Modeling the losses of soil-applied chemicals in runoff: Lateral irrigation versus precipitation

The differences between rainfall- and surface irrigation-induced runoff are described in relation to the transfer of the surface-applied chemicals from soil solution to surface runoff water in the absence of infiltration. The different processes inducing runoff are characterized by the residence time distribution (RTD) of the water in the system. The time-dependent chemical concentration at the field outlet is affected by the RTD, even for the case of instantaneous equilibrium between chemicals in runoff water and soil solution. In this approach the water and chemical transport process in the field is represented by linear well-stirred mixing cells in series, for which a general solution for the effluent concentration is available. The rainfall- and the lateral irrigation-induced runoff processes, which are represented by complete mixing and plug flow models, respectively, form the two limiting cases for the actual chemical transport process by runoff. For each of these two extremes, mixing in the system is characterized by the parameter n, which is the number of cells in the series chosen to represent the actual transport process over the field.