RAINFALL ENERGY EFFECTS ON RUNOFF AND INTERRILL EROSION IN EFFLUENT IRRIGATED SOILS

Abstract

Seal formation at soil surfaces is significantly affected by raindrop kinetic energy (KE). Soil sealing enhances runoff and soil erosion. It has been hypothesized that in soils exposed to rain (i) runoff and interrill erosion levels in soils previously irrigated with effluents differ from those in soils previously irrigated with fresh water (FW), and (ii) that the difference in runoff and interrill erosion between the previously irrigated effluent and FW samples depends on rain KE. The effects offour droplet KE levels (3.6, 8.0, 12.4, and 15.9 kJ m -3 ) of distilled water (DW) on runoff and wash erosion of four smectitic soils that had been irrigated for many years with fresh water or effluents was studied with a drip-type rain simulator. The soils used were a loamy loess (Calcic Haploxeralf) from Be'er Sheva Valley, a dark brown sandy clay vertisol (Chromic Haploxerert) from the Pleshet Plains (vertisol HH), and two dark brown heavy clay vertisols (Chromic Haploxerert) from Zevulun Valley (vertisol Y) and Western Galilee (vertisol E), Israel. For all soils, total runoff and wash erosion values increased with the increase in rain KE. Total runoff levels were lower for the FW-irrigated samples than for the effluent-irrigated soils, except for the highest KE where no differences in runoff between the two types of soils was noted. Conversely, for each soil and each rain KE, wash erosion was higher in the effluent-irrigated samples than in the FW-irrigated ones. Total runoff was affected mainly by changes in rain KE at the lower to medium range of the KE studied. Thus, the effects of irrigation water quality (i.e., effluents vs. FW) on total runoff was pronounced mainly at the low-to-moderate rain KE range; at high rain KE, the effects of irrigation water quality on runoff were negligible. Wash erosion was affected mainly by changes in rain KE at the medium-to-high KE range studied. That wash erosion increased markedly at the high rain KE values, despite the negligible change in runoff level, suggested that for KE > 12.4 kJ m -3 , seal formation was completed, and the contribution of runoff in facilitating transport for the entrained material was secondary to the role of soil detachment in determining wash erosion. The dependence of wash erosion (through soil detachment) on rain KE was greater than its dependence on the quality of the irrigation water that the soil was exposed to in the past (FW vs. effluent).