Water resource vulnerability due to uneven precipitation and water allocations is a significant issue in many regions of the world, including the Mekong Delta. Although numerous studies have already evaluated drought tendencies in many parts of the Mekong Delta, some areas have been excluded. This study targeted these excluded areas, including one inland, one coastal area, and two islands, which are defined as “understudied regions.” The meteorological drought intensity, frequency, and duration in the study areas in the Mekong Delta were evaluated using different time scales of the Standardized Precipitation Index (SPI) between 1994–2020. Unique contrasts in drought features were found between the study areas, indicating that severe drought events occupied the highest percentages inland from 2011 onward, while the most extreme drought events occurred in the coastal areas. Furthermore, trends in drought intensity, tendency, frequency, and duration were identified within the same delta region, demonstrating that combining SPI with other indicators can detect drought patterns in the Mekong Delta. These findings emphasize the importance of comprehensive evaluation of drought tendencies, including in understudied regions, for a better understanding of the features and future of water resources management.

In the renowned Kushiro wetland, Japan, the government has been undertaking a nature restoration project to counter the impact of post-WW2 restoration land development. The recent typhoon landfalls in Hokkaido has triggered concerns about the potential impact of climate change. In response, the government plans to incorporate nature restoration as an adaptation strategy to mitigate climate risks. To support this initiative, which involves an assessment of climate change impacts, we improved the high-resolution climate change projections and hydrological model to predict future impacts on wetlands in terms of river discharge and sediment runoff. The results show that although there is no significant increase in the risk of drought, intense rainfall and flooding are likely. In addition, simulated sediment runoff increased by mean of 2–3 folds due to increased summer flooding, and the frequency of intense sediment runoff events increased by 10 fold. These results show that countermeasures against sediments are important for protecting wetland ecosystems from climate change.

Historical climatological upper percentile rainfall over Japan from the Database for Policy Decision-making for Future climate changes (d4PDF) was compared with that from 121 rain-gauges for the 1952–2010 period. Percentiles calculated excluding or including values below 1 mm, “wet-extremes” and “all-extremes” were compared. The 5 km d4PDF hourly wet (all) extremes matched rain-gauges at 72% (45%) of the locations, but for daily extremes this was 39% (33%). Further comparison with radar-AMeDAS (Automated Meteorological Data Acquisition System) was made for hourly extremes in the 2006–2021 period. Wet (all) extremes matched for 84% (79%) of rain-gauges, and 80% (51%) also matched radar-AMeDAS at the same locations. Exceedance probabilities were calculated for historical and 4 K warmer climates using thresholds based on a non-warming climate. Historically, hourly (daily) extremes were more frequent over most of (eastern) Japan. In a 4 K warmer climate, daily extremes would become more frequent over most of Japan. Rainfall from 5 km d4PDF at hourly and daily resolutions are provided in prefecture and river basin subsets, with compressed sizes of each below 1 Gigabyte.

Localized heavy rainfall events have caused an increasing number of water-related disasters in recent years, particularly in urban areas where drainage capacity is limited. Accurate prediction of such events requires high-resolution, high-frequency three-dimensional (3D) observation data to capture rapidly evolving atmospheric structures. The Multi-Parameter Phased Array Weather Radar (MP-PAWR) provides rapid, high-density observations, offering significant advantages over conventional parabolic radars. However, rainfall prediction methods such as Vertically Integrated Liquid water content (VIL) Nowcast (VIL Nowcast; VILNC), which traditionally rely on limited-resolution radar data, often overestimate rainfall intensities, especially during the dissipating phase of cumulonimbus clouds. In this study, we propose an enhanced VILNC model that incorporates 3D wind field estimations derived from MP-PAWR to better represent the development and decay of rainfall areas. By integrating vertical wind information, the modified VILNC aims to mitigate overestimations associated with downdrafts. Comparative analysis of several localized heavy rainfall events demonstrates that the proposed method improves prediction accuracy, particularly at shorter lead times. These results suggest that the utilization of high-resolution 3D wind fields from MP-PAWR contributes to more reliable short-term rainfall predictions.

