A comprehensive mathematical model for transport of soil-dissolved chemicals by overland flow

The model developed in this study simulates the contamination of overland f low by soil chemicals that reside near its surface during a surface runoff event. The model includes mass-balance equations for both water flow and ch emical transport in the soil profile and surface runoff. A rate-limited mas s transfer through an overland-flow boundary layer at the soil overland how interface controls the dissolved chemical transfer from soil solution to o verland flow, once formed. The model predicts water flow and chemical trans port in the soil profile prior to the rainfall pending (when overland flow starts) and during the surface runoff event. The predictions of these varia bles, together with the total load to the surface runoff, were successfully compared with the measured data of Hubbard et al. [Trans. ASAE, 32(4) (198 9) 1239]. Being physically based, the model was used to investigate the dep endence of surface runoff pollution and its extent on the system hydrologic al parameters. A key factor on the availability of soil chemicals to pollut e the overland flow is their displacement by infiltrating water prior to ru noff initiation. Being dependent on soil moisture prior to rainfall initiat ion and on rainfall intensity, a lower chemical concentration and a lower l oad in surface runoff are obtained for longer pending times, ones that art: associated with lower rainfall rates and initially drier soil profiles. Du ring the surface runoff flow, the chemical concentration in overland flow a t the slope outlet is affected by the contact time of an overland flow parc el with the soil surface. Thus, it increases for higher values of equilibri um time - t(E), lower rainfall rates, slope gradients, and higher soil-surf ace 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 betwee n the relevant dynamic processes throughout the storm event is achieved by studying the transient variation of soil chemical flux to overland flow, th e chemical flux at the slope outlet, and the change of chemical mass in the overland flow. (C) 2001 Elsevier Science B.V. All rights reserved.