Surface Hydrology: V—In-Surface Transient Flow

Numerical computation of nonsteady flow in the transition layer at the soil surface was carried out with various geometric dimensions and rain regimes. The main conclusions are: (1)Lateral flow in the transition layer is proportional to the rain and the slope; (2)moisture concentration increases with the concavity of the soil surface and continues long after the rain stops occurring mainly in unsaturated soil; (3)saturation initially occurs at the most concave point within the transition layer; and (4)the saturation zone may spread to the soil surface and cause seepage runoff and erosion. Both saturation and seepage occur after a certain amount of rain has fallen and are little dependent on its intensity. At very low rates of rainfall, no seepage will occur if the transition layer is sufficiently thin. Gullies and rills are not merely the result of seepage at concave spot—they themselves produce more and earlier seepage runoff and erosion; seepage of rain and runoff from concave spots will occur when the amount of rain is either greater or less than the infiltration capacity.