Estimation of reference evapotranspiration (ET₀) is important in hydrological studies. As climate change is predicted to cause changes in water resources and agriculture sectors, the possible implications of changes in different climate variables on ET₀ need to be understood. Sensitivity analysis was employed to illustrate the effects of perturbation in meteorological parameters (maximum and minimum air temperature (T_max and T_min), sunshine hours (n), maximum and minimum relative humidity (RH_max and RH_min)) and wind speed (u_z) on ET₀. ET₀ was calculated by The Food and Agriculture Organization of the United Nations (FAO) 56 Penman-Monteith approach using climate data from 1979–2017 for the Enbara and Futatsumori watersheds in forested mountain areas in Japan. We quantified the contributions of climatic factors to ET₀ at daily, monthly, seasonal and annual time scales. Daily results showed large fluctuations. According to the monthly and seasonal analysis, during warm seasons, T_max and n had more influence on ET₀, especially in May, while for the cold seasons, RH effect was dominant, especially in December. Based on the annual results, the factors most influencing ET₀ were T_max followed by n, u_z, RH and T_min. We also found that the response of ET₀ to changes in climatic parameters differs for sites with different topographic and geographic characteristics.

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.