Thinning of cypress forest increases subsurface runoff but reduces peak storm-runoff: a lysimeter observation

Changes in runoff caused by forest management prac‐ tices such as thinning need to be better understood for effective water resource management. We established matched (20° slope) 62%-thinning treatment and grassland control lysimeter plots in a 22-year-old cypress plantation in the Inuyama Research Forest of the Ecohydrology Research Institute, Japan. Runoff (surface and subsurface) was directed into a collection tank with a 90° v-notch weir outlet. Measurements were made before and after the thin‐ ning treatment and were compared with grassland control. Monthly manual measurements of subsurface runoff (March 2011–December 2014) performed via a measuring cylinder and stopwatch yielded 18 pre-thinning and 24 post-thinning observations. In addition, 26 pre-thinning and 24 post-thinning sets of storm-event measurements were continuously recorded via a water level data logger. Fol‐ lowing thinning, subsurface runoff and peak storm-runoff changed by up to +133% and –80% respectively. By con‐ trolling the geology, soil characteristics and hydrological pathways, we were able to attribute these outcomes to reduced transpiration and increased ground resistance from felled logs respectively, suggesting that well-managed high-intensity thinning may be beneficial for increasing water supply and controlling floods. However, this is only achievable if felled logs are aligned along contour lines on the hillslopes.