Modeling Soil Solute Release into Runoff and Transport with Runoff on a Loess Slope

Abstract

Rainfall results in the transfer of chemicals from soil to surface runoff. A physically-based solute transport model was developed for estimating the solute concentration in runoff originating from the soil surface. The model accounts for the effects of soil infiltration, raindrops, the water runoff rate, and the return flow, all of which influence the concentration of the solutes in the runoff. It was assumed that the depth of mixing zone changed with the varieties of the raindrop hits, return flow, and overland flow. It was also assumed that runoff and soil in the mixing zone mixed instantaneously and that the solute in the soil beneath the mixing zone was moved to the mixing zone by diffusion. The mixing zone was included in the model and was based on the deposited layer or shield concept. To test the model, laboratory experiments were carried out that used two soil types that were exposed to simulated rainfall. The results simulated by the model were highly correlated with the experimental data. In the first few minutes after rainfall began, the solute concentration in the runoff was mainly controlled by the rainfall rate and solute concentration in the mixing zone; higher solute levels in the mixing zone resulted in higher solute concentrations in runoff. When the solute concentration in the runoff stabilized, the solute concentration in the runoff was mainly controlled by the diffusion of solutes from the soil beneath the mixing zone. The simulated data showed a high level of correlation with the measured data for both runoff volume and solute concentration in the runoff. This demonstrates that the model captured the temporal behavior of the runoff and solute transport in the runoff.