Water flow and nutrient transport in a layered silt loam soil

English. Theory, numerical models, and field and laboratory measurements are used to describe and predict water flow and nutrient transport in a layered silt loam soil. One- and two-dimensional models based on the Darcy equation for water flow and the convection-dispersion equation for solute transport are evaluated. Pressure heads simulated with the one-dimensional water balance model SWATRE are too large. The two-dimensional SWMS_2D model simulates water flow well for the winter leaching periods. The layering of the soil profile and the height of the phreatic surface determine flow paths to the subsurface drain. The hydraulic conductivity at saturation ranged from 200 cm d -1in the 0-25 cm depth topsoil to 10 cm d -1in the 95-120 cm depth subsoil. Under wet conditions solutes are transported laterally from the 0-25 cm topsoil towards the drain. Under drier conditions drainage water originates mainly from the 75-120 cm depth soil layers. Water flow and solute transport through the larger pores and solute exchange between the soil and water in these pores appear to be important, which was confirmed by a bromide tracer experiment. The upward diffusion of 220 kg ha -1yr -1chloride (Cl) was a major term in the Cl mass balance. Estimates of yearly averaged nitrogen (N) losses for integrated and conventional arable farming plots were 45 and 72 kg N ha -1yr -1respectively. At an integrated plot measured N leaching ranged from 0 to 50 kg ha -1yr -1. High nitrate (N0 3 ) concentrations in the drainage water were often measured during wet periods with shallow phreatic surfaces. Spatial differences in N content of 30 kg ha -1were measured as a function of the distance from the drain, probably as a result of denitrification. Smaller drain spacing, precise application of fertilisers and irrigation water, and controlled drainage can reduce N0 3 leaching to groundwater and surface water.

Dutch; Flemish. De laatste twintig jaren hebben milieuproblemen in relatie tot landbouw veel aandacht gekregen (Hoofdstuk 1). Uitspoeling van nitraat (NO3) en fosfaat (P04) vanuit de bodem naar grond- en oppervlaktewater is een groot probleem, zeker in Nederland. Het bodemwater is het transportmedium voor de nutrienten. Dit proefschrift combineert theorie, numerieke modellen, en veld- en laboratoriummetingen om waterstroming en nutriententransport te begrijpen, te beschrijven en te voorspellen voor de gelaagde zavelgrond van de proefboerderij Dr. H.J. Lovinkhoeve in de Noordoostpolder.