Role of Wetting Rate and Rain Energy in Seal Formation and Erosion 1

Seal formation, runoff, and erosion are controlled by soil texture, rain kinetic energy (KE), and wetting rate (WR) of soils aggregates. The objective of this study was to determine the relative importance of WR and rain KE in seal formation, runoff production, and erosion, in soils varying in their clay and silt content, using a drip-type rain simulator. Four soils, ranging in clay content from 22.5 to 61.2%, were packed in 0.2×0.4-m trays, pre-wetted at WRs of 2, 8, or 64 mm h(-1), and exposed to 60 mm of simulated distilled water (DW) rainfall with two KEs (8 and 15.9 kJ m(-3)). Runoff and erosion increased as rain KE and WR increased; however, the magnitude of change depended on clay content. In the loam (22.5% clay), the effect of rain KE on seal formation and runoff was significant, and the effect of WR was small. Conversely, in the clay soils (51.3 and 61.2% clay), the effect of WR on seal formation was significant and the effect of rain KE was negligible. In the sandy clay soil (38.1% clay), the effects of both WR and rain KE on seal formation and runoff were significant. The effect of rain energy on soil erosion was significant in all soils. Since water erosion is the product of sediment detachment and transport of the particles by overland flow, increase in the impact of rain drops increases both soil detachment and transport capacity and increases soil erosion in all soils. In clay soils, erosion also increased with the increase in WR. Disintegration of the aggregates by fast WR combined with detachment by rain impact increased erosion from clay soils. The conclusion reached is that for seal formation and runoff production, rain KE predominates in medium- and light-textured soils and WR predominates in heavy-textured soils. Conversely, for soil erosion from laboratory trays, detachment by rain KE is essential in all soils.