Until recently, it has been challenging to couple hydrological and biogeochemical processes at the watershed
scale. We have coupled two models, WTB and MEL, to simulate lateral water and nutrient fluxes and their
influence on ecosystem functioning. WTB is a spatially explicit water balance model. Vertical flow was
simulated using a capacitance model with lateral flow dependent on head development and the local slope of
the confining layer. The Multiple Element Limitation (MEL) model is an ecosystem model, developed to
examine limitation in vegetation acclimating to changes in the availability of two resources (carbon and
nitrogen). MEL also incorporates the recycling of resources through the soil. In our coupled model, nutrients
are treated as inert solutes and are transported vertically as well as laterally using a mixing model. Nutrients
moving down the slope are repeatedly taken up, cycled through vegetation and soils, and released back into
the soil solution. We are currently identifying the possibilities for incorporating flood dynamics into the
model. We evaluated the impact of adding lateral nutrient fluxes to the original MEL model using a virtual
experiment. The model (coupled and MEL only) was applied to a small, well defined catchment. After a
simulation period of three years, we detect a redistribution of the stock of inorganic N. A larger amount of N is
present near the river than at the top of the slopes of the catchment, largely due to lateral fluxes. Comparing
the coupled model to the MEL model, we also find large losses of inorganic N in the coupled model due to
large vertical fluxes out of the root zone. These vertical out-fluxes cause a smaller N uptake by plants. To
detect if Carbon (C) uptake by plants is affected due to the changes in N distribution, the simulation period
has to be increased due to a lag time in the optimization of the C:N ratio in plant biomass