Abstract
Controls of floods and water quality are of great concern in Shiraoi town, Hokkaido. The function of soil and water conservation in three sub-watersheds in the town was evaluated and its reliability was tested. The methods to evaluate forest function, which was developed by Hokkaido prefecture government, are easy to employ, and the results are easy to understand for local people, and therefore we used them. The chronological changes in those functions were compared among three sub-watersheds in the following three years. They are 1948 representing the period of postwar rehabilitation, 1976 being post rapid growth period in Japanese economy, and 2006, the period after the collapse of bubble economy. Agreements of the scores and vegetation changes due to the change of social situation were examined. In results, 1) function of soil and water conservation were degraded temporarily after clear-cutting, 2) conversion of land use to the quarries caused a decline in the function of soil and water conservation, which may continue for a long time, 3) function of mitigating drought and flood was most sensitively affected by forest exploitation, 4) function of controlling water quality greatly declined when riparian zone having steep slopes was broadly developed. The scores were mostly explained by vegetation changes due to the change of social situation. While comparing scores of other sub-watersheds pointed out problems to be solved. It is necessary to simplify the evaluation procedure and show standards of judgment with regard to scores to make the guideline more lucid one.