Accurate simulation of gross primary productivity (GPP) and evapotranspiration (ET) is essential for understanding global carbon and water cycles. However, land surface models (LSMs), while widely used, still exhibit substantial uncertainties, largely due to poorly calibrated parameters. This study integrated the Morris sensitivity analysis with Differential Evolution calibration (MD framework) to improve GPP and ET simulations in the MATSIRO LSM across twenty FLUXNET sites representing diverse ecosystem types. Results demonstrated that the MD framework led to marked improvements, with ensemble mean Kling-Gupta Efficiency (KGE) for monthly GPP and ET increasing by 43% and 30%, respectively. Among all parameters, GPP and ET were most sensitive to the maximum carboxylation rate of RuBisCO (VMAX0), which was underestimated in the original model. Along with VMAX0, the MD framework identified the top eight sensitive parameters for each site and provided site-specific optimal values, enabling tailored parameterization to improve simulation performance across diverse ecosystems. In addition, our results showed that leaf area index (LAI) is a non-negligible source of uncertainty for GPP simulations in cropland sites. Multiple-cropping systems amplify the importance of accurately capturing the abrupt LAI. These findings offer valuable guidance for improving GPP and ET simulation in LSMs, particularly in managed ecosystems.

Increases in water temperature due to climate change have been reported in many parts of the world, raising concerns about potential impacts on ecosystems, human water use, and water quality. In Japan, it is difficult to obtain trends in water temperature changes over a wide area because only a limited number of locations have long-term water temperature observations. Here, we analyzed historical changes in water temperature in Japan using both in situ observation data and satellite-derived surface water temperature (SWT). The annual mean water temperature change in Japan from 1982 to 2022, based on in situ observations, was 0.27 ± 0.19°C decade^–1. This is slightly lower than the estimate for the shorter period from 1982 to 2016 (0.30 ± 0.22°C decade^–1), but the number of sites showing a significant increasing trend was higher (85 sites) compared to the shorter period (66 sites). Similarly, the number of basins with a significant increase in SWT rose from 447 to 520 out of 675 with the inclusion of recent years. The nationwide average satellite-derived annual SWT trend from 1985 to 2022 was 0.47 ± 0.25°C decade^–1, with a particularly strong increasing trend observed in central Japan.

This study investigated the potential impacts of Long Lead-time Flood Forecasting Systems (LFS) on disaster response and societal changes through focus groups involving administrative stakeholders and researchers in Nagano, Japan. LFS has been shown to enhance proactive disaster responses, such as improved evacuation support and shelter environments in flood-prone areas, while fostering long-term societal transformations, including increased resilience and adaptive capacity. However, the study also identified potential negative impacts, such as overreliance on forecasts, psychological biases, and reduced infrastructure investments, underscoring the need for responsible LFS implementation.

Bimodal raindrop size distributions (RDSD) cannot be adequately represented by conventional exponential or gamma distributions. This could affect quantitative precipitation estimation (QPE) using meteorological radars. In this study, we developed a QPE method using a gamma raindrop distribution combined with an exponential (GRACE) distribution, which can represent bimodal RDSDs, and examined its utility. R−K_DP was calculated using the parameters obtained by fitting the GRACE distribution to the RDSD observed using a two-dimensional video disdrometer (2DVD). Estimates based on the GRACE distribution provided results closer to the precipitation intensity observed by rain gauges than estimates based on the gamma distribution or the estimation method used in the extended radar information network (XRAIN), especially during heavy rainfall. The utility of the GRACE distribution for QPE has been clarified; however, it is necessary to investigate the optimal estimation method for various types of rainfall.

Agricultural water infrastructure management by land improvement districts (LIDs) has the primary purpose of irrigation and drainage for agricultural production. However, recent climate change and urbanization have necessitated additional operations for flood control, becoming an additional and significant burden to LIDs. Therefore, this study aimed to evaluate the changes in operational efforts of agricultural water infrastructure resulting from urbanization. The number of gate operations of the main canals was proposed as a quantitative index of the operational efforts. The number of gate operations in the past five years (1994–1998) and the present (2017–2021) were extracted from the management logbooks and classified by operation purpose based on rainfall intensity to analyze the effects of urbanization. The results showed that the number of gate operations can be applied as an indicator of operational effort. The operational efforts for flood control purposes have increased because of urbanization. In contrast, the operation effort for irrigation remained the same as before, even if the number of paddies decreased due to urbanization. These results prove that urbanization is increasing the operational efforts of agricultural water infrastructure by LID.