A Study of Factors Affecting Infiltration

Infiltration of water into soil is related to irrigation methods, to erosion control, and to water-spreading. In recent studies of infiltration of water into soils of Northwestern United States, attention has been given to the various factors involved. Major use has been made of the ring or cylinder method of study. Lysimeter studies and measurements of losses from small erosion plats and irrigation basins have been found useful. At this time an attempt will be made to group and chart the factors previously reported in an orderly concise way and then to discuss briefly some factors encountered and studied which do not seem to have been previously reported or investigated. As is true in any rapidly expanding field of study there is some confusion as to causes and effects and their relative sequence. Rate of entrance of water at the soil surface or infiltration should be distinguished from ultimate rate of movement through a wet soil mass, which may be designated as percolation. In the following chart it has seemed logical to make the major distinction between, (a) those factors influencing the infiltration capacity at a given time and point and (b) those influencing the average infiltration capacity over a considerable area and period of time. The present discussion will be devoted chiefly to the first group. The flow of any fluid is 'governed by three elements. These are (l) the crosssectional area and character of the channel through ¥/hich the flow takes place; (g) the energy gradient under which flow takes place, and, (3) the viscosity of the fluid. Translated into terms of soil and water, these three elements are the soil pores, the hydraulic gradient, and the viscosity of water, and these are influenced by many factors, some of which have been studied in considerable detail and others only indirectly. The numbers under the different factors in the outline indicate the items in the list of references in which studies of those factors have been reported. The chart was proposed by M. R. Lewis. It would be difficult to list factors affecting rate of infiltration in the order of their importance. Almost any one factor may be of controlling importance in a given case. The list of factors is incomplete. Aside from conditions at the soil surface, the soil depth, substrata, and under-drainage conditions become increasingly important with duration of tests. Some factors which have been recognized at the Oregon Agricultural Experiment Station as of importance in their effect on infiltration have been studied using six-inch rings and results will now be briefly reviewed; Table 1: Depth of penetration of water was noted after tests by taking soil moisture samples.