Castanopsis sieboldii (Makino) Hatus. is one of the most popular evergreen broad-leaved species, distributing from Okinawa Pref. to the area beyond Kanto region, Japan. However, there are limited estimations of transpiration amounts with sap flow measurements, thereby very few knowledge of its characteristics of water use. Here we evaluated transpiration amounts of three trees of C. sieboldii based on the thermal dissipation method for 12 months from October 2020 to September 2021 within Forestry and Forest Products Research Institute, Ibaraki Pref., Japan. The minimum and maximum of transpiration amounts were found in mid-winter (January 2021) and early summer (June–July), respectively. The crown conductance, which indicates the degree of stomatal openness for whole leaves of a tree, was saturated when daytime mean of solar radiation was more than 200 W m⁻². The crown conductance showed a decreasing trend with increasing daytime mean vapor pressure deficit. The daily mean value of soil water content to the depth of 40 cm was about 0.3 to 0.5 during the observation period, resulting in its less effect on the conductance. The optimum value of daytime mean air temperature for the conductance was found for the range of 10–20°C. The three trees showed isohydric behaviors, revealed by the analysis for the response of the conductance to vapor pressure deficit. Therefore, these test trees would avoid hydraulic failure with maintaining steady midday leaf water potential by closing stomata under extremely drought conditions.

In the Suizenji and Lake Ezu regions in Kumamoto City, changes in groundwater discharge rate were measured both before and after the 2016 Kumamoto earthquake that occurred on April 14th and 16th. The study area was divided into four zones and only positive values in the rate of groundwater discharge within each zone were accumulated. As a result, discharge rate averaged approximately 410,000 m³/day from May 2015 to April 2016, and then increased significantly to approximately 620,000 m³/day from May 2016 to April 2018. The reasons for the large increase after the earthquake are that groundwater discharge to the lake greatly increased in the Kami-Ezu and Naka-Ezu zones. While lake seepage into groundwater greatly increased in the Shimo-Ezu zone. Since 2018, groundwater discharge decreased to average approximately 510,000 m³/day from May 2018 to April 2020, and became approximately 550,000 m³/day after May 2020. It is possible that the observed river discharge rates after the earthquake might be affected by restoration works carried out predominantly in the Kami-Ezu region.

This overview paper discusses the potential of hydrology toward the creation of comprehensive knowledge which is necessary for sustainable society. This paper discusses four characters of water, history of hydrology, and the trend of integration between global environmental studies and sustainability studies. Moreover, this paper summarizes the relationship between the change of hydrology and the social transformations, such as industrialization, urbanization and green revolution. In addition to these, this paper discusses the hydrology as discipline, inter-discipline and trans-discipline, as well as natural hydrology, socio-hydrology and human hydrology. At the end, this paper overviews the potential of hydrology toward the creation of comprehensive knowledge, as a history embedded in the earth-human system.