Simulation of water and solute transport in field soils with the LEACHP model

LEACHP is a modular package for calculating the one-dimensional vertical water and solute flux in horizontally layered soils under transient conditions. Data from field studies conducted in a sandy soil (Vredepeel, The Netherlands) and in a loamy soil (Weiherbach, Germany) were used by five groups to simulate water flow and bromide and pesticide transport with the LEACHP model. Calibrated outputs were compared to the actual field values. Soil hydraulic properties derived from laboratory measurements performed best to predict soil moisture profiles of field soils. For small-scale lysimeters calibration was necessary to simulate drainage fluxes that were within the wide range of experimental values. These calibrated parameters failed to predict increased drainage volumes observed under additional irrigation. Measurement of all soil water balance terms would allow a more thorough evaluation of the hydraulic component of LEACHP. Bromide profiles were not well simulated on the sandy soil where considerable plant uptake was observed. Additionally, zones of immobile soil water might have been present. Residue profiles of the volatile pesticide ethoprophos in soil were best simulated by groups that accounted for volatilisation in their simulations. Different descriptions of the soil sorption process for the mobile pesticide bentazone between groups were dominated by different input of half-life values and hydraulic properties. Although bromide residue profiles were predicted reasonably well in the loamy soil, it was not possible to predict isoproturon dissipation during summer with degradation parameters calibrated in a winter simulation. Predictions of soil water content profiles and leaching volumes can be used with confidence especially after calibration given that preferential flow processes are not predominant. Although important input data for pesticide transformation and transport could be derived from extensive laboratory scale experiments, these did not represent all processes that could affect pesticide fate and behaviour under field conditions. Calibration did not significantly enhance the predictive capability of the solute transport simulations. (C) 2000 Elsevier Science B.V. All rights reserved.