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Show streams. Such systems offer an outstanding op- portunity fo»r integrated use of water resources on a very large scale for a variety of purposes, such as domestic and industrial water supply, irrigated agriculture, mineral and industrial development, the generation of hydroelectric power, and navi- gation. Need for Integrated Program Reviewing these considerations and oppor- tunities, we begin to see in rough outline the im- mense possibilities of a thoroughly integrated water management program. These opportuni- ties will increase as we improve knowledge of geo- logical conditions, climate, and the habits of water. Development programs will utilize ground water as well as surface water reservoirs to provide carry-over storage to equalize supplies not only between wet seasons and dry seasons, but also between years of heavy precipitation and those of drought. Again to quote the Geological Survey:3 Though we face the future with confidence that, over the long term, the Nation will receive about the same precipitation as it has in the past, we must study the effects of observed climate fluctuations on our water s-upply and take them into account in planning waiter use for the future. It does the aver- age farmer-or the resident of New York City- no good when rain is insufficient this year to reflect that the 10-year average precipitation probably will be close to normal. He needs water this year, and information should be gathered to show how storage facilities or supplemental sources of water can be made available to bridge the dry years. It is proper here to note that evaporation and replenishment of soil moisture have the highest natural priorities in their demands on the avail- able water from precipitation. Only when these demands are satisfied or the maximum at which they can occur is temporarily exceeded by rain- fall or snow-melt is water available to run off over the land surface or recharge the under- ground rese rvoirs. This means that the fluctua- tions in runoff and recharge of ground water «Id., p. 29. reservoirs will be more extreme than the fluctua- tions in climatic conditions. Precipitation in excess of replenishment of soil moisture is divided between ground water re- charge and surface runoff into streams. The proportion between recharge and runoff will de- pend upon the relation of the rate at which water becomes available at the surface to the rate at which the soil and subsoil can transmit it down- ward to the ground water. Application of management to the combined surface and underground water resources of a region will therefore be guided by the physical characteristics of that region. In the humid areas, for instance, ground water recharge is greatest during the spring, and the amount of water flow stored reaches a maximum as spring gives way to summer. The rains may continue or even increase, but evaporation and consumption by vegetation increase even more rapidly and soon dispose of all or nearly all of the rainfall. Meanwhile, because ground water continues to drain out during the summer, when not much recharge is occurring, the amount of stored water is diminished and the water table declines. By fall, it is generally at a low stage and, although a slight increase may occur as evaporation and use of water by vegetation declines, winter brings a new downward trend to a low in early spring. The recharge in the spring months may, how- ever, vary greatly from year to year because, if the melting of snow takes place too quickly, much surface runoff may occur before the soil is completely thawed and capable of transmitting water downward. Under such conditions dis- astrous floods may result. Heavy rains occurring at the same time may add to the floods, as in the Northeast in 1936. Meanwhile, the replenish- ment of ground water may to that extent be reduced. Studies might well be undertaken to deter- mine the possibility of stabilizing the recharge of ground waters from year to year through sur- face and ground water management programs that provide a safe yield. Surface reservoirs would be constructed to conserve the excessive runoff in years of quick melt. Careful geological studies would determine the possibility, through 118 |
