NONBENEFICIAL VEGETATION AND WATER RESOURCES

Abstract

Summary Revealing of the unused reserves of water resources and the fight against non-productive water losses assume ever greater importance with every passing day. Major kinds of water losses are evaporation and water effusion into saline water-bodies. The fight against non-productive water losses should be firstly directed to reduction of the non-productive evaporation, transpiration of weeds and water surface evaporation. The more intensive is the territory used for agriculture, the less is the specific portion of non-productive evaporation. The more intensive is the territory used for agriculture, the less is the specific portion of non-productive evaporation. Many areas of the USSR lose a great amount of water through non-productive evaporation. According to data of the Institute of Geography of the USSR Academy of Sciences this share of non-productive evaporation of the USSR territory comprises 58 per cent. The main volume of the non-productive evaporation falls on inconvenient soils, meadows and pastures covered with vegetation uneatable by cattle. A great portion of evaporation from inconvenient soils is taken by evaporation from cane under-growths and other pheatophytes growing in valleys and deltas of rivers in the southern and south-eastern part of the country. These plants are not suitable for cattle forage. Total area of cane undergrowths in these areas is about 4 million hectares. There are different opinions about the value of evaporation from cane undergrowths. Some explorers think that evaporation from these undergrowghs is less than that of the open water surfaces. Investigations of the rates Kara-Kum Canal crossing the sand desert for a lengthy distance showed that under such conditions the evaporation from the cane undergrowths could be five times higher than that from the water surface and comprise 9 000 mm per year. A study of the heat balance showed that the heat losses for evaporation greatly exceed the residual radiation and that the heat source for evaporation will be mostly the flow of the heated air masses from the surrounding desert. Assuming that in arid zones of the USSR evaporation from cane undergrowths and other pheatophyte plants in flood-lands and river deltas requires all the heat from the sun radiation, then the mean value of evaporation will be 2 200 mm per year and the total value of non-productive evaporation will comprise 180 km3 i.e. three times higher than the value withdrawn for irrigation. Reduction of non-productive evaporation by means of storing the flood-flow in reservoirs and by using the flood-lands and river deltas for agricultural crops will allow to additionally use the considerable reserves of water resources in arid areas of the country.