Effect of soil wetting conditions on seal formation, runoff, and soil loss in arid and semiarid soils - a review

Soil surface sealing is one of the main causes for low infiltration rate (IR) and high runoff and soil loss under raindrop impact conditions in arid and semiarid regions. Many studies have focused on the effects of soil properties on seal formation under fast wetting conditions. However, in the field, soils can be exposed to different wetting conditions, before an intense rainfall event, which can affect the role of the soil properties on seal formation. The present paper reviews the effects of different initial wetting conditions and their interactions with soil properties on seal formation, IR, runoff, and soil loss in smectitic soils. Fast wetting of soil causes aggregate slaking, which enhances seal formation, runoff, and soil loss under rainfall, mainly in soils with >40% clay content. An increase in clay content of the soil increases aggregate strength, but at the same time increases the slaking forces. Hence, in soils with low clay content (<40%) and low aggregate stability, raindrop impact alone was sufficient to break down the aggregates and to develop a seal. In contrast, in soils with >40% clay content and high aggregate stability, slaking plays an important role in aggregate breakdown and seal formation. An increase of raindrop kinetic energy, from 8 to 15.9 kJ/m(3), decreased the effect of the slaking forces on seal formation and runoff. It was suggested that the effects of raindrop kinetic energy and of the slaking forces on aggregate disintegration and seal formation are complementary. An increase in soil exchangeable sodium percentage (ESP), from 0.9 to 20.4%, decreased the effect of slaking forces on seal formation and runoff production under rainfall with 15.9 kJ/m(3) kinetic energy. Probably, increasing the ESP increased the soil dispersivity, and therefore diminished the effect of the slaking forces on aggregate disintegration and seal formation. Aging (the time since wetting) of soil increased the stability of soil structure, decreased the seal formation, maintained high IR, and diminished soil loss amounts. These effects of soil aging depend on both the prewetting rate of the soil and soil texture.