Splashability Models of Some Tropical Soils

Abstract

Falling raindrops are the major agents responsible for initiating soil erosion. They cause soil detachment from its original position. The collision between dry soil particles and raindrop is essentially a modelled collision between two elastic bodies. Water, of course, has a high volumetric surface tension. It is precisely this tension which forces the falling drop of water to take on ·a spherical shape which offers strong resistance to deformation. Water from a raindrop acts both as an energy source and wetting agent. The energy is transmitted to the aggregate or soil clod. The detached particles are transported by two mechanisms; displacement caused by physical impact and particle entrainment caused by overland flow. Rain splash has now been recognized as an important erosion agent. Rain splash erosion information from aggregated tropical soils is under-represented in literature. The splashability of twelve different soils from the Cape Coast area was investigated The soils are associated with a variety of vegetation types under different cultivated crops and different parent materials. Undisturbed core samples of the soils were taken carefully using deep steel cylinders. Using artificial rainfall characteristics and soil properties as input we simulated raindrop-induced soil splash rates for these soils. Five of the soil samples used in the study were classified as sandy clay loam, four as sandy loam and the remaining three samples, as sandy clay, loamy sand and clay respectively. The total elemental analysis indicated that the major element in the soils was iron. A linear plot was used to establish a relationship between soil splashed and rainfall intensity. The relationship was observed to be approximately linear with highly significant correlation. Aggregate stability and organic content did not correlate with soil splashed, implying that these indices are not reliable for assessing the splashability of the soils in this area. Mechanical ratio, iron content, and kinetic energy of rainfall strongly correlated with soil splashed. The main physical aspects of soil splash caused by raindrops were modelled. Two physically based models were proposed. The models required input parameter of soil detachment due to the action and interaction of raindrop. The models relate the splashability of soils to specific hydraulic and mechanical properties of the initially undisturbed soil as well as the physical characteristics of the rainfall applied, under given initial and boundary conditions defining the flow system. The dynamics of the splashability of the soil at the surface are found to be related to the following variables: the rainfall intensity, maximal drop diameter, initial shear strength and water content. The results show that the proposed models address the main factors affecting soil splashability. It also shows that incorporating intensity effect and kinetic energy of rainfall into the basic soil erosion model can represent raindrop-induced soil detachment processes.