Global urbanization, population growth, and climate change have considerably impacted water resources, making sustainable water resource management (WRM) essential. Understanding the changes in hydrological components is important for effective WRM, particularly in cities such as Higashi-Hiroshima, which is known for its saké brewing industry. This study used the Soil and Water Assessment Tool (SWAT) with Hydrological Response Units (HRUs) to achieve high spatial precision in assessing the impacts of land use change and climate variability on hydrological components in a suburban catchment in western Japan. Over the 30-year study period (1980s–2000s), land use change was the main driver of hydrological variability, whereas climate change played a minor role. Increased surface runoff, along with decrease in groundwater recharge, evapotranspiration, and baseflow, resulted in an overall reduction in water yield, with a 34.9% decrease in groundwater recharge attributed to the transformation of paddy fields into residential areas. Sustainable WRM practices, including water conservation, recharge zone protection, and green infrastructure, are recommended to balance urban development with water sustainability. These findings offer valuable insights into the strategies for managing water resources in rapidly urbanizing regions worldwide, emphasizing the need for an integrated WRM system that considers both land use and climate change impacts.

Japanese government recommend farmers to cultivate upland crops in paddy fields (“converted fields”) to suppress the overproduction of rice. Converted fields are subject to excessively wet and dry conditions that reduce the yield of non-rice crops. Drought and wet stresses are critical to crop growth within specific growth periods. To provide data for use in mitigating crop yield reduction, we evaluated drought and wet stresses in maize (Zea mays L.). A SWAP (soil–water–atmosphere–plant) model was applied to a converted maize field. Observations were carried out in 2019 and 2018 for model calibration and validation. Thereafter, we evaluated the water stress of maize in 2019 (actual conditions) and at a tillage depth 11 cm deeper (scenario conditions). We found that (1) drought and wet stresses occurred within the relevant critical growth periods under actual conditions; (2) in the critical periods, the drought and wet stresses under scenario conditions were 33%–75% and 10%–82%, respectively, of those under actual conditions; (3) water stress at depths of 10 and 20 cm was lower under the scenario conditions than under the actual conditions. These results indicate that deeper tillage may mitigate both drought and wet stresses and can be used to reduce water stress damage in converted fields.

In recent years, there has been an increasing number of cases in Japan where countermeasures against environmental pollution caused by natural heavy metals contained in soil and wastewater generated during construction work have become a problem. For example, when arsenic, which is often used, exceeds 0.01 mg/L, which is the criterion in the elution test, the generated soil is covered with low-permeability (permeability coefficient ≤1×10^-6 cm/s) soil mixed with bentonite to prevent heavy metal leakage. Although the impact on the surrounding environment is predicted in the design of such a disposal site, it does not accurately reflect the behavior of leachate in the embankment and the elution characteristics of heavy metals. Therefore, in this study, for the purpose of optimal design of the disposal site, the distribution of heavy metals in the embankment, behavior of leachate, and elution amount of heavy metals were investigated, and a numerical analysis method using the advection-dispersion equation was examined. In addition, a large column test using soil at the site was conducted to reproduce the site. The parameters related to the permeation characteristics obtained from the column test agreed with the field test; however, the unit elution amount, which is a parameter related to the elution characteristics, was 7.5 times that of the field test. This result indicates the possibility of overestimating the elution amount of heavy metals in the test modeling of the disposal site.