Climatological Methods forManaging Farm Water Resources

Almost all of the water available on the earth, 97 percent, occurs as saltwater in the oceans. Of the remaining 3 percent, 66 percent occurs as snow and ice in polar and mountainous regions, which leaves only about 1 percent of the global water as liquid freshwater. More than 98 percent of freshwater occurs as groundwater, while less than 2 percent occurs in rivers and lakes. Groundwater is formed by excess rainfall (total precipitation minus surface runoff and evapotranspiration) that infiltrates deeper into the ground and eventually percolates down to groundwater formations (aquifers). For temperate, humid climates, about 50 percent of precipitation ends up in the groundwater. For Mediterranean-type climates, this figure is 10 to 20 percent, and for dry climates it can be as little as 1 percent or even less. The global renewable water supply is about 7,000 m3 per person per year (present population). The per capita minimum water requirement is estimated at 1,200 m3 annually, of which 50 m3 is for domestic use and 1,150 m3 is for food production (Food and Agriculture Organization [FAO], 1994). In Western and industrialized countries, a renewable water supply of at least 2,000 m3 per person per year is necessary for adequate living standards (Bouwerg, 2000). These figures suggest that enough water is available for at least three times the present world population. Hence, water shortages are due to imbalances between population and precipitation distributions.

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