Integrated Action Plan for Inba-numa Lake Watershed Management

Lake Inba-numa is located in the northwestern part of Chiba Prefecture in Japan and drains a 541-square-kilometer watershed. The population in the watershed is 722,000, which accounts for about 12 percent of the total population in the prefecture. The water quality of Lake Inba-numa has been deteriorating rapidly since around the mid-1970s mainly because of the urbanization in the watershed. Today, Lake Inba-numa is ranked as a lake with the worst water quality among lakes which are being used as water supply sources in Japan (Figure 1 & Table 1). Chiba Prefectural Government and other organizations concerned have drawn up and implemented a number of plans for the lake’s sewerage system construction and lake water quality conservation. Despite various measures taken under those plans, the water quality has not yet improved significantly.