RAINDROP ENERGY EFFECTS ON CHEMICAL AND SEDIMENT TRANSPORT

Abstract

In this study, effects of raindrop kinetic energy on the transport of soluble chemicals and sediment to runoff and infiltration into a silt loam claypan soil were quantified. The chemical and sediment transfer processes were investigated in saturated soil beds for two soil surface conditions (bare and 80% covered), and three rainfall rates (2.8, 5.0, and 9.1 cm/h) using a stationary rainfall simulator. The rainfall simulator produced terminal velocities and droplet size distributions similar to natural rainfall at 5.1 cm/h. The bromide transport in surface runoff was best described by a power decay model with the decay coefficients related to rainfall intensity. Bromide concentrations in runoff were higher for bare soil surfaces at all intensity rates than for the surfaces covered with residue. The vertical transport of bromide within the soil matrix was strongly dependent upon the raindrop impact. The amount of bromide lost to surface runoff and retained in the soil profile was related to droplet energy flux (DE) as a function of rainfall intensity and initial surface conditions. At calculated DE levels of 0.199, 0.355, and 0.645 W/m 2 corresponding to rainfall rates of 2.8, 5.0, and 9.1 cm/h, respectively, surface residue reduced the total surface losses of Br by 74%, 40%, and 34%, respectively. Sediment masses in runoff were reduced by 99.8%, 91.5%, and 90.4%, respectively, with 80% surface cover for these three respective rainfall rates.