EROSION OF LAND

Waikato Times, Volume 127, Issue 21199, 23 August 1940, Page 15 (Supplement)

EROSION OF LAND

Effect of Slope Degree.

AMERICAN EXPERIMENT. At a time when the subject of run-off and erosion is much under discussion in New Zealand, it is of interest to read of an American experiment conducted chiefly to endeavour to decide the effect of the degree of slope of the land on the run-off of the rainfall and on soil erosion. The experiment was conducted by Mr J. H. Neal, assistant professor of agricultural engineering at the University of Minnesota. The experiment was necessarily on a small scale. The "paddock” was contained in a wooden soil tank 12ft. long by 3.63 ft. wide, giving an area of one-thousandth part of an acre. In it was placed meadow soil l 6in. deep, under which was a.6in. stratum of sand and gravel to give good drainage. Means were provided to give the surface of the soil any desired slope, and sprinklers above simulated rainfall intensities of a few points per hour and upwards. There were also devices for catching and measuring the water run-off and the eroded soil. After a long series of tests, the following conclusions wera arrived at: Infiltration was not affected by either the slope or the rainfall intensity, but varied inversely as the initial soil moisture content. j Percentage of slope had no apparent effect on the percentage of run-off for slopes above 1 per cent. Percentage of run-off increased as the rain intensity increased, but at a decreasing rate. When the soil was dry before a rain, run-off did not occur until several minutes after the rain started. The time elapsing between the beginning of the rain and the time when run-off occurred, decreased as both the slope and the rain intensity increased. After run-off started there was a continual increase in the rate until the infiltration rate had become approximately constant. This occurred one to two hours after the beginning of the rain. Density of the run-off material decreased during the first hour of a rain. When the rain continued longer, the density remained approximately constant. From one and a half to two times as much run-off was required to remove a pound of soil at the end of one hour as at the beginning of the rain. Relative density of the run-off material increased as both the slope and the rainfall intensity increased. Soil Moisture Content The amount of erosion from a soil which was in a dry condition at the beginning of the rain was affected by the initial soil moisture content and the condition of the soil surface, in addition to the degree of slope, the rain intensity, and the duration of the rain. A soil in a dry pulverised condition, or one in a dry, rough condition, will absorb much more rainfall than one in a smooth, hard, baked condition. From the point of view of New Zealand arable land husbandry, the interesting points in these conclusions are those which show that the nature of the surface of the soil has an important bearing upon infiltration and the soaking of the subsoil. They bear out the teachings of our trained agricultural experts in relation to the necessity for keeping the top soil well mulched, and preferably in a cloudy condition, in order to get the best results in the conservation of moisture in the soil from the available rainfall. The problem of erosion, siltation and channelling arising from deforestation was, of course, not touched by this experiment.