ABSTRACT
Part of the amount of water that reaches the ground as precipitation is held by the canopy of plants covering the ground. The amount of water intercepted in this way is not constant but is dependent on the plant type and the percentage of cover in the area and the characteristics of the precipitation. Some amount of water is retained by soil irregularities; part of this quantity goes back into the atmosphere through the process of evaporation and transpiration. The remaining moves on the ground or percolates into the soil. One part of the infiltrated water moves laterally, immediately beneath the surface, and reappears downstream on the surface or stream banks (interflow). The remaining moves into deeper layers and enriches the aquifers, becoming part of groundwater; the groundwater, again moving sideways, can reach the bed of a stream or even move outside the boundaries of the basin. Above-ground runoff starts almost immediately when precipitation intensity exceeds infiltration and surface detention capacities or is delayed and starts in accordance with the upper soil layer saturation level provided the precipitation continues unabated. The water moving above and below the ground surface following any of these paths forms the runoff. In particular, the part of the water moving on the surface makes the surface runoff, which combines with interflow to form the direct runoff. The water moving underground as groundwater also flows to stream beds, forming the main runoff or baseflow.
