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Design of Sustainable W ater Management Systems 197

pores. When a soil is saturated with water, the larger pores drain first
due to gravitational forces. As the larger pores drain, the menisci
begin to retreat toward smaller pores. As the pores get smaller, the
capillary forces become larger and it becomes increasingly difficult to
drain freely. The volume of water within the pore space as a ratio of
the total volume of the soil is defined as volumetric water content.
When soil is thoroughly wetted and allowed to drain under the
force of gravity, all of the large pores will drain. The volumetric water
content after two days of free drainage of saturated soil is defined as
the field capacity (FC) (Soil Science Society of America 1984). All of the
remaining water is held within the pore space by capillary action.
As plants begin to uptake this water, the menisci continue to recede
to release the water. However, as the menisci become smaller, the
capillary forces increase making it more difficult for plants to uptake
the remaining water. If this process continues, there will reach a stage
when the lowest pair of leaves of a particular kind of plant wilts and
fails to recover when placed back in a water-saturated environment.
The volumetric water content at which this happens is called the
permanent wilting point (PWP) (Soil Science Society of America 1984).
The volume of water between FC and PWP is available for plant
growth.

6.3 Water Management

6.3.1 Plant Water Relations
Plants take up water through their roots, which is transported through
the stems to the leaves and eventually lost to the atmosphere via the
stomata. This process is known as transpiration, which is affected, by
soil water content and atmospheric conditions. Water will also evap-
orate from the soil and the plant surface. These two processes, tran-
spiration and evaporation, are combined and termed evapotranspiration
(ET) and is often difficult to distinguish (FAO 1998). Dry and windy
conditions promote higher ET. During hot weather conditions, the
evapotranspiration process helps to cool the plant. However, if
the evapotranspiration rate is larger than the plant water uptake, the
plant will be subjected to water stress, signaling the plant to conserve
water by closing its stomata. The plant will also show signs of wilting
whereby the leaves droop, reduce exposure to solar radiation, and
minimize ET.
Consequently, the gaseous exchange between the atmosphere
and the leaf cells are reduced. The wilted leaves do not capture the
solar radiation effectively. The reduced gaseous exchange and wilting
of the leaves will lead to a reduction in yield. Therefore, it is impor-
tant to maintain adequate root–zone moisture levels to help keep the
stomata open.