Effects of reforestation on saturated soil hydraulic conductivity and surface runoff in upland Eastern Madagascar
Monday, 6 June 2016
Ilja H.J. van Meerveld1, Chandra Prasad Ghimire2, Bob W Zwartendijk1, Maafaka Ravelona3, Jaona Lahitiana4 and L.a. (Sampurno) Bruijnzeel5, (1)University of Zurich, Department of Geography, Zurich, Switzerland, (2)University of Twente, Faculty of Geo-Information and Earth Observation, Enschede, Netherlands, (3)Laboratoire des Radio-Isotopes, University of Antananarivo, Antananarivo, Madagascar, (4)Laboratoire des Radio-Isotopes, Antananarivo, Madagascar, (5)King's College London, Geography, London, United Kingdom
Abstract:
Secondary forests are ubiquitous across the tropics but their hydrological effects are still poorly understood. As part of a larger project investigating the effects of land use on hydrological processes in upland Eastern Madagascar, we studied the differences in soil hydraulic conductivity (Ksat), preferential flow pathways and surface runoff at a degraded grassland site (DG), a young secondary forest site (YSF, 5–7 years; LAI: 1.83) and a semi-mature secondary forest site (SMF, ~20 years; LAI: 3.39). Soil hydraulic conductivity was measured at the surface using a double ring infiltrometer and at 10-20 and 20-30 cm below the surface using a constant head permeameter. Preferential flow pathways were determined using blue dye sprinkling experiments. Surface runoff was measured between October 2014 and September 2015 at two (both forest sites) or three (degraded site) 3 m by 10 m plots. Ksat declined sharply from >100 mm hr-1 at the surface to <2 mm hr-1 at 20–30 cm depth at all sites. Surface Ksat was significantly higher at the forested sites than at DG. Infiltration was dominated by preferential flow at SMF, by matrix flow at DG and by a mixture of matrix flow and preferential flow at YSF. Surface runoff was 11% of precipitation at DG vs. 2% at the two forested sites. Event runoff coefficients were as high as 45% for DG, 12% for YSF and 10% for SMF. Comparison of surface runoff occurrence with the temporal variation of perched groundwater levels and soil water content showed that surface runoff was generated after saturation of the top soil layer (0–20 cm). The results of this study, thus, show that reforestation can increase surface Ksat and reduce surface runoff. However, the net effect of reforestation on groundwater recharge and baseflow will depend on the balance between the decrease in surface runoff (and associated increase in recharge) and the higher interception and transpiration losses from the forested sites compared to the degraded grassland.
