Where runoff water is used for livestock and other farm purposes, it is usually stored in ponds or reservoirs. We estimate that in 1943 there were more than a million farm and ranch ponds in the United States. A great deal of work should be done to improve those existing ponds, and many more are needed. Reports of the postwar planning boards of 12 States called for a total of more than 250,000 new ponds and. reservoirs. The need for farm ponds and reservoirs is by no means limited to the and and semiarid parts of the country; the postwar planning board of Georgia, for example, estimates that 34,000 are needed in that State.

In planning a farm pond, we need to assure ourselves that the water supply will be sufficient for the intended purpose after evaporation and seepage take their toll. We cannot do much to control evaporation, and very seldom can we entirely eliminate seepage. To estimate properly how much usable water we can expect to obtain from the pond, we must set up a sort of bookkeeping, based on records of past rainfall, runoff, and evaporation, and on characteristics of the proposed pond site. Our ledger must show, for any given period, how much water would flow into the proposed pond and how much rain and snow would fall on it, how much water would go over the spillway, how much would evaporate from the pond surface, and how much would be lost by seepage through the bottom and through the dam or dyke. If the balance is too small for the intended purpose, and we cannot make up the deficit by building a higher dam, we shall need to increase either the drainage area or the quantity of runoff. One way to increase the drainage area is to locate the pond farther downstream; another is to divert water from an adjoining drainage basin by means of terraces or ditches. The amount of runoff can sometimes be increased by putting a larger portion of the drainage area in cultivated crops. Care must be taken to protect the pond from silting. This is done by following conservation practices on the cultivated land within the drainage area and by providing a protective strip of grass around the pond.

The bookkeeping suggested cannot safely be done on the basis of average conditions. Rainfall, runoff, and evaporation at any place vary greatly from time to time; and when rainfall and runoff are least, evaporation and the quantity of water needed for farm purposes may be greatest. Obviously, the balance may be favorable in 1 year and unfavorable in another. How often the balance must meet farm needs if the pond is to be worth what it cost depends on how badly the water is needed, whether the pond will be the only source of water on the farm, the losses that would result from lack of water, and the cost of providing water from other sources. With these practical considerations in mind, the farmer can decide whether the proposed pond must provide this balance in 4 out of 5, 9 out of 10, 14 out of 15, or 24 out of 25 years.

Records of runoff and of evaporation are still meager and far too short. However, with the understanding of surface runoff and of related factors gained through research, these limited records are being put to good use.

Technical reports giving information needed in planning farm ponds in the claypan prairies of Missouri, Oklahoma, Iowa, Kansas, Illinois, and Indiana have been issued by the Soil Conservation Service in cooperation with State agricultural experiment stations.

Because the information we now have to guide us in controlling and utilizing runoff water is so limited, we have to play safe and use structures and practices that often turn out to be more expensive than necessary. However, if adequate records of surface runoff and information on evaporation and seepage are obtained, in all parts of the country, we can look forward to managing runoff better and more cheaply as time goes on.

THE AUTHOR

D. B. Krimgold, a soil conservationist, joined the Soil Conservation Service shortly after its establishment. He helped set up the Hydrologic Division and with C. E. Ramsey wrote the working plan for the experimental watersheds. He directed the selection of the watersheds in Ohio, Texas, and Nebraska, and the technical phases of the runoff studies in some 22 States.