Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 85

Scale
2 cm

Vadose
zone

Water
table

Capillary
fringe

FIGURE 3.21
Photograph showing vadose zone, capillary fringe, and
water table surface. (Photo by Daniel T. Rogers.)

variations in the thickness of the vadose zone. During prolonged periods of drought, the
vadose zone may become much thicker as opposed to periods of prolonged rainfall when
the thickness of the vadose zone may shrink significantly.
As surface water continues its journey downward through the unsaturated zone, it
encounters the capillary fringe. The capillary fringe is the subsurface layer where ground-
water seeps up from the water table by capillary action to fill pore spaces (USGS 1999). At
the base of the capillary fringe, the pore spaces are filled with water due to tension satura-
tion. The thickness of the capillary fringe is dependent upon the balance between the soil/
sediment's adhesion binding capability and surface tension, and the gravitational force of
the lifted water mass. What factors prevail here? In most cases, the thickness of the capil-
lary fringe is less in course-grained materials than finer-grained materials (Payne et al.
2008). Figure 3.21 shows the vertical arrangement of the subsurface zones just described.
Once water reaches the water table, it migrates downward through the available inter-
connected pore spaces under the force of gravity and from zones of higher pressure to
areas of lower pressure. Groundwater can also move upward from areas of higher pres-
sure to areas of lower pressure. Imagine a U-shaped tube partially filled with water. If
pressure is exerted on one side of the tube, the water on the other side rises proportionally
in response to the pressure exerted on the other side. This one-sided pressure is provided
by the cork in the image on the right side of Figure 3.22.
Similar variations in pressure occur beneath the surface of the Earth. Pressure is higher
beneath hills and lower in valleys (Heath 1983). Elevated land contains more mass than
valleys and therefore exerts more force per unit area—the definition of pressure. The lower
pressure below valleys creates a pressure gradient toward the surface, allowing deeper

Atmospheric
pressure

Cork

FIGURE 3.22
Water movement under pressure.