Half-Century Analysis of Climate Trends and Soil Water Storage in a Steep Forested Catchment in Kanto Metropolitan Area

Decadal Changes in Soil Water Storage Characteristics Linked to Forest Management in a Steep Watershed

“…In fact, by observing the number of correlation points at R > 70 mm alone, only decades A, B, and C can be comparable for having a precise number of points ranging from 27 to 30. Decades D and E have higher numbers of correlation points with 42 and 40, respectively, due to the increase in the number of successive wet days in their inclusive years [44].…”

“…The second approach utilizes the calibration of flow-related parameters in the physically based Soil and Water Assessment Tool (SWAT) model to illustrate the trends in runoff potential and soil available water capacity. The results from the first approach, briefly explained in the study of Gunay et al [44], conclude that longterm and sustainable forest management in the watershed has likely increased the soil water capacity through the decades. A parallel analysis utilizing the parameter adjustment function in a physically based hydrologic model is necessary to strengthen the claim and validate the earlier hypothesis.…”

“…The air temperature at the Dam station measured a daily average of 11.9 °C based on the data in the past five decades, while the precipitation from the weather stations estimated an annual basin-average amount of 1480 mm. Despite being in the center of metropolises in the Kanto region, the watershed is reported to be resilient to urban heat as the reservoir sustains a cooling effect on the microclimate of its surrounding environment [44]. The forested area in the Ogouchi Dam watershed covers more than half of the Tama River water conservation forest and is being subjected to sustainable management.…”

“…The daily basin-average rainfall was estimated using the Thiessen polygon method. Linear correlation between rainfall and surface runoff, which was calculated using three-stage tank model [45] Gunay et al [44] reported a non-significant trend and large variability of rainfall in the watershed. To neglect the effect of such great variability in precipitation, only the datasets collected during the wet months were considered.…”

“…To neglect the effect of such great variability in precipitation, only the datasets collected during the wet months were considered. To further reduce the large number of correlation points that may potentially cause significant differences among the sample sizes for each decade, the cumulative amount of rainfall collected from continuous Gunay et al [44] reported a non-significant trend and large variability of rainfall in the watershed. To neglect the effect of such great variability in precipitation, only the datasets collected during the wet months were considered.…”