[AHW20-P04] Attempt to quantify the water resource storage function of company-owned forests —A case study of "Asahi no Mori"—
Keywords:water resource storage function, water balance analysis, company-owned forests, CSR
"Asahi no Mori" is generic name for fifteen forests owned by Asahi GHD, and its total area is 21.7 km2. The bedrock of the forests are largely classified as granite and Takata-rhyolite. Flow rate observations of major rivers in forests were conducted from April 2017 to November 2017. From observation results, it became clear that the flow rate varies depending on the type of bedrock. Water balance analysis was conducted for Gono-kawa River basin that encloses "Asahi no Mori" for the purpose of estimating groundwater recharge in forest area. The method suggested by Ogata et al. (2017) is suitable for planar evaluation of evapotranspiration rate in forest, we used it for water balance analysis. This method can consider canopy interception evaporation, soil evaporation and transpiration and be suitable for the target area where much snowfall and snow cover is observed in winter because snowfall, snow accumulation and snowmelt process were modeled. In water balance analysis, the target area was divided into 1 km mesh and precipitation amount, evapotranspiration rate and groundwater recharge were estimated by month. Groundwater permeability necessary for estimating the amount of water infiltrating the bedrock was set based on field observation data and runoff analysis results by the tank-model. The validity of the water balance analysis model has been confirmed by obtaining discharge observation data for Gono-kawa River and comparing observed value and estimated value.
As a result of the water balance analysis carried out based on the field survey results, annual groundwater recharge throughout "Asahi no Mori" was estimated at 9.58 million m3. This is about 40% of the water resources consumption of Asahi GHD in 2016. Also, groundwater recharge in the forest where bedrock is granite resulted in high amount (average 684 mm/yr), whereas groundwater recharge in the forest where bedrock is Takata-rhyolite resulted in low amount (average 395 mm/yr). It is thought that this is because weathering of granite is more advanced than Takata-rhyolite and granite is a structure that is easy to penetrate inside the bedrock through cracks. Thus, according to this study, the importance of considering geology in the evaluation of the water resource storage function became clear.