Battling Drought: The Science of Water Management

Generally about three-fourths of the applied water is lost to evapotranspiration. The rest, which contains all the originally dissolved salts (except for the tiny amounts incorporated in the crop itself), percolates downward and laterally through the soil. It may enter an underground aquifer or it may reappear somewhere downstream as seepage into a

most parts of the world, rainfall and snow accumulation—and runoff to rivers—peak during the nongrowing seasons of the year, when demand for irrigation water is lowest.

Industrial water requirements for washing, cooling and the circulation of materials range from one to two tons per ton of product in the manufacture of brick to 250 tons per ton of paper and 600 tons per ton of nitrate fertilizer.

The amount of water that must be applied annually to irrigated land is equivalent to between one foot and five feet of water covering the area cultivated, depending on the crop and the climate.

From 60 to 80 percent of the industrial demand is for cooling water, mainly in electricpower generation.

Egyptian records show an average of one plague every 11 years.

billion on the ocean and 80 billion on the land. Over the ocean 9 per cent more water evaporates

by an equal excess of precipitation over evaporation on land; consequently the volume of water carried to the sea by glaciers, rivers and coastal springs is close to 27 billion acre-feet per year. About 13 billion acre-feet is carried by 68 major river systems from a drainage area

In humid regions the water table may have to be lowered in order to provide aerated soil around plant roots and to increase the firmness of the soil for tillage and other farm operations. For this purpose in such regions a network of ditches or tile drains is normally laid out at a depth of three to five feet below the surface.