Environmental Engineering Reference
In-Depth Information
water is removed from the large pores by gravitational forces, and air
takes its place. Gravitational forces remove water only until an equilib-
rium is reached at which capillary forces (the force of cohesion between
water molecules and the force of adhesion between water molecules and
soil particles) are sufficient to balance gravitational forces and prevent
further drainage. As soil water content continues to decrease, mostly as a
result of evapotranspiration, soil capillary forces become strong enough
to counteract roots' ability to absorb water. Finally the point is reached at
which the turf-grass plant wilts because its roots are unable to absorb the
remaining water, which is held too tightly by the small pores. If the soil
is not then recharged with rain or irrigation water, the turf-grass plant
may die.
One of the main objectives in turf-grass irrigation is to balance soil
capillary forces (soil suction) and root water-absorption forces (root suc-
tion). The amount of irrigation should be such that the soil always
maintains enough suction to prevent water drainage but not enough to
counteract root suction and subject plants to water stress.
The ease of achieving this objective depends on soil texture, structure
and organic-matter content. Sandy soils are the most challenging. They
generally hold only a few per cent of water, which may be used up by turf
during a few hours of intensive evapotranspiration, so sands may require
irrigation several times a day. Heavy clays are also difficult. They may
hold the largest amount of water, up to 30-40%, but very strong capillary
forces often prevent most of it from being taken up by roots, so availabil-
ity of water to plants may be limited. Loamy soils are usually the best.
They hold large amounts of water readily available to plants.
Soils with well-developed granular structure hold more water avail-
able for plants than unstructural soils. Soil organic matter helps to retain
moisture as well.
As mentioned above, after heavy rainfall water moves downwards,
passing mainly through the large soil pores. If the soil is porous and
uniform, this type of water movement, called saturated water flow, will
continue until the large pores are empty. Once that point is reached,
movement of water in the soil is described as unsaturated water flow.
In such situations, water movement is subject to capillary forces, and flow
proceeds in all directions: downward, upward and sideways. These
unique properties of soil capillaries can sometimes be used to our advan-
tage. For example, if a layer of sand is placed on top of a layer of gravel,
capillary forces in the coarser soil (gravel) are too weak to draw water
from the finer soil (sand), and movement will either stop or slow down
considerably. This hydrological phenomenon can be utilized in designing
soil media for specific turf purposes. For example, in construction of golf
course greens and some athletic fields, a layer of sand mixed with a little
loam is spread over a layer of very coarse sand or gravel. If a small
