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The source of life is water. Life began in the
oceans, which represent the largest stock of water
on Earth. Much less water is stored below the land
surface in the form of fresh groundwater, amounting
to not quite 0.8% of the earth’s total water
reserves, while lakes and rivers combined only
contribute a further 0.007%. Therefore terrestrial
life depends primarily on the global water cycle.
This cycle makes the land productive by the infusion
of fresh water precipitation, originating from
the salt water of the oceans. However, in many
regions the precipitation does not provide a sufficient
or reliable source for the sustained presence
of plants and animals. In fact, in all regions, precipitation
proves to be highly variable in time and
space, and human activities that have led to global
warming have also increased the variability and
intensity of rainfall.
Assimilation and biomass production in natural
plant communities are intimately linked to
water use through transpiration. The same is true
for agricultural crops. However, extreme weather
events like drought and torrential storms threaten
agricultural enterprises and the well-being of
an ever increasing world population. Three-quarters
of the renewable fresh water resources used
by mankind are consumed in irrigated agriculture,
but such practices are at risk in several regions for
varying reasons. These include climate change,
weather variability, decline in groundwater
reserves owing to over-utilization, and the degradation
of soil and water quality. The 21st century
has been referred to as the ‘century of water’.
At the world food summit held in Rome in 1996,
water was identified as the major threat to food
security. The global water crisis is predicted to
intensify within the coming decades. Areas of
acute water shortage are expected to spread,
particularly to large regions of Africa and the
Middle East.
In an age characterized by an increasing
demand for fresh water and at the same time by
an actual decline in reliable water resources for
both rainfed and irrigated agriculture, conservation
of water is essential. Agricultural water
management must aim to eliminate unproductive
water losses and optimize transpirational water
use. The goal is to achieve optimum economical
yields per unit of water used without compromising
the environment. The development of an
understanding how to approach such a goal is a
central theme of this book. |
