Genji Kurima’s research while affiliated with Kagoshima University and other places

Forest management practices influence the availability and quality of water by changing the characteristics of forest site, which in turn affect runoff, infiltration and evaporation rates. Therefore, understanding how forest vegetation impacts the relationship between rainfall and runoff over time is essential for sustainably managing forest for freshwater supplies, especially on remote islands due to their size, locations, geology and topography. However, the tool and /or data for studying a hydrological process of a particular site are/is often not available and even a basic hydrological process of a particular site is often little known. In this study, in the aim of improving our understanding of the hydrological process in an essential watershed on Okinawa Island, we use a simulation model that incorporates a kinematic wave approach to examine rainfall-runoff relationships. We tested the simulation model under various rainfall events to determine its accuracy for projecting runoff discharge. After comparing predicted and observed values, we found the model accurately predicted discharge under moderate and heavy precipitation, but were less accurate for lighter precipitation events. We also explored the modelʼs parameters to identify which values most accurately represent the watershedʼs environmental characteristics. We finally discussed the limitations and the provision for exploring various forest management in using our simulation model and implications for forest management.

Makabe, which is located in the southern-most part of the main Okinawa Island, is an upland agricultural area that produces sugarcane and vegetables. Geographic features in Makabe-South have created a flood-prone basin that make it one of the most flood-damaged areas in Okinawa. For example, in August 2007 a heavy rainfall event resulted in flood damage over an area of 20 ha. Several measures have been proposed to help mitigate flood damage in this region, which features a relatively complex drainage system. To evaluate and compare these proposed measures, it is important to understand the infiltration mechanisms at work in the area. We use a surface runoff model to compute rainfall outflow for our study site. To simulate the study site's complex infiltration and drainage system, we model two adjacent infiltration ponds with different infiltration capabilities. Among other factors, infiltration rates depend on overflow between these two adjacent ponds, which occurs through both a subterranean and pipe flow drainage system. The simulation results indicate that our modeling approach is useful for evaluating the flood mitigation measures proposed in this region.

In coral limestone regions, river systems are often primitive and water is drained through underground infiltration from a doline in watershed areas. In these areas, if the infiltration capability is lowered, for example, by trash and dirt piled up at the mouth of a doline, heavy rain often triggers flood damages. In order to mitigate flood damages in a district of Itoman City, Okinawa Prefecture, a tunnel drainage was constructed. In this study, we conduct simulation analysis to examine the effect of the tunnel drainage on the degree and extent of flood damages. Our simulation model utilizes overland flow model and analysis of unsteady flow to assess rainfall outflow into the doline and applies Torricelli's theorem to model drainage capacity by explicitly considering both seepage holes and hydraulic pressure. Our simulation model can quantify the effect of tunnel drainage on flood damages and is deemed useful for a doline improvement plan intended to mitigate the damage from submergence in the areas where the infiltration capability has deteriorated.

A Feng Shui village landscape, which embodies the symbiosis of nature and man, might be re-evaluated as an ideal landscape model in East Asia. Ho:go is one essential word for a Feng Shui village in Ryukyu Islands. The literal meaning of Ho:go is to embrace and protect by forest planting in order to retain the living energy. Ho:go also refers to a forest belt that encircles a house, a village, several neighbouring villages, or the coastline, and is called House Ho:go (habitat-embracing forest), Village Ho:go, District Ho:go, and Coastline Ho:go, respectively. However, such Feng Shui village landscapes have disappeared rapidly since Word War II because of the changing life styles. In order to preserve the traditional Feng Shui village landscape, our primary research focus concerns the actual structure, management, and regeneration of house-embracing Garcinia subelliptica Merr. trees. We chose to survey the two best preserved villages of Tonaki Island and Bise village in northern Okinawa Island. We reproduced the actual distribution and sizes of house-embracing G. subelliptica trees by HO CAD software. We found tree lines were much thicker in the borderline of the village, in particular, those either facing the coast or standing in the north. In contrast, there was usually one tree line inside the village. The surveyed G. subelliptica trees on Tonaki Island were much smaller than those in Bise Village. More demand of G. subelliptica trees for timber use in this small isolated island and better maintenance might be assumed to be the reasons for the difference in tree height between the two surveyed villages. Thus, proper maintenance in terms of cutting and cleaning are necessary to preserve house-embracing G. subelliptica tree lines in a traditional Feng Shui village. A traditional village landscape might also serve purposes for forest tourism and environmental education.