Kristina Isberg

The effect of re-wetting ditch-affected forest on peat was investigated here with model calculations for the whole of Sweden to see if soil moisture, groundwater level as well as downstream flows and concentration of the nutrient nitrogen, phosphorus and organic carbon could be significantly affected. The study was carried out by introducing improved information about ditches in the hydrological model S-HYPE and then calculating flows and transport of nutrients with the model, with and without re-wetting. For ditches, information about placement was collected from a new survey. It was assumed here that all the ditches had a depth of 0.7 m and that groundwater could be affected within 20 m of the ditches. In the hydrological model, calculations were carried out in about 40,000 subcatchment covering the whole country. These have an average size of about 10 km2. Information on the proportion of land area covered by different combinations of land uses and soil types, as well as information on flows and concentrations of substances in watercourses and lakes were calculated and represented on this scale. However, model results for e.g. groundwater levels and soil moisture exist for each soil and land use class, such as forest on peat. 

According to the calculations, the groundwater level in drained forest on peat during re-wetting was usually raised up to about 14 cm, which corresponds to an average of about 2 cm increase for forest on peat where even peat with no ditching is included. Changes in soil moisture as well as in flows (low, medium and high flows) and concentrations of nitrogen, phosphorus and organic carbon at the outlet of each sub-basin in the country, were below 2% when all drained forest on peat was rewetted. The effect in watercourses on the scale of sub-catchment areas was thus small. 

The main reason why the impact is so small on the scale of sub-catchments is the small proportion of drained forest on peat on this scale according to the implementation of information on ditches, usually less than 1%. However, there are factors that could increase the impact downstream and that have not been included in the calculations, such as runoff from surrounding soil to ditch-affected peat, and the possibility of having deeper ditches than 0.7 m. The effect of this could be investigated through model development and sensitivity analysis. There is also uncertainty in the simulation of organic carbon where processes in surface water were not included in this study, and where processes in the riparian zone can be further developed. More observations of flows and substance concentrations before and after rewetting would also be of great value in increasing knowledge about the important processes and providing better support for decision-making.