A COMPREHENSIVE MATHEMATICAL MODEL FOR TRANSPORT OF SOIL-DISSOLVED CHEMICALS BY OVERLAND FLOW

Abstract

The model developed in this study simulates the contamination of overland flow by soil chemicals that reside near its surface during a surface runoff event. The model includes mass-balance equations for both water flow and chemical transport in the soil profile and surface runoff. A rate-limited mass transfer through an overland-flow boundary layer at the soil overland flow interface controls the dissolved chemical transfer from soil solution to overland flow, once formed. The model predicts water flow and chemical transport in the soil profile prior to the rainfall ponding (when overland flow starts) and during the surface runoff event. The predictions of these variables, together with the total load to the surface runoff, were successfully compared with the measured data of Hubbard et al. [Trans. ASAE, 32(4) (1989) 1239]. Being physically based, the model was used to investigate the dependence of surface runoff pollution and its extent on the system hydrological parameters. A key factor on the availability of soil chemicals to pollute the overland flow is their displacement by infiltrating water prior to runoff initiation. Being dependent on soil moisture prior to rainfall initiation and on rainfall intensity, a lower chemical concentration and a lower load in surface runoff are obtained for longer ponding times, ones that are associated with lower rainfall rates and initially drier soil profiles. During the surface runoff flow, the chemical concentration in overland flow at the slope outlet is affected by the contact time of an overland flow parcel with the soil surface. Thus, it increases for higher values of equilibrium time - tE, lower rainfall rates, slope gradients, and higher soil-surface roughness coefficients. These parameters have an inverse effect on the surface runoff concentration by affecting the transfer coefficient of soil chemical to overland flow. A different insight into the relationship between the relevant dynamic processes throughout the storm event is achieved by studying the transient variation of soil chemical flux to overland flow, the chemical flux at the slope outlet, and the change of chemical mass in the overland flow.