Rainfall induced chemical transport from soil to runoff: theory and experiments Bin Gao a , M. Todd Walter a, * , Tammo S. Steenhuis a , William L. Hogarth b , J.-Yves Parlange a a Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853-5701, USA b Science and Information Technology The University of Newcastle, Callaghan, NWS 2308, Australia Received 15 August 2003; revised 7 March 2004; accepted 19 March 2004 Abstract Although both raindrop driven processes and diffusion play important roles in the transfer of chemicals from soil to surface runoff, current transport models either do not consider the two processes together, or use ‘effective’ parameters with uncertain physical definitions. We developed a physically based, solute transport model that couples both mechanisms and tested it with laboratory experiments. One unique aspect of this study is that all the parameters were either directly measured or previously published, that is, there was no model ‘calibration’ or ‘fitting.’ Our model assumes that chemicals near the surface of the soil are ejected into runoff by raindrop impact and chemicals deeper in the soil diffuse into a surface layer, or ‘exchange layer,’ via diffusion. The exchange layer depth and transfer processes are derived from the ‘shield’ concept in the Rose soil erosion model (e.g. Rose, 1985). The model’s governing equations were solved numerically and the results agreed well with experimental data ðR 2 . 0:90Þ: The model’s sensitivity to various physical and chemical parameters illuminated the importance of both raindrop controlled processes and diffusion on chemical transport from soil to surface runoff. q 2004 Elsevier B.V. All rights reserved. Keywords: Raindrop impact; Exchange layer; Diffusion; Phosphorus; Chloride; Physically based model; Rose model 1. Introduction Accurate, physically based modeling of soil chemical transfer into runoff is essential for develop- ing predictive non-point source pollution models. Several dynamic, inter-related processes control chemical transport from soil to runoff during rainfall including ejection of soil water by raindrop impacts, diffusion of chemicals out of soil water, infiltration, and soil water-runoff mixing (Ahuja and Lehman, 1983; Ahuja, 1990; Zhang et al., 1997; Gao et al., 2004). Although many models have been developed to predict chemical transport from soil to surface runoff, they generally assume one process dominates the chemical transport or that multiple mechanisms can be characterized as a single effective process, like diffusion (Ahuja, 1990). These models can generally be fitted to experimental data by calibrating one or 0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2004.03.026 Journal of Hydrology 295 (2004) 291–304 www.elsevier.com/locate/jhydrol * Corresponding author. Tel.: þ1-607-255-2488; fax: þ 1-607- 255-4080. E-mail address: mtw5@cornell.edu (M. Todd Walter).