Application of a two-dimensional model to simulate flow and transport in amacroporous agricultural soil with tile drains

It is essential that important field processes are taken into account to mo del water flow and chemical transport accurately in agricultural fields. Re cent field studies indicate that transport through macropores can play a ma jor role in the export of solutes and particulates from drained agricultura l land into surface water. Non-ideal drain behaviour may further modify the flow and transport. We extended an existing two-dimensional flow and trans port model for variably saturated soils (SWMS_2D) by adding a macropore dom ain and an additional Hooghoudt drain boundary condition. The Hooghoudt bou ndary condition accounts for an entrance head needed to initiate flow into the drains. This paper presents the application of the new model (M-2D) to an agricultural field in Switzerland. To understand interactions between ma cropore flow and drains better we simulated water flow and bromide transpor t for four different field scenarios. We considered both collector drains o nly with an ideal drain boundary condition (with and without macropores) an d collectors and laterals with a Hooghoudt boundary condition (also with an d without macropores). For each scenario, inverse modelling was used to ide ntify model parameters using 150 days of data on observed cumulative discha rge, water table depth, and tracer concentration. The models were subsequen tly tested against a 390-day validation data set. We found that the two add itional components (macropore flow, drain entrance head) of the M-2D model were essential to describe adequately the flow regime and the tracer transp ort data in the field.