Water

Ground Water.

It has been estimated that 5o% of the pop ulation of the U.S. depends directly upon wells for its water supply and the proportion for the world is no doubt nearly or quite as great. The practicable use of this water is measured by the extent to which it may be replaced by precipitation. The water table may be progressively lowered by excessive pumping. In some places the rate of recharge of the underground water from surface streams may be increased to offset the lowering of the water table, but excessive drafts on the supply inevitably reduce it.

Water-power.

The developed water-power of the world, ac cording to estimates made by the U.S. Geological Survey, was about 23,000,00o h.p. in 192o, 46,000,000 h.p. in 1930 and 63,500, 00o h.p. at the end of 1938, an increase of 18o% in 18 years.

Summary of Developed Water-power of the World in 1938, and Potential Water-power at Ordinary Low Water in Horse-power The estimates of potential water-power are based on data of widely varying completeness and accuracy but they afford a gen eral idea of the water-power resources of the world. It appears that Africa's potential water-power is nearly equal to that of any other three continents. Africa is essentially a great plateau on which there are large rivers that fall rather abruptly to the sea. Tropical Africa, particularly the Congo, contains a large land area within the belt of equatorial rains, and this fact taken in connection with its characteristic topography gives it the great est concentration of potential water-power on the globe. Asia comprises vast areas of high mountains and plateaus but in its northern, western and central parts the precipitation is low and in its southern and eastern parts the rainfall is irregularly dis tributed through the year.

Irrigation.

Nearly one-third of the earth's surface receives only io in. of rain or less annually and for another third the annual rainfall is between Io and 20 inches. On the area receiving less than Io in. and on much of that receiving up to 20 in., irri

gation is generally essential to the production of a profitable crop. The earliest civilization of which we have any knowledge was de veloped by means of irrigated agriculture. Not until near the Christian era did the world dominion and leadership in art and culture pass to nations inhabiting other than great irrigated low lands. During the dark ages, irrigation systems were neglected and some were entirely destroyed. With the advent of the Renais sance and the gradual growth of modern civilization, the centres of population gradually shifted to more humid regions and ir rigation became relatively less important in the growing of the world food supply.

After 1875 new influences in world affairs gave a decided im petus to irrigation development. Railroad construction has opened up vast areas of arid and semi-arid lands requiring irrigation to bring them to a high degree of crop production. Construction equipment and methods have been greatly improved. Projects are now feasible which, a half century ago, would have been entirely impracticable. Dams are projected and under construction of a height and size undreamed of a few decades ago. Improved standards of living have increased the demand for certain prod ucts of irrigated lands such as citrus and deciduous fruits, and increased transportation facilities have made possible their wide spread distribution. As a result, the irrigated areas in the newly developed countries of the Western Hemisphere have increased rapidly, and in 1940 it was probable that one-fourth of the globe is dependent for food on the products of irrigated land.

The irrigated area of the world by countries in 1926 was estimated by the U.S. Bureau of Reclamation as practically 200, 000,000 ac., made up by continents as follows: North America, 26,834,00o ac. ; South America, 6,613,00o ac. ; Europe, 14,798,000 ac. ; Asia, 140,760,000 ac. ; Africa I o,3io,000 ac. ; Oceania, 1,2 70, 00o ac.; grand total 200,585,000 acres. (N. C. G.)