Reducing the Error in Infiltration Determinations by means of Buffer Areas 1

NVESTIGATORS interested in the use, disposal, or storage of I rainwater have made many attempts to estimate field infiltration rates by means of artificial applications of water to small plots. Different types of equipment and methods have been employed for the purpose. The size of the areas to which water has been applied has varied from a few square inches to several hundred square feet. The methods of applying water have varied from irrigation by flooding with a constant small head over the entire surface to various methods of sprinkling, using a wide range of intensities and size of drops. Certain fundamental principles concerning the relative importance of various surface factors which may affect the rate of intake have been ascertained through the use of these small areas. The results of most infiltration tests as determined by the use of small areas indicate the capacity of the surface to transmit water under different conditions of soil or surface cover. This information has been of great value, but the infiltration rates obtained do not necessarily represent the absorption that would occur during rainfall. With small areas such as those where tubes have been driven into the ground, or even with small or narrow plots, there may be so much lateral seepage beneath and beyond the plot boundaries that the indicated intake is much higher than would occur over an entire watershed under rainfall conditions. Excavations across small plots after water has been applied for a few hours have shown that the wet soil tends to assume an irregular elongated, globular shape, and may have a mean horizontal cross section several times as large as the plot itself. Katchinsky ( I ) ~ attempted to calculate the true permeability of a soil from the extent of the wetted material below and beyond the plot boundaries. Several other investigators ( 2 , 3, 4, 5 ) have sought to avoid the error in infiltration rates due to lateral seepage by increasing the width or area of the plot or by using concentric areas or multiple square areas where only the inner rings or squares are used for determining intake rates. In some plot tests a larger area has been sprinkled than the plot on which the infiltration measurements have been made. Any of these methods may serve to reduce the variability between duplicate tests, but probably cannot eliminate the fundamental error due to subsurface lateral seepage from any small plot of whatever shape even though it may be several feet in its narrowest dimension. When water is applied to a small or narrow plot on land that is not saturated, the water descends into the soil as an expanding ‘