Simulation of bromide tracer and nitrogen transport in a cracked clay soilwith the FLOCR/ANIMO model combination

A modified version of the FLOCR/ANIMO model combination was developed and t ested on simulating solute transport through a cracked macroporous clay soi l. The improved model combination was calibrated against measurements of gr oundwater level, soil moisture content, bromide concentrations in the groun dwater, and bromide contents in the soil profile. The contribution of the m odifications made to the model on the overall model combination's performan ce was analysed. Nitrate transport was simulated with minor model calibrati on. Within three weeks after bromide application at a wet soil surface with out visible shrinkage cracks, high bromide concentrations in the groundwate r were measured, showing clear evidence of preferential flow through perman ent macropores. The modified model combination was able to adequately simul ate the bromide concentrations in the groundwater and the bromide contents in the soil profile. The most important modifications made to the model com bination which allowed for these results were the inclusion of permanent ma cropores and internal catchment domains in the model. Shrinkage cracks, int erflow into macropores, and diffusive exchange between macropore water and soil water also contributed positively to the model combination's performan ce. The contribution of shrinkage cracks to transport of solutes applied un der dry conditions could not be tested in this study because of the lack of relevant field-data. For complete testing of models like the FLOCR/ANIMO m odel combination, which include transport through permanent macropores and shrinkage cracks, results of tracer experiments executed under dry and wet conditions are required. The model combination's performance on simulating nitrate concentration in the groundwater was moderate. For a more accurate simulation of nitrate transport additional model adjustments need to be con sidered, especially concerning the oxygen diffusion through macropores. (C) 1999 Elsevier Science B.V. All rights reserved.