Geoscience Reference
In-Depth Information
Chapter 10
Soil Regulates Circulation of Water
on the Planet Earth
The circulation of water on Earth is composed of several basic processes. Water
vapor existing in the air condenses into small waterdrops that fall to the Earth's
surface as rain or snow. The water either soaks into the ground or it fl ows on the
surface into rivers - the majority of which empties into the ocean. Moreover, por-
tions of infi ltrated water fl ow to the rivers as groundwater. Water evaporates from
oceans, from rivers on its run to the ocean, and directly from the soil surface or
indirectly from the plants as transpiration. When water circulates back into the
atmosphere as water vapor, it is ready to take another run within the above pro-
cesses, collectively known as the hydrologic cycle. If there were no soil, infi ltration
would not exist. A net of dry riverbeds, called wadi in a desert, would quickly con-
duct rainwater. During the great majority of time, landscapes would be without
water, similar to today's desert regions. Without any water on land, there would be
no chance for plants to grow and animal life would not exist. Indeed, life on land
would be restricted to some microbes resistant to dryness existing for the majority
of times when rainfall does not occur. With rain seldom appearing, large territories
without rain could exist for several years. Frequent rains happening several or more
times in the month would occur only in narrow strips of land at the shore. The
remaining regions of continents without soils that have no porosity to catch and
store water in its pores during rainless periods would always remain dry and could
not contribute to evaporation.
By retarding some of the rain falling on land, soil maintains a water supply that
is absolutely essential for life on the land. Soil infl uences the rate of surface runoff
and is a decisive factor together with the weather about the occurrence of fl oods.
The deterioration of soil properties combined with variable weather conditions is
responsible for occasional dry seasons that last long enough to produce extreme
food shortages and famine in agricultural regions. We have no tools for making vital
changes in the weather as, for example, how to bring rains during long-lasting dry
periods or within specifi c seasons or years suffering excessive dryness. But we can
change the soil properties either in a favorable or unfavorable direction for agricul-
ture and for society in general. Being unaware of the fi nal consequences of our
