Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 133

4
Soil Water Flow
4.1 Introduction
Compared to the height of the atmosphere, the depth of the ocean and the thickness
of Earth's crust, the permeable soil above the bedrock is an amazingly thin body -
typically not much more than a few metres and often less than 1 m. Yet this thin layer
of soil is indispensable to sustain terrestrial life. Soil contains a rich mix of mineral
particles, organic matter, gases, and soluble compounds. When infused with water,
soil constitutes a substrate for the initiation and maintenance of plant and animal life.
Precipitation falls intermittently and irregularly, although plants require a continuous
supply of water to meet their evaporative demand. The ability of soil to retain soil
moisture (and nutrients) is crucial for vegetation to overcome drought periods. Soil
determines the fate of rainfall and snowfall reaching the ground surface - whether
the water thus received will low over the land as runoff, causing loods, or percolate
downward to the subsurface reservoir called groundwater, which in turn maintains
the steady low of springs and streams. The volume of moisture retained in the soil
at any time, though seemingly small, greatly exceeds the volume in all the world's
rivers (Hillel, 1998 ). Without the soil, rain falling over the continents would run off
immediately, producing devastating loods, rather than sustaining stream low. The
normally loose and porous condition of the soil allows plant roots to penetrate and
develop within it so as to obtain anchorage and nutrition, and to extract stored mois-
ture during dry spells between rains. But the soil is a leaky reservoir, which loses
water downward by seepage and upward by evaporation. Managing the top system
in water deicit regions so as to ensure the survival of native vegetation as well as to
maximize water productivity by crops requires monitoring the water balance and the
consequent change of moisture storage (as well as nutrient storage) in the root zone
(Hillel, 1998 ). Soil regulates the amount of evapotranspiration, which is with rainfall
the largest component of the hydrological cycle. In weather prediction, climate and
environmental research and groundwater recharge, the amount of evapotranspiration
plays a key role. Therefore not only a qualitative understanding of the soil water
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