Agriculture Reference
In-Depth Information
crop is growing. The third is to reduce evaporation of water during the period when
a crop is not growing so that more water will be retained in the soil for use by a sub-
sequent crop. Fourth, the harvestable portion of the crop can be increased. The fifth
is to increase the portion of the field water supply that is used for evapotranspiration
by reducing percolation, runoff, and evaporation.
IntRoductIon
Water is often called the blood of life, but for an increasing portion of the world's
population, it is life itself. Prema Ram, head of a village council in the Rajastan
Desert of India, said “Ask for blood and we'll readily give it to you, but don't ask for
water because our lives depend on every drop that falls from the sky” (Caryl, 2007).
Almost 30% of the world's population lives in arid and semiarid environments where
water is a severe constraint for producing food and fiber (Stewart et al., 2006). More
than 1 billion people across the globe lack enough safe water to meet minimum lev-
els of health and income (Rosengrant et al., 2002). Water will become increasingly
limited as population increases to a projected 8 billion by 2025.
Agriculture is by far the biggest user of water. Although the amount of water
required to produce cereal grain depends somewhat on the species and the climate
in which it is grown, an average of 1000 tons of water to produce 1 ton of cereal is
reasonable. Cereal production is an important benchmark because cereals directly
supplied 57% of the calories in the global human diet in 2000 (Food and Agriculture
Organization [FAO], 2003). Postel (1999) stated that crops get about 70% of their
water directly from rainfall and 30% from irrigation. However, irrigation develop-
ment is the primary reason that world food production has increased faster than
world population during the past several decades. Between 1961 and 1997, world
population increased by 89%, food production per person increased by 24%, while
food prices fell by 40% in real terms (Wood et al., 2000). Worldwide, about 17%
of the cropland is irrigated and accounts for 40% of the food and fiber production.
Irrigation, however, uses much of the water abstracted from aquifers and surface
water supplies such as lakes and rivers. On average, irrigation accounts for nearly
70% of world water abstraction—more than 90% in agricultural economies in the
arid and semiarid tropics, but less than 40% in industrial economies in the humid
temperate regions (FAO, 1996).
Irrigated area expanded rapidly during the twentieth century, and for much of the
time it increased at a rate faster than the population (Table 19.1). Per capita area of
irrigated land increased from 0.027 ha person
−1
in 1900 to 0.38 in 1950 and reached
a peak of 0.047 in 1980. Since 1980, population has increased at a faster rate than
irrigated area, as reflected by the per capita area dropping back to 0.044 ha per-
son
−1
. This downward trend is of concern because there are a number of reasons that
irrigation area will become increasingly difficult to increase. Postel (1999) stated
that groundwater is being pumped faster than nature is recharging it in many of the
world's most important food-producing regions, including parts of India, Pakistan,
the north China plain, and the western United States. Postel further stated that there
is simply little undeveloped water to tap, and that one of every 5 ha of irrigated
land is losing productivity because of spreading soil salinization. There is general
