Long-term monitoring of the effects of forest management on stream discharge and water quality in the Oborasawa Experimental Watershed, Japan

The Oborasawa Experimental Watershed is located in the Tanzawa Mountains in central Japan (35° 28' N, 139° 12'E). It consists of three mountainous forest headwaters: catchments 1 (49 ha), 3 (7 ha), and 4 (5 ha). The goals of this research are to understand the hydrological and biogeochemical processes within the watershed and to quantify the impact of forest management on stream discharge and water quality. The Kanagawa Prefecture government started monitoring rainfall and stream discharge in the 1980s and improved the observation system in 2009. Catchment 3 was enclosed by a deer fence in 2012. We monitored the impact of the deer fence on stream discharge and water quality, and also conducted extensive monitoring in four small plots; thinning is scheduled for one of these plots in 2021. We are presently observing the rainfall, meteorological conditions, interception and transpiration rates, soil moisture content, groundwater level, stream and sediment discharge, and water chemistry in the catchments.

The climate of the catchment is warm and humid, with an average annual rainfall of 2,950 mm and an average annual temperature of 12.1°C. Large rainfall events occur mainly in summer, especially in June and July (rainy season) and September and October (typhoon season). Snow occurs only once or twice a year from January to March, and accounts for a small percentage of the total annual rainfall. The underlying bedrock is tuff and tuff breccia from the Cenozoic and the soils are classified as Cambisols. Two-thirds of the catchments is covered by Cryptomeria japonica and Chamaecyparis obtusa and the remaining third is covered by broad-leaf trees.

Here, we present the long-term observational data and major results. We found that the rate of groundwater movement across the catchment boundary was extremely high in the catchments and greatly affected the stream discharge and water chemistry. No effects of the deer fences on stream discharge or water quality have been detected. The nitrate concentration in the stream water has decreased in recent years, which may be due to the long-term decrease in atmospheric deposition.