Forest ecosystem processes at the watershed scale: hydrological and ecological controls of nitrogen export

We present a hierarchically distributed model of catchment forest hydrology and biogeochemistry. The goal of the model is to evaluate and predict the distribution of water, carbon and nitrogen cycling within a forested watershed, as well as the export of nitrate. We use a spatial hierarchy of land elements arranged through the stream network, and successively containing catchments, subcatchments, hillslopes, then patches (within each hillslope) to distribute key processes. Time scales of active processes range from those forced by daily meteorology to canopy growth and soil development occurring over decades. This allows us to follow the hydrological and biogeochemical dynamics of the landscape over a range of spatial and temporal scales. A key assumption of the model is that a substantial amount of the water movement through the unsaturated zone occurs as preferential flow. Nitrate is then transported downslope by rising saturated zones during storms. We test the predictions of the model with preliminary data generated as part of the Baltimore Long-Term Ecological Research site. The hydrological modules of the model are calibrated with rainfall/runoff data from the 1980s, while the nitrogen cycling modules are uncalibrated. The model generated good correspondence to the seasonal dynamics of stream water nitrate concentration over the 1998-1999 water year. Copyright (C) 2001 John Wiley & Sons, Ltd.