Surface Hydrology: III°Causes of Lateral Flow

Streamlines bend after they enter a nonuniform, sloping soil. On penetrating to a more permeable soil layer they bend downstream. This can occur in unsaturated soil above a sloping water table or in a layer in which water accumulates because of the presence of less permeable underlying layer. We considered the general case of a nonuniform soil with rotational symmetry at an angle to the vertical, i.e., a sloping soil. General proof is provided that the soil generally behaves as a anisotropic medium so that infiltration also invloves a horizontal net flow component. This net flow component is proportional to the slope, the vertical flow, and the coefficient of anisotropy. The existence of such a mechanism can account for the concentration of water in concave spots, possible seepage, and runoff and erosion phenomena at low rates of rainfall. It can also explain how a previous rainfall leads to earlier onset of runoff during the next rain, etc.