Many studies have proved that hydrological extreme values estimated from decadal observation data and river inundation simulations are associated with various uncertainties; however, few studies have evaluated the uncertainties associated with internal climate variability. We used large-ensemble river inundation simulations to quantitatively evaluate uncertainties in river depth at the Takahama monitoring station and flood extent in the Yodo River basin. Using a single 60-year ensemble, the river depth for a 1,000-year return period (RP) flood scale have uncertainty between –11.7% and +9.2% in a 3,000-year flood simulation. Thus, the RP of the simulated river depth ranges from 207–3,441 years. To maintain the RP uncertainty within ±300 years would require a simulation of ≥1,200 years. The flood extent uncertainty with an RP of 1,000 years was found to be –8.4% and +7.6% based on a 3,000-year simulation for the lower Yodo River basin. According to this result, the RP of the simulated flood extent ranges from 340–3,060 years. These results suggest that the decadal data used in conventional flood risk analyses potentially contain large uncertainty related to internal climate variability in the RP for water depth and flood extent by approximately 0.3–3-fold.

Accurate soil erosion estimations are required for both economic and ecological purposes. In sugarcane fields on Ishigaki Island, Japan, several types of cropping practices are undertaken in the same region and annual cropping practice distribution changes (ADCs) occur. ADCs suggest that the dynamics of soil erosion from sugarcane fields differ year by year. In this study, we evaluated the annual soil erosion errors derived from ADCs by reconstructing a map showing the distribution of cropping practices among sugarcane fields, calculating soil erosion using the Universal Soil Loss Equation (USLE), and determining the coefficient of variation (CoV) as an indicator of errors. The CoV was large, especially at the scale of 0.001–3 km². A comparison with previous research also indicated that the errors derived from ADCs had a larger impact on soil erosion estimations than those from the USLE properties, at least at scales smaller than 0.1 km², suggesting that an appropriate consideration of ADCs is required for accurate soil erosion estimations.

This study aims to clarify the contributions of pre-event water to storm runoff using environmental tracers (dissolved inorganic ions and stable isotopes) in a tropical forested catchment in Puchong, Selangor, Malaysia. We performed intensive sampling campaigns of stream water and throughfall for two storm events in July and November 2018. The discharge showed a low peak of 0.13 mm/h in event 1, with 18 mm of total rainfall, whereas event 2, with 50 mm of total rainfall, showed a quick discharge peak of 1.17 mm/h and a slow recovery of 0.39 mm/h. The nitrate concentration in the stream water during event 2 was higher than that in event 1. The temporal variations in nitrate ions indicate that subsurface water provided a dominant stormflow in event 2. Hydrograph separations using silicate as a tracer revealed that pre-event water was the dominant component of the storm hydrograph (58–98%). Our results suggest that pre-event water plays an essential role in storm runoff of headwaters in humid tropical regions.

Various models have been developed to predict rainfall-runoff. However, practical models often simplify the actual phenomena and do not always provide sufficient accuracy for field-observed parameter values, such as soil water retention and permeability. Other models based on Richards’ equation can directly account for these factors but are not practical because of their huge computational cost. In this study, we developed a vertical quasi-two-dimensional surface-subsurface flow model (quasi-2D model) based on Richards’ equation, in which the hydraulic gradient in the downward direction was approximated by the slope gradient. This method makes it possible to consider soil moisture distribution perpendicular to the slope and simplify the modeling of the runoff process. Rainfall-runoff simulations were conducted on a single slope using the quasi-2D model and compared with the results computed under the same conditions using a detailed model solving the two-dimensional Richards’ equation (2D model). For both subsurface and surface flows, the quasi-2D model reproduced the results of the 2D model well (NSE > 0.99), and performed particularly well on steeper slopes. The computation time of the quasi-2D model was reduced to less than 1/10 of that of the 2D model, confirming the usefulness of the quasi-2D model.

In this study, groundwater potential and aquifer characteristics were analysed and investigated for the first time in an integrated manner using pumping test and resistivity method in all areas within Makassar City, Indonesia. It is identified that the soil layer in Makassar consists of alluvial sediment, clay sand, sandy clay, tuff, and volcanic breccia. Productive aquifers were found at a depth of 15 to 50 m in the northern, southern, and eastern areas of the city and at a depth of 51 to 120 m in the central areas. The storativity values showed that the type of aquifer in Makassar is dominated by unconfined and semi-confined. The largest optimum pumping discharge was identified in Panakkukang and Manggala Districts with values of 0.102 and 0.061 m³/min, respectively. It was found that the distribution of aquifer transmissivity corresponds to the distribution of the optimum pumping discharge where the largest transmissivity values are located in Panakkukang and Manggala Districts. Among all districts in Makassar City, Panakkukang and Manggala Districts have the greatest groundwater potential. This is most likely due to the position of these two areas which are situated in a groundwater discharge zone identified in the previous study.

Meromictic lakes are lakes that do not have intermixed layers, and thus have unique physical, chemical, and biological properties. Lake Oigon is one lake with such a meromictic nature. However, a detailed study of the physico-chemical characteristics of Lake Oigon hasn’t been conducted. In this study, we investigated the physico-chemical characteristics of the water of Lake Oigon from the surface to bottom at one-meter depth intervals and compared parameter trends in summer and winter. The predominant hydrochemical type of lake water was Na-Cl-SO₄. The salinity of the lake’s water was mesosaline to hypersaline. The concentrations of major ions (except sulphate) and microelements in summer were lower than that in winter. The suboxic layers (chemocline), and anoxic layer (monimolimnion) were often rich in nitrate, sulphide, ammonium, and phosphate despite the season. We hope that this study will show the characteristics of this meromictic lake and provide underlying information for future research on the chemical composition of Lake Oigon.