Effects of vegetation on soil slippage by pore pressure modification

Influences of vegetation on shallow (< 1 m) soil slip formation through modification of soil water was investigated on hillsides covered by verdant chaparral (dense shrubland), and burned vegetation in the Transverse Ranges of California. Per cent available water and hydraulic potentials were obtained from electrical resistance blocks and tensiometers for one year in soils under burned and unburned vegetation on three slopes. Soil remained moister during a dry period under burned vegetation than under unburned chaparral on two of the three slopes studied. Daily increases in per cent available water and hydraulic potential of soils were greatest for a given storm where soil was driest prior to the storm. Furthermore, water levels in soil tended to be greatest for a given storm where soil water levels had been lowest prior to the storm. These two findings were corroborated by laboratory wetting trials on undisturbed soils of vastly differing mechanical properties in that initially drier soils always absorbed water faster and became wetter than initially moister soils. In the field, soil water levels became similarly high under all vegetation after several storms and varied little throughout the remainder of the wet season. These results contradict the common assumption that depletion of soil water by vegetation would result in slower saturation rates and hence greater resistance of a soil mass to slippage.