The extensive watershed forests managed by the Yokohama Waterworks Bureau are located in the village of Doshi, which is situated at Minami-Tsuru region of Yamanashi prefecture at the northern part of the Tanzawa mountains.
Because Japan faces water shortages constantly, there has been a new appreciation for the cultivation of watershed forests and a greater emphasis is on protecting and nurturing such forests. Forest policy takes a long time to show fruit because steady efforts are required to cultivate a tree since the growth cycle of a tree requires several score of years. Source: Yokohama Waterworks
36 percent of a total area of Doshi village are watershed cultivation forests covering an area of 2,872 hectares in the Minami-Tsuru area of Yamanashi prefecture.
Yokohama has continued its efforts to develop these watershed cultivation forests for almost 100 years, and has a management policy to further improve the watershed functions that such forests provide. The watershed cultivation forest policy focuses on enhancing the natural, public-benefit functions of forests to hold water and prevent soil erosion-and to a limited extent to provide lumber as a secondary product. However, artificially planted forests, which account for some 36 percent of the total watershed acreage, experience a decline in their water-retention capacity without proper forest management after planting. The people of Doshi village voluntarily carry out the necessary cultivation work by cutting undergrowth, trimming branches, thinning, etc. As artificially planted forests at higher altitudes and the upper reaches of the streams can quickly become overgrown by less desirable vegetation without proper forest management, our goal in such areas is to cultivate larger trees such as conifers by repeated thinning and cutting, while at the same time promoting a balance with the natural growth of broadleaf trees. This policy of promoting the proper mix of trees is also pursued in areas that provide an ideal topography for tiered, multi-layer forests (i.e., those containing trees planted in different years).
The function of watershed forests:
|To store and purify water and prevent flooding. Even when dry weather continues, the rivers continue to flow. The reason for this seeming anomaly can actually be found in the forests. Rainfall in the forests is absorbed by the thick mat of forest soil, which is able to retain water for a long period of time, and slowly penetrates into the ground.|
|Watershed forests are those that are able to positively utilize this water-retention capacity, serving almost as “invisible reservoirs” to provide a constant source of fresh, clear water.|
|Forest soil is an accumulation of years and years of fallen leaves and branches that creates a thick humus layer. This layer acts like a sponge, which can hold many times its own weight in water as it slowly penetrates into the ground. And with every rainfall, the process starts all over again-absorbing the rainfall and slowly releasing it into the ground.|
|The water that penetrates into the ground appears again on the surface in the form of water from springs, which flows along the contours of the ground to become rivers.|
|Without forests in the mountains, rainfall would gush down the sides of mountains and race towards the sea-taking with it soil, plants and almost everything else it encounters.|
|This is why watershed cultivation forests are so important in controlling the flow of rainfall into the rivers, providing protection against soil erosion and landslides, and functioning to purify our water supplies.|