Development of Innovative Technologies for Increasing in Watershed Runoff and Improving River Environment by the Management Practice of Devastated Forest Plantation

Sixty-five percent of Japanese land is covered by forests. More than 40% of the forest is consists by Japanese cedar and cypress plantation. Because plantation typically concentrate in headwater watershed which are an important reservoir basin to the densely populated region in downstream, water resources in Japan primary depends on water supply from the headwaters covered by forest plantations. Despite the importance of forest management, the area of abandoned forest plantation has been increased because of declining domestic forest industry and market due to low timber prices, high labor cost, and shortage of forest operators. Due to the low light conditions, understory vegetation cover tends to be sparse in the dense, unmanaged plantations, particularly Japanese cypress forests. Under such forest, infiltration excess overland flow and resultant soil surface erosion occurs. Such overland flow and soil erosion affect runoff and water quality at the downstream counterparts. Therefore, unmanaged and devastated plantations hamper the functions of forest for regulating discharge and recharge of water resources. Increasing the frequency of drought and flashy flood due potentially to global climate change also threaten the stable water supply and flood management in watersheds.

Forest thinning is essential for maintaining forest stand conditions and hydrological processes in devastated plantation forest. Recent studies demonstrated that intensive 50-60 % thinning can increase infiltration rate and reducing the opportunities of overland flow and soil surface erosion by recovering understory vegetation. Thinning generally reduces canopy interception and evapotranspiration. Thus, net precipitation reaching ground surface after thinning increases compared to the pre-thinning condition. Such increases in effective rainfall promote greater amount of groundwater recharge after the thinning. However, no quantitative assessment has been conducted for evaluating the effects of intensive thinning (removal of stems > 50%) on runoff and sediment discharge at a watershed scale. This project specifically aims to examine the influence of intensive thinning practice on the variation of low flow (drought period)discharge. We also focused on the examining production of turbidity material associated with thinning operation and forest management from the forested watershed. We finally develop innovative technique of water resources management which can equalize water supply and improve river water quality in devastated forest plantations.

We conduct intensive field observation campaign in 5 watersheds across Japan. Three watersheds located in Aichi, Mie, and Kochi had been monitored for the last five years and continuous runoff data have been accumulated which are essential for thinning experiment. Intensive field observation is also carried out in the newly established monitoring watersheds in Tochigi and Fukuoka. Water and sediment discharges are continuously monitored before and after intensive thinning practices in all of the watersheds. The changes of runoff processes before and after the thinning are examined for each hydrological components including rainfall, throughfall, evaporation, soil moisture, groundwater, Hortonian overland flow, stream runoff. We also sampled both low and high flow water for analyzing water chemistry and stable isotopes. Soil erosion at hillslope scale and suspended sediment discharge at watershed scale are also observed. Sources of suspended sediment entering channels are determined by using fallout radionuclides as a tracer of soil particles. Because methods of thinning operation vary, we also examined the effects of methods (random removal and line removal) of thinning are compared between watersheds. We then investigate the tree species (cedar and Japanese cypress) and effect of thinning on hydrological processes. Based on these filed information, we applied rainfall-runoff model for clarifying the effects of intensive thinning practice on water and sediment discharge a under various forest management conditions.

Forest conditions due to forest management practice are spatially and temporally varied. Thus, airborne laser scanning with developed remote sensing technique is applied for estimate spatial patterns of canopy structure, relative light condition under canopy, and understory vegetation cover. These remote sensing technique and rainfall-response model are combined to simulate water and sediment discharge from watersheds. We finally integrate these hydrological models into the forest growth model which can project future change of forest stand conditions and runoff under different thinning rates. The ultimate objective of this integrated model provide us potential scenario for maximize low flow discharge in drought period and reduce turbidity material in high flow period.