This study evaluated the Advanced Microwave Scanning Radiometer 2 (AMSR2) L2 soil moisture product (ver. 3) using in situ hydrological observational data, acquired over 7 years (2012–2018), from a 50 × 50 km flat area of the Mongolian Plateau covered with bare soil, pasture and shrubs. Although AMSR2 slightly underestimated soil moisture content at 3-cm depth, satisfactory timing was observed in both the response patterns and the in situ soil moisture data, and the differences between these factors were not large. In terms of the relationship between AMSR2 soil moisture from descending orbits and in situ measured soil moisture at 3-cm depth, the values of the RMSE (m³/m³) and the bias (m³/m³) varied from 0.028 to 0.063 and from 0.011 to − 0.001 m³/m³, respectively. The values of the RMSE and bias depended on rainfall condition. The mean value of the RMSE for the 7-year period was 0.042 m³/m³, i.e., lower than the target accuracy 0.050 m³/m³. The validation results for descending orbits were found slightly better than for ascending orbits. Comparison of the Soil Moisture and Ocean Salinity (SMOS) soil moisture product with the AMSR2 L2 soil moisture product showed that AMSR2 could observe surface soil moisture with nearly same accuracy and stability. However, the bias of the AMSR2 soil moisture measurement was slightly negative and poorer than that of SMOS with deeper soil moisture measurement. It means that AMSR2 cannot effectively measure soil moisture at 3-cm depth. In situ soil temperature at 3-cm depth and surface vegetation (normalized difference vegetation index) did not influence the underestimation of AMSR2 soil moisture measurements. These results suggest that a possible cause of the underestimation of AMSR2 soil moisture measurements is the difference between the depth of the AMSR2 observations and in situ soil moisture measurements. Overall, this study proved the AMSR2 L2 soil moisture product has been useful for monitoring daily surface soil moisture over large grassland areas and it clearly demonstrated the high-performance capability of AMSR2 since 2012.

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.

A coil probe (CP) for time-domain reflectometry (TDR) with a sensor length of 40 mm (CP₄₀) was developed for long-term field soil moisture measurements in a thin surface soil layer (0–3 cm depth). In laboratory, soil moisture measurements of CP₄₀ were nearly identical to those of a traditional two-rod type TDR (2RTDR) probe (rod length: 15 cm, diameter: 0.3 cm, spacing: 3 cm). The CP₄₀ measurement accuracy was between 0.01 m³/m³ and 0.03 m³/m³. For long-term field soil moisture measurements, five CP₄₀ units were installed in the highly wetted soil at the Sanzai site (SS), which is in the permafrost area of the Taiga, and dry soil at the Mandalgobi site (MGS) in the semi-arid area of Mongolia. Four units accurately measured soil moisture at both sites over six years (2002–2007and 2008–2009). Three units succeeded in conducting precisely continuous soil moisture measurements between 2008 and 2022 at the MGS. Two units successfully measured soil moisture over 21 years at both the sites. The representativeness of the CP₄₀ soil moisture measurements in highly wetted soils was low because of heavy rainfall and soil heterogeneity. The accuracy of CP₄₀ soil moisture measurements in highly wetted soils was slightly lower than that of the traditional 2RTDR probe. However, the accuracy of the CP₄₀ soil-moisture measurements in the dry soil was comparable to that of the traditional 2RTDR probe. CP₄₀ units are durable and their field soil moisture measurements demonstrate stable and precise performance (bias, RMSE), even under severe environmental conditions.