Irrigation with effluents: effects of prewetting rate and clay content on runoff and soil loss.

Stability of soil surface aggregates and irrigation water quality significantly affect runoff and soil erosion. Slow wetting of aggregates prevents slaking. We hypothesized that wetting rate will affect a soil's susceptibility to seal formation and soil erosion, and that this susceptibility will differ between effluent- and fresh water (FW)-irrigated soils. Effects of prewetting rate (2, 4, 8, and 64 mm h(-1)) on runoff and interrill erosion from five Israeli soils exposed to 60 mm of simulated rain were studied in the laboratory. Soils were taken from fields irrigated with FW or effluents for >15 yr. In general, for effluent- and FW-irrigated samples, runoff and soil loss were greatest for the soil with 22.5% clay; at lower or higher clay contents, less runoff and soil losses were noted. Runoff and soil loss decreased with decreasing prewetting rate (PWR) mainly in soils with clay content > or =38%. Total runoff and soil loss were higher in effluent-irrigated soils than in FW-irrigated ones in the loamy sand (8% clay) only. Greater soil losses occurred from effluent-irrigated soils exposed to fast PWR (64 m h(-1)) compared with FW-irrigated soils. In general, PWR had similar effects on total runoff and soil loss for effluent- and FW-irrigated soils. Use of wetting rates < or =8 mm h(-1) to prevent aggregate slaking decreased runoff and soil loss from loam and clayey soils exposed to simulated rainfall of high kinetic energy (15.9 kJ m(-3)). Long term irrigation with effluents in soils containing >20% clay did not seem to adversely affect soil susceptibility to runoff and soil loss in soils exposed to simulated rainfall, beyond that observed in FW-irrigated soils.