ABSTRACT
When a storm occurs, some of the rainfall inltrates into the ground and some of it evaporates. The rest ows as a thin sheet of water over the land surface, called overland ow. If there is a relatively impermeable stratum in the subsoil, the inltrating water moves laterally in the surface soil and joins the stream ow. This is called underow, subsurface ow, or interow (Figure 5.1). If there is no impervious layer in the subsoil, the inltrating water percolates into the ground as deep seepage and builds up the groundwater table (GWT or phreatic surface). The groundwater may also contribute to the stream ow if the GWT is higher than the water surface level of the stream. Low ow is the overland ow that reaches the stream channel rst and contributes to the stream ow. The interow is slower and reaches the stream channel after a few hours, and the groundwater ow is the slowest and reaches the stream channel some time after surface runoff. The term “direct runoff” is used to include the overland ow and the interow.
