Freshwaters provide valuable habitat and important ecosystem services but are threatened worldwide by habitat loss and degradation. In Southeast Asia, rainforest streams are particularly threatened by logging and conversion to oil palm, but we lack information on the impacts of this on freshwater environmental conditions, and the relative importance of catchment versus riparian‐scale disturbance. We studied 16 streams in Sabah, Borneo, including old‐growth forest, logged forest, and oil palm sites. We assessed forest quality in riparian zones and across the whole catchment and compared it with stream environmental conditions including water quality, structural complexity, and organic inputs. We found that streams with the highest riparian forest quality were nearly 4 °C cooler, over 20 cm deeper, had over 40% less sand, greater canopy cover, more stored leaf litter, and wider channels than oil palm streams with the lowest riparian forest quality. Other variables were significantly related to catchment‐scale forest quality, with streams in the highest quality forest catchments having 40% more bedrock and 20 times more dead wood, along with higher phosphorus, and lower nitrate‐N levels compared to streams with the lowest catchment‐scale forest quality. Although riparian buffer strips went some way to protecting waterways, they did not maintain fully forest‐like stream conditions. In addition, logged forest streams still showed signs of disturbance 10–15 years after selective logging. Our results suggest that maintenance and restoration of buffer strips can help to protect healthy freshwater ecosystems but logging practices and catchment‐scale forest management also need to be considered.

Relatively minimal attention has been given to the hydrology of natural broadleaf forests compared to conifer plantations in Japan. We investigated the impacts of ground litter removal and forest clearing on surface runoff using the paired runoff plot approach. Plot A (7.4 m²) was maintained as a control while plot B (8.1 m²) was manipulated. Surface runoff was measured by a tipping-bucket recorder, and rainfall by a tipping-bucket rain gauge. From May 2016 to July 2019, 20, 54, and 42 runoff events were recorded in the no-treatment (NT), litter removed before clearcutting (LRBC), and after clearcutting (AC) phases, respectively. Surface runoff increased 4× when moving from the NT to LRBC phase, and 4.4× when moving from the LRBC to AC phase. Antecedent precipitation index (API₁₁) had a significant influence on surface runoff in the LRBC phase but not in the NT and AC phases. Surface runoff in the AC phase was high regardless of API₁₁. The rainfall required for initiating surface runoff is 38% and 56% less when moving from the NT to LRBC, and LRBC to AC phases, respectively. Ground litter and canopy function to reduce surface runoff in regenerated broadleaf forests.

Changes in runoff caused by forest management practices 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 thinning 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. Following thinning, subsurface runoff and peak storm-runoff changed by up to +133% and –80% respectively. By controlling 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.