Solute dissipation regimes in soil: Exponential loss resulting from non-linear process interactions

Solute dissipation from the top soil is a result of a set of complex interactions of several non-linear processes related to soil water flow in heterogeneous soil and solute sorption and decay under highly variable atmospheric forcing. Solute dissipation rates have been used as key indicators for pesticide and contaminant transport. It has been often observed that field dissipation follows exponential decay functions, despite the fact that water flow is highly non-linear and variable. To date, field dissipation rates have been modelled using simple models ignoring the non-linearity of unsaturated water flow and the complexity of atmospheric forcing. Accordingly, our goal focusses on analysing the interactions of unsaturated water flow in structured soil (as represented by the mobile-immobile water concept) with the sorption and decay of a pesticide as a model compound for distinct wet and dry periods given by intermittent rainfall and different evaporation rates. The process interactions are investigated by solving non-equilibrium advection-dispersion transport and Richards equation in a mobile-immobile soil setting in Hydrus1D. We perform an extensive analysis of the sensitivity of solute dissipation rates from the top soil in response to all permutations of a parameter space comprised of soil properties (immobile fraction, mass transfer coefficient), solute properties (decay coefficient, adsorption coefficient), rainfall parameters (total precipitation, duration, frequency) and the presence or absence of evaporation. The results of over 90000 simulations are assessed in terms of dissipation curves. Although these curves have different behaviours, their average behaviour remains exponential for all cases. We identify three dissipation regimes which exhibit characteristic time scales and dissipation curve shapes: an advection dominated regime under particular rainfall conditions, an evaporation dominated regime under low rainfall volume and intensity and a decay-dominated regime, in which the bio- or chemical- decay rate of the substance is large and therefore dominant.