HALF-CENTURY ANALYSIS OF CLIMATE TRENDS AND SOIL WATER STORAGE IN A STEEP FORESTED CATCHMENT IN KANTO METROPOLITAN AREA

Abstract

 Significant changes and variability in climate are likely to influence the hydrological balance in small and steep Japanese watersheds. Previous studies had confirmed that in the past decades, rapid increase in population and expansion of industrial zones had developed ‘urban heat islands’ in large cities of the country. Hence, this study recognizes the need to analyze the hydroclimatic features of a relatively small, steep, and forested watershed, which lies in the center of Kantō metropolises. Upon applying trend test and determining the rates of change (slope) to the 1965–2015 dataset of the Ogouchi Dam watershed, results showed non-significant trends in rainfall and peak discharge, indicating their high variability in the half-century study period. On the other hand, significantly increasing trends were observed on the annual and seasonal average air temperatures, with the increase in winter being the most drastic. Comparing the rate of temperature increase per month with the rates of increase from its nearby urban stations, local climate in the watershed exhibited milder warming during hot months (Apr–Sep). Meanwhile, the amounts of water stored in the soil for each decade were ably estimated from the rainfall-runoff correlation plots developed during wet season (May–Oct). Results proved a gradual increase in soil water storage capacity, which could be the long-term impact of the continuous efforts of the Government to conserve the forested catchment.