Managing Surface Runoff

by D. B. KRIMGOLD

RAIN OR SNOW WATER that runs off the surface of farm land instead of sinking into the soil may serve a useful purpose if it is properly managed, but it may cause great damage if it is not controlled. Many farmers and ranchers in the United States have no springs, no wells, and no streams on their land, and must depend on impounded runoff water for livestock and other purposes at all times. On the other hand, water flowing over unprotected land loosens and carries away topsoil; further damage results when this soil is deposited in stream channels and reservoirs and on valley floors and when fertile bottom land is flooded.

Our studies have shown that more of the rainfall runs off where soils are tight, shallow, and wet, vegetation is poor and thin, and rainfall is heavy and intense; and that the larger the amount of runoff water and the faster it moves, the greater is its cutting power and the greater is the load of sediment and debris it can carry. The speed with which surface water runs off a small agricultural area depends on its amount, the slope along which it flows, whether its course is straight, the size and shape of the drainage area, the number of watercourses, and whether its path is obstructed.

We have learned to reduce the amount of surface runoff by rotating crops and protecting the soil with trees and grasses and with crop residues, and by practicing contour cultivation, strip cropping, terracing, pasture furrowing, and basin listing. To limit the speed of flow and protect the soil against erosion, we use drainage terraces, diversion ditches, spillways, gullet' plugs, culverts, and channels lined with vegetation and with 'masonry. The size and cost of such structures, the space they require, and the extent to which they interfere with farming operations vary according to the quantities of water expected.

To control and make use of surface runoff from agricultural areas we must find out how much water flows off, in terms of gallons, acre-feet, or cubic feet, and how fast it flows, in terms of gallons per minute or cubic feet per second. A great part of this task has been to develop suitable instruments and procedures. Through laboratory experiments, weirs and flumes have now been developed with which we can accurately measure flows ranging from less than one-half gallon a minute to 800,000 gallons a minute. These devices can be used for measuring all flows except those carrying exceptionally heavy loads of debris.

Rates of runoff and total amount of water running off an area within a given time are calculated from the depth of water flowing over a weir or through a flume. Rate of surface runoff from a small area changes rapidly and irregularly. Therefore, to determine amounts of surface runoff from such an area we must have a record of the depth of flow for each 10-, 5-, 2-, or even 1-minute interval. A new type of water level recorder has been devised that gives a continuous record of the depth of flow to the nearest 0.01 foot for every minute. In this recorder a chart is mounted on a cylinder, which is rotated by a fast moving clockwork. A pen moved by a float rests on the chart with the result that a continuous record of depth of flow appears on the chart.

Since the development of adequate devices for measuring runoff from small areas, studies of runoff have been undertaken in some of the major agricultural areas of the United States, on small drainage basins of various sizes that are typical as to soils, vegetation, and other factors. Altogether, more than 100 experimental watersheds have been used.

The runoff from small areas is extremely variable. A question therefore arises as to what sorts of flow should be provided for in designing control structures. When we build terraces, diversion ditches, and other structures for control of runoff, should we make them large enough to carry the greatest flows expected at any time? or should we make them only large enough to carry flows that, on an average, are expected once in 10, in 15, in 25, or in 50 years? On a small agricultural area, the farmer must ask himself which would in the long run be more economical a smaller structure that might overtop or even fail at long W intervals, or a larger one that might cost a good deal more, might occupy more land, and might interfere more with farming operations. Soil conservation structures on the farm are usually made large enough to carry flows expected once in 10 or 25 years. Occasionally, one is made large enough for the flow expected once in 50 years.

Early results of the runoff studies already mentioned showed that rainfall of a certain intensity on a certain area does not always result in the same rate of runoff; that, for instance, the most intense rainfall in 15 years on a certain area does not necessarily produce the heaviest runoff in 15 years from that area. Accordingly, in designing measures for control of runoff we use runoff records rather than rainfall records. Any reliable estimate as to how heavy a runoff should be expected from a certain area once in 10 or 25 years, for example, must be based on records of runoff over a long period. The longer the record, the more reliably can such things be estimated.