Impacts of ditch cleaning on hydrological processes in a drained peatland forest

Abstract. One fourth of the forests in Finland are growing on drained peatlands. Forestry operations such as ditch network maintenance increase the export of suspended solids and nutrients, and deteriorate water quality in lakes and rivers. Water protection presupposes an understanding of how forestry operations affect peatland hydrology. The objective was to study the hydrological impacts of ditch cleaning on the basis of water table level and runoff measurements from two pairs of artificially delineated catchments in drained peatland forests in Finland. Data from treated and control catchments indicated that ditch cleaning lowered the level of the water table in sites where a shallow peat layer was underlain by mineral soil. In sites with deep peat formation, the water table showed no detectable response to ditch cleaning. Runoff data suggested that annual runoff clearly increased after ditch cleaning, which was in conflict with the previously reported small impacts of ditch network maintenance. The hydrological model FEMMA was calibrated and applied to assess the conformity of the data and the experimental setup. In the model application, the catchments were assumed to behave as independent hydrological units. However, assessment of the model results and the measurements suggested that ditch cleaning had an impact on hydrological measurements in both treated and control catchments. It appeared that the independence assumption was violated and there was a hydrological connection between the artificial catchments and, therefore, the results of the data analysis were considered misleading. Finally, a numerical experiment based on the model simulations was conducted to explain how the assumed relationship between soil moisture and transpiration is reflected in the modelled runoff. Modelled runoff decreases and evaporation increases when ditches are cleaned in poorly drained sites, where the initial ditch depth is small and the depth of a highly conductive topsoil layer is low. The numerical experiment can be applied to assess when ditch cleaning does not improve evapotranspiration and is unnecessary.