Tokyo's Water Conservation Forests — a Century-Old 'Green Dam' for Source-Water Protection (Japan)

Since the Meiji era, the Tokyo Metropolitan Government's Bureau of Waterworks has owned and managed forest land along the upper reaches of the Tama River, stretching from Okutama in western Tokyo into neighbouring Yamanashi Prefecture, explicitly to protect the quality and quantity of the water supplying the capital. As of April 2024, these "suigen-rin" (water-source forests) covered approximately 25,000 hectares — about half of the roughly 45,000 hectares of forest across the whole upper Tama basin, and, per the Bureau's own comparison, an area about four times the size of Manhattan — making it the largest forest holding of any water utility in Japan. The Bureau describes the forest's function as a "green dam": tree cover and soil structure absorb and gradually release rainwater, reduce soil runoff and help purify water before it reaches the city's reservoirs.

Because outright purchase of scattered private forest parcels is costly, the Bureau added a land-acquisition programme in 2010 aimed at forest owners in the Ogouchi Reservoir watershed who wished to relinquish their holdings, and has separately built a corporate funding model: companies can sponsor forest maintenance either through short-term "naming rights" agreements or longer annual sponsorships. As of April 2023, 12 companies held naming-rights agreements and 14 were annual sponsors; one documented example is Casio Computer, which signed a naming-rights agreement in 2018 to sponsor 2.46 hectares near Koshu City, sending staff for annual planting, thinning and seedling-raising visits, with volunteer participation growing from roughly a dozen employees to over 20.

Public sources describe the forest's water-retention and erosion-control role qualitatively but do not report an independent, quantified evaluation of water-quality outcomes or an avoided-cost figure comparable to Seattle's or New York City's EPA filtration-avoidance determinations. The corporate naming-rights and sponsorship scheme, while a genuine funding innovation, covers only a small fraction of the roughly 25,000-hectare estate (a few dozen sponsored hectares against the total); the bulk of management costs are still borne by the Bureau of Waterworks and, ultimately, Tokyo's water ratepayers.

Read the full analysis: https://jstories.media/article/tokyo-updates-water-supply