
controlling soil water behavior.
9.2
Flow of Water in Saturated and Unsaturated Soils
When water fl ows on the Earth's surface, people often comment that it fl ows down-
hill or downslope. However, this remark is not exact. If it were indeed correct, water
could not fl ow in a river having a mound or bump across its bottom with a slope in
the opposite direction of the general slope of the riverbed. A more exact remark
would say that water fl ows in the direction of the slope of the water level of the river.
Even this latter selection of words is still only approximate because below the water
level of a river we fi nd transverse as well as reverse fl uxes. Transverse river fl uxes
are roughly perpendicular to the direction of the main river fl ow. Reverse fl uxes
commonly exist below a weir with water moving back along the bottom without
regard to the direction of the local bottom slope. The local gradient of the potential
always plays the decisive role in the local fl uxes. This is why we will speak about
potential and its slope (gradient) when we are dealing with fl ow within the pore
systems of soils. In order to accentuate the important role of potential upon the fl ow
on another much bigger scale, we describe a well-known ocean stream. Bulky and
intense streams caused by differences in potential are ubiquitous in oceans. Let us
start with easily observed and understood causative forces in order to identify the
nature of their active potential gradients.
Essentially as a continuation of the Equatorial Currents, the well-known Gulf
Stream in the Atlantic Ocean starts in the tropical area of the Mexican Gulf. We dem-
onstrate its size by comparing it to the Amazon River. With the narrowest portion of
the Gulf Stream representing about 20 Amazons, its biggest part grows and reaches a
size of 200 Amazons. After circulating in the warm waters of the Gulf of Mexico, the
Gulf Stream exits through the Straits of Florida, fl ows north parallel to the East Coast
of the USA, enters the deep Atlantic Ocean after passing Cape Hatteras, and brings
relatively warm water to the west coast of Europe. As it continues to fl ow further
northward, it gives us the impression that it disappears in the Arctic zone. But it does
not actually disappear - it simply dives to the depths of the ocean bottom. This sud-
den change from horizontal to nearly vertical downward streaming is simply caused
