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Peatland forest management is an important source of human-induced additional nitrogen, phosphorus, and organic carbon loads to surface waters in Finland. Ditch network maintenance (DNM) is typically used to ensure sufficient soil drainage for profitable timber production on peatland sites, but nutrient mobilization, particularly of phosphorus, and suspended sediment loads from ditches can cause harmful ecological effects on the quality of water courses. Various water protection structures have been developed to capture the loads released after treatments and to decrease the management impacts in forestry. Overland flow fields are efficient mechanisms for water purification, but they are rarely used in practice due to difficulties in finding suitable locations. In addition, leaving breaks between cleaned ditch sections is recommended wherever possible, as uncleaned sections decrease flow velocity and enhance sediment deposition from runoff water. To plan the most adequate protection methods in DNM, new GIS-based tools are needed to propose e.g. appropriate locations for water protection structures. We developed a spatial analysis to suggest locations for overland flow fields and uncleaned ditch sections within the catchment of lake Kovesjärvi (Parkano, Western Finland). Airborne lidar data consisting of 5 pulses m−2 enabled detailed terrain analysis and finding strictly specified terrain properties that satisfied the conditions of the water protection structures. Half of the field-reviewed overland flow fields were evaluated as good suggestions by forest management professionals and researchers. The model for uncleaned ditch sections worked well on the relatively flat study area, as 77% of the field-reviewed suggestions were rated good by expert opinions. Overall, the approach was evaluated to be useful in peatland forest management. Model parameters can be modified for various topographies, but more studies are needed prior to wider use of the model. • We developed a workflow to place water protection structures within a catchment. • Lidar data enable finding strictly specified terrain properties from the landscape. • Suggested locations for water protection structures were promising in the study area. • The method is useful for forest managers planning ditch network maintenance. [ABSTRACT FROM AUTHOR]

Ditch network maintenance in drained peatland forests increases sediment load and causes harmful effects on downstream watercourses. The objective of this study was to identify the immediate erosion processes in the ditch network and quantify the impact of the ditch network maintenance on suspended solids (SS) load at the catchment outlet. Ditch level processes were measured by conducting field campaigns and catchment-level responses were obtained by using the paired catchment approach. Measurements were conducted at two nested catchments in Koivupuro, Eastern Finland, where runoff and turbidity were monitored continuously at the catchment outlets and erosion and deposition were measured in selected ditches with a pin meter, erosion pins and sediment collectors. According to the results, the sediment load was at its highest during the first year after the ditch network maintenance and especially high during the first spring snow-melt period. Pin-meter measurements, sediment-collector data and results of ditch-bed erosion pins showed that the erosion was more severe in the ditch cut into mineral subsoil than in the ditch cut into thick peat. The SS load from the whole Koivupuro catchment during the first year after the ditch network maintenance was, depending on the estimation method, 185 or 250 kg treated-ha −1 a −1 (0.56 or 0.75 kg ditch-m −1 ), while the net erosion within the ditch network was much higher, 6300 kg treated-ha −1 a −1 (19 kg ditch-m −1 ), indicating that suspended solids were deposited before reaching the outlet of the catchment. Compared to the earlier studies which have focused on the catchment-level effects of ditch network maintenance on the SS loads, the current study succeeded in showing erosion processes and their variability within the ditch network and provided information for improving water protection during ditch network maintenance. [ABSTRACT FROM AUTHOR]