Detached stemflow has been defined as rainwater that breaks away from the stemflow and falls around the trees as throughfall. Quantitative measurements of detached stemflow were taken for two sample broadleaf trees on the university campus. Zelkova, with smooth bark, has a tree structure that concentrates rainwater, producing a large amount of stemflow. A rainwater collection system installed around the trunk can capture large amounts of throughfall as detached stemflow. The detached stemflow amount had almost doubled in water height equivalent to throughfall at the tree stand. Therefore, some trees generate much throughfall in the forest near the trunk. In the case of the Katsura tree, however, the stemflow was low. The throughfall attributable to the detached stemflow was less than the average throughfall. This low stemflow generation was assumed to be due to the roughness of the Katsura bark. The rainwater which attaches to the trunk and branches breaks away easily. Presumably, the leaves near the trunk intercept raindrops and disperse the rainwater to the surroundings. The detached stemflow can constitute a large quantity. It can be expected to be related closely to the stemflow generation mechanism.

Future expansion of salt-damaged areas is anticipated in Northeast Thailand. We conducted a field investigation of paddy fields from 2016 to 2019 in Ban Phai district, Khon Kaen province in Northeast Thailand to evaluate yearly changes in the effect of salinity damage on rice production. The investigation area was classified into severely salt-affected areas (2^nd of 5 classes) based on the definition used in Thailand. Since salinity severely damages rice production, rice cultivation was abandoned in some fields, although some were still planted. The soil electrical conductivity (EC) in the rice-planted paddy fields changed yearly in association with the amount of precipitation. The effect of the difference in EC on rice yield was moderate, suggesting that rice yield was mediated by surface water. Some areas in the abandoned fields did not have any vegetation, and quite high soil EC values were observed. The non-vegetated areas evaluated based on yearly unmanned aerial vehicle (UAV) images changed partly due to the amount of precipitation. However, some non-vegetated areas decreased in contrast to the decrease in precipitation, probably because of the effect of groundwater. Although the continuous expansion of severely salt-damaged areas was not observed, the monitoring of salinity levels is recommended for the future.

A unique freshwater lens shape observed in Tarama Island, Japan, in which hydraulic conductivity is on the order of 10⁻² m s⁻¹, has posed a question as to how well we can estimate the fresh groundwater volumes in extremely permeable aquifers. We applied both an analytical model and numerical simulations with various hydraulic conductivities, including extremely permeable conditions, and compared their results. The simulation showed that, when the hydraulic conductivity was extremely high, saline groundwater existed near the coast. The analytical model overestimated the freshwater volume compared with those estimated from the numerical simulations, and the discrepancy became more significant with increasing hydraulic conductivity. These findings imply that, when hydraulic conductivity is extremely high, numerical simulations considering density-dependent flow and dispersive mass transport processes should be applied to better assess the shapes and volumes of freshwater lenses.

Monitoring and quantifying hydrological flows in the vadose zone is complicated to analyze due to the effects of rainfall in the tropics, the dynamic interactions among rains, the vegetation layer, moisture in the soil, and the entire regolith. Quantifying subsurface hydrological flows at specific scales and high resolution presents further difficulties. To overcome these issues, resistivity methods can play an important role. This paper examines the results of gravimetric moisture content monitoring in the Panamanian tropics through time-lapse electrical resistivity tomography analysis. Changes in the electrical properties of soil were quantified through six tomographic tests performed between February 2012 and March 2013 along with a profile. Significant changes in resistivity were identified between February (dry season) and May, and August and October (rainy season), with negative percentages (–60%) indicating the effects of rain infiltration at the surface and positive percentages (60%) linked to moisture absorption in the soil, electrode relocation for each test or inversion processes. Additional laboratory analyses of soil samples were carried out to obtain gravimetric moisture content tomograms. The changes of this parameter in the subsurface horizons, and the percentage differences in the calculated resistivity values, are helpful for determining the impact of rain on the soils.

Oxygen and hydrogen stable isotopes are key hydrological tracers for identifying groundwater flow paths, groundwater recharge sources, and related hydrogeological processes. This study aims to identify the spatiotemporal variability in isotopic compositions of hydrogen (δ²H) measured in shallow groundwater, river water, paddy water, and precipitation collected from two catchments in Tottori Prefecture, Japan. A total of 613 surface water and groundwater samples were collected from 45 sites (wells) in five different landscapes during a two-year survey. The enriched groundwater δ²H values were in the range –52‰ to −36‰, and large isotopic variations were observed in the coastal regions of the study area. The lower groundwater δ²H compositions were in the range −61‰ to −53‰ in high elevation areas. Our study shows that the spatial variation in groundwater δ²H (23‰) was larger than the temporal variation (12‰) within the 3,500 km² total area of Tottori Prefecture. At this scale, clear correspondence between the temporal standard deviations of the δ²H values and landscape characteristics was not found, suggesting that spatial scales should be considered carefully when analyzing the temporal evolution of hydrological tracers.

The Asian monsoon and Japan’s steep terrain structure make it a flood-prone country. These natural, flood-prone features have prompted Japan to develop unique social norms for flood risk management. Over the last decade, human-flood interaction models have been developed in socio-hydrology (SH), being applied and validated in various countries. This study applied the SH model for the Naganuma District of Nagano City, Japan, an area that was affected by Typhoon Hagibis in 2019. Additionally, the SH model was examined for its applicability in Japan using sensitivity analysis. To the best of our knowledge, this is the first study to apply the SH model in a real-life scenario in Japan. The results suggest that there are differences between the output from the existing SH model and the actual human-flood interactions in Japanese society. This paper also provides recommendations to improve the Japanese SH model and inform associated future research agendas in Japan.