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

Since the Meiji era (from 1901), the Tokyo Metropolitan Government's Bureau of Waterworks has managed forest land along the upper Tama River as "suizen-rin" (water-source forests) to protect the quality and quantity of the water supply serving the capital. The forest estate covers approximately 25,000 hectares — about half of the roughly 45,000 hectares of forest in the upper Tama basin — making it the largest forest holding of any water utility in Japan. Trees and soil act as a "green dam," absorbing rainwater and releasing it gradually while reducing soil runoff.

Protect the quality and quantity of source water feeding Tokyo's water supply by maintaining forest cover and soil structure that regulate water retention and control erosion in the upper Tama river basin.

The Bureau of Waterworks acquires and maintains forest land (a land-acquisition programme targeting private forest parcels in the Ogouchi Reservoir watershed launched in 2010) and has introduced a corporate-sponsorship model, with naming-rights agreements and annual sponsorships from companies such as Casio Computer, which has sponsored 2.46 hectares since 2018.

As of April 2023, 12 companies held forest naming-rights agreements and 14 companies were annual sponsors; Casio's volunteer participation grew from about 12 to over 20 employees per year. However, no independent, quantified evaluation of water-quality outcomes or avoided-cost figures (comparable to determinations made for Seattle or New York City) was found in available sources.

Corporate sponsorship covers only a small fraction of the 25,000-hectare estate; the bulk of management costs continue to be borne by the Bureau of Waterworks and, ultimately, Tokyo's water ratepayers, making this primarily a publicly funded, century-long source-water protection programme rather than a corporate-financed one.