Infiltration increases when there is vegetative cover because vegetation slows the surface flow of water. This process allows the water additional time to enter the soil. The root systems of these plants also make the soil increasingly pervious. Foliage also shields the soil, thereby reducing the packing of the surface soil due to direct rainfall. Porosity, the ratio of the pore volume to the total volume of the soil, is one of the most important factors affecting infiltration.
Infiltration tends to increase with porosity.
The quantity of runoff from a storm relates basically to two conditions, the soil moisture conditions of a region prior to the storm, and the overall characteristics of the storm. The most profound of storm characteristics would be the duration and intensity of the rainfall, and the precipitation amount. The Sacramento Soil Moisture Accounting Model simulates runoff and places it into one of two response categories. Slow response runoff reaches the river channel in a much longer time period, from days to even months later. This runoff is received only from the pervious layers of the soil.