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

H Koivusalo,R Nevalainen,L Finér,T Kokkonen,T Karvonen,E Ahti,A Laurén

doi:10.5194/hess-12-1211-2008

H Koivusalo, R Nevalainen + Show 5 more

Open Access

https://doi.org/10.5194/hess-12-1211-2008

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Abstract

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.

Highlights

IntroductionDrainage may decrease or increase low flow and peak flow volumes depending on peat type and structure, hydraulic characteristics of drainage network, location of drained area within the catchment, and vegetation response to drainage
Assuming dependency between transpiration and tree growth the results suggest that ditch cleaning may not be necessary when stand volume of the forest is sufficiently 5 large (50–100 m3/ha) or top soil layer with a high hydraulic conductivity is sufficiently deep (0.3 m or deeper in the current model parameterisation)
The model application is subject to a number of assumptions: 1) the effect of ditch cleaning on the growth of the forest biomass is neglected during the five-year study period, 2) excessively high 15 or low soil moisture limits transpirations in the root zone, 3) the structure of the soil is not affected by ditch cleaning, and 4) there is no degeneration of peatland vegetation species or invasion of new species after the maintenance

Summary

Introduction

Drainage may decrease or increase low flow and peak flow volumes depending on peat type and structure, hydraulic characteristics of drainage network, location of drained area within the catchment, and vegetation response to drainage. Increased height and leaf area index (LAI) of the tree stand result in higher canopy interception and transpiration, which gradually leads to a decrease in total runoff volumes. Paivanen and Sarkkola (2000) suggested that maintenance of a ditch network is not necessarily required, when the volume of the growing stock is sufficiently large for efficient interception and transpiration. A question of major importance for practical forestry is to assess whether ditch network maintenance is required, or whether the water uptake of the forest stand is large enough to maintain favourable moisture conditions in the root zone. Decreasing soil moisture content enhances subsidence and decomposition of peat and can alter the soil hydraulic prop erties (Holden et al, 2004)

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