Modeling impacts of forest management practices on water and nitrogen fluxes within a first-order catchment

Forest management practices such as clear-cutting, site preparation, ditching, and ditch network maintenance increase nitrogen (N) load to streams. In environmentally sustainable forestry, water protection actions are required to decrease the nutrient loading. These include e.g. use of uncut buffer zones between a clear-cut area and a waterbody, sedimentation ponds in the ditch network and overland flow fields. In practical forestry, there is an increasing need for developing design criteria for effective protective actions. Clearly, the efficiency of a protective construction depends on a complex interaction between the catchment, the treatment area, atmosphere, vegetation, microbes and soil. We constructed a modeling tool (KUNTO) by integrating hydrological, ecological and water quality models. KUNTO describes a forested first-order catchment as a two-dimensional simplification (Fig. 1). The area of the first-order catchments may vary from tens of hectares to a few square kilometers. Boreal forested catchments usually contain upland mineral soil areas and peatland areas. Peatland areas can be ditched to enhance forest growth. The development of KUNTO aims at combining computation schemes that describe the most important processes controlling the N export from a catchment. The models associated with KUNTO describe hydrological processes above the soil surface and along the hillslope. Net primary production and litter input to soil are described for trees and ground vegetation. Decomposition of organic matter on upland mineral soil and on peatsoil are assessed with separate submodels. Nitrification, denitrification, N uptake and transport with surface and ground waters, retention of N in soils, immobilisation of N into litter and logging residues are also calculated. Water and N input from different parts of the catchment are fed into an outlet stream or ditch network, where erosion, transport of suspended solids and sedimentation are calculated. With KUNTO we can assess the impacts of cutting, site preparation, ditching and ditch network maintenance on N export to water bodies and on catchment N fluxes. Furthermore, we can assess efficiency of buffer zones and sedimentation ponds.