Upscaling Field Scale Hydrology and Water Quality Modelling to Catchment Scale

The aim of the research presented in this manuscript is to model the outflow discharge and nutrient load at the outlet of small scale, mainly agricultural catchments. There to two approaches for the simulation of the transport of water and the transport and transformation of nitrogen in the stream were tested and compared. Both approaches use the DRAINMOD and the DRAINMOD-N models to simulate the hydrology and the nitrogen balance of the land phase at the scale of a field/field block/sub-catchment. Both models are used to generate the drain outflow and the nitrate concentration of the drainage water of the field unit considered. The contribution of the field units to the nutrient load of the river are calculated by multiplying the simulated flow weighted N concentrations with drain outflows. In a first approach, called the lumped approach, the water discharge and the nutrient load of field blocks are routed through the river using an exponential model. In this model the nitrate contribution of an individual field block to the nitrate load in the river outlet is calculated assuming first order nutrient decay/attenuation during the transport of the drainage water from the field outlet to the river outlet. The arrival at the outlet section of the nitrate plumes of the field blocks are phased in time based on the velocity profile in the river. The second approach, herein called the complex approach is using the hydraulic river modeling code MIKE 11. This model is using a complex process ADR (advective-dispersive-reactive) equation to calculate the chemical changes in the river water. The comparative analysis between both routing approaches reveals that the lumped approach is able to predict sufficiently accurate nutrient load at the catchment outlet. The complex approach has the advantage of giving a more accurate estimate of the nutrient load at the catchment outlet, resulting in a more precise modeling of the transport and transformation of the nutrient load in streams. Copyright Springer Science+Business Media, Inc. 2007