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drought,abnormally long period of insufficient rainfall. Drought cannot be defined in terms of inches of rainfall or number of days without rain, since it is determined by such variable factors as the distribution in time and area of precipitation during and before the dry period. Since ancient times droughts have had far-reaching effects on humankind by causing the failure of crops, decreasing natural vegetation, and depleting water supplies. Livestock and wildlife, as well as humans, die of thirst and famine; large land areas often suffer damage from dust storms or fire. Drought is thought by some to have caused migrations of early humans. In India and China drought has periodically brought widespread privation and death. In 1930 lack of rainfall devastated the Great Plains of the United States; called the Dust BowlDust Bowl,
the name given to areas of the U.S. prairie states that suffered ecological devastation in the 1930s and then to a lesser extent in the mid-1950s. The problem began during World War I, when the high price of wheat and the needs of Allied troops encouraged farmers to
..... Click the link for more information. , its area spread to alarming dimensions (about 50 million acres). During 1962 much of the eastern part of the United States experienced the worst drought in more than 50 years. Some two thirds of the United States experienced drought that combined with some of warmest temperatures on record in the summer of 2012, and in 2012–16 below normal rainfall and warmer temperatures result in severe drought at times in California. Since the 1960s severe, sometimes recurring droughts have afflicted countries in many parts of Africa. Clearcutting of trees for firewood, overgrazing, and overcultivation, which lead to land degredation, contribute to this drought cycle.
See C. S. Russell et al., Drought and Water Supply (1970); W. C. Palmer and L. M. Denny, Drought Bibliography (1971); R. V. Garcia and J. Escudero, Drought and Man (1986).
a prolonged and significant shortage of rainfall, often accompanied by increased temperature and reduced moisture-content of the air, which causes the deposits of moisture stored in the soil to dry up, thereby leading to a reduction of crop yield or loss of harvest. The beginning of a drought is usually associated with the formation of an anticyclone. Abundant solar heat and dryness of the air bring about an increase in the rate of evaporation (atmospheric drought), and stored soil moisture is exhausted when it is not replenished by additional rain (soil drought). During a drought, water intake through the root systems of plants becomes difficult, the rate of transpiration begins to exceed moisture inflow from the soil, water saturation of tissues declines, and the normal conditions of photosynthesis and carbon nutrition are disrupted.
Droughts may occur during the spring, summer, and autumn. Spring droughts are especially hazardous for early grain crops; summer droughts cause heavy damage to both early and late grain crops and other annual crops, including fruit plants; and autumn droughts are a hazard for the sprouts of winter crops. Spring-summer and summer-autumn droughts are the most destructive dry periods. Drought occurs mostly in the steppe zone, less often in the forest-steppe zone; two or three times in a century, a drought may occur even in the forest zone. The concept of drought is inapplicable to regions with rainless summers and extremely small amounts of precipitation, where farming is possible only with irrigation, as in the Sahara and Gobi deserts.
In the USSR, a dry climate is typical for the southwestern and central parts of Kazakhstan, the Middle Asian republics (with the exception of the high mountain regions), and the southeastern part of the European USSR. It is difficult to establish the boundaries of the regions of periodic drought in the USSR since there are almost no localities in which drought has not occurred. The worst drought years were 1891, 1911, 1921, 1931, 1936, 1946, 1954, 1957, 1967, and 1971. Approximately every three years, the USSR loses up to 1.5 billion poods (24,584,958.7 tons) of grain because of drought. Drought most often strikes the lands along the Middle and Lower Volga regions and the basin of the Ural River. Only isolated indicators are available for forecasting the probability of drought. For example, if the autumn moisture deposit in a layer of soil one meter deep is less than 50 per-cent of the average over several years, a future deficiency of soil moisture may be expected. If the height of the snow cover and the moisture stored in it amount to no more than half of the average for several years, there is a strong probability of a forthcoming spring drought.
Drought control includes the use of agrotechnical and reclamation measures designed to increase the water-absorbing and water-retaining properties of soil and the retention of snow in fields. The most efficient agrotechnical measure is basic deep tillage, particularly of soils with a highly compacted soil horizon, such as chestnut and solonets soils. Soil tillage with flat-cutting implements, leaving the stubble on the surface of the field, is suitable in northern Kazakhstan and in the steppe regions of Western Siberia. Special soil tillage methods regulating surface discharge must be used for soils on slopes: plowing transversely to the slope, contour plowing (along contour lines), and altering the microrelief of the surface of the plowed land (by making hollows, small estuaries, and discontinuous furrows). In order to reduce moisture evaporation, the soil of fallow and of land where crops have been sown in wide rows must be kept loose so that a soil crust does not form. Harrowing, smoothing, cultivation, and interrow cultivation are used for this purpose. Also of great importance are the extermination of weeds, spring snowmelt control, the application of fertilizers, the presowing preparation of soil and the planting of crops in the shortest possible time. An efficient method is to combine the sowing of winter crops, which make good use of autumn rainfall and are resistant to spring-summer drought, with the sowing of early summer grain crops, which require rainfall during the first half of the summer, as well as with sowings of corn, millet, sorghum, and other late crops that need rainfall in the second half of the summer and can withstand spring drought with relative ease. The introduction of drought-resistant plant strains plays an important role in arid regions. Another agrotechnical drought-control measure is the adoption of proper crop rotation on clean fallow in arid regions and on occupied fallow in regions with more precipitation. Clean fallow land (with windbreak strips) in arid regions may be equated with fields having water-supply irrigation.
Important reclamation measures for controlling drought include field-protecting forestation and the conservation and expansion of water-protective forest areas.
Since the early days of the Soviet state, the party and the government have carried out important measures for increasing agricultural production and land reclamation. After the severe drought of 1921, V. I. Lenin signed the special decree On Drought Control, which called for giving special care to forests important for field protection and water conservation, the stabilization of sands and ravines, and the establishment of snow-collecting belts and fences. Many provisions on drought control were subsequently made at the Seventeenth Party Congress in 1934, at the February plenum of the Central Committee of the ACP (Bolshevik) in 1947, and in the Directives of the Nineteenth Party Congress on the 1951–55 Five-Year Plan. The resolutions of the March 1965 and May 1966 plenums of the Central Committee of the CPSU and those of the Twenty-Fourth Congress of the CPSU were of particular importance for the implementation of drought-control measures, the strengthening of the material and technical resources of agriculture, increasing the overall efficiency of farming, and the development of reclamation work in order to obtain high and stable yields of agricultural crops.
REFERENCESTimiriazev, K. A. “Bor’ba rasteniia s zasukhoi.” Izbr. soch., vol. 2. Moscow, 1948.
Dokuchaev, V. V. “Nashi stepi prezhde i teper’.” Izbr. trudy. Moscow, 1949.
Izmail’skii, A. A. “Kak vysokhla nasha step’.”Iz&r. soch. Moscow, 1949.
Zasukhi v SSSR, ikh proiskhozhdenie, povtoriaemost’ i vliianie na urozhai. Edited by A. I. Rudenko. Leningrad, 1958. (Collection of material.)
Bor’ba za vlagu—bor’ba za urozhai. Edited by P. F. Kotov. Voronezh, 1969. (Collection of material.)
Preobrazovanie prirody v Kamennoi Stepi. Moscow, 1970. (Collection of material.)
I. A. SKACHKOV