How landslide-triggering rainfall conditions vary with geology is unclear. The effects of three different geological conditions (gneiss, GN; granite, GR; sedimentary rock, SR) on variations in intensity–duration (I-D) conditions and rainfall characteristics responsible for initiating shallow landslides were examined using data from 476 landslides in South Korea from 1963 to 2018 and detailed statistical analyses. Results from quantile regression and one-way analysis of variance analyses clearly showed that impermeable SR slopes result in smaller critical rainfall than permeable GN and GR slopes do, indicating a relatively high occurrence exceedance probability and susceptibility to landslides in SR slope. These findings suggest that geological conditions, particularly the relatively high susceptibility of SR slopes, should be considered when establishing rainfall information-based landslide warning criteria for South Korea. Our findings can contribute to the assessment of landslide susceptibility and probability based on geological conditions; however, they should be further investigated through in situ observations.

We examined changes in water temperature in relation to air temperature in the two upper, two middle and lower reaches of the Han River basin, Republic of Korea. Air and water temperatures were monitored from 2020 to 2025, with analyses restricted to the May–October period of each year. T_W exhibited pronounced spatial variability, whereas T_A exhibited relatively low spatial variability. Hysteresis loops were analyzed separately for the warming (May–July) and cooling (August–October) phases using the air–water temperature relationship. Distinct phase-dependent patterns in air–water temperature differences were observed. Air–water temperature differences were greater in streams than in rivers, with values 4–5 °C higher during the warming phase and 4–6 °C lower during the cooling phase. Water temperature responses to air temperature showed phase-dependent spatial patterns, with regression slopes varying across stream and river systems and temperature elasticity decreasing from streams to rivers. Therefore, the results indicate that water temperature responses to air temperature vary both spatially and temporally, with sensitivity differing along the longitudinal gradient.

Identifying potential sources of suspended sediment (SS) in headwater catchments is crucial for water quality management. To differentiate these potential SS sources, we investigated the distribution of two fallout radionuclides (FRNs), ¹³⁷Cs and ²¹⁰Pb_ex, using gamma spectrometry along with soil organic matter (SOM) analysis in a headwater catchment with five potential SS sources: cultivated land, non-harvested forest floor, eroded hillslope, harvested forest floor, and stream bank. The ¹³⁷Cs and ²¹⁰Pb_ex concentrations and the SOM content were considerably higher in the harvested forest floor materials than in the other four potential SS source materials. FRN concentrations revealed distinct properties according to the type of potential SS sources. Specifically, the combination of FRNs (with the effect of SOM content removed) associated with the mineral fraction and SOM showed distinguishable differences among the potential SS sources, except for no difference between cultivated land and eroded hillslope. Therefore, SOM and FRNs, or their combination, can be effective indices to differentiate or trace potential SS sources on various land use/land cover types within a catchment. Further field tests will allow the tracing techniques that bind FRNs with SOM to contribute to understanding SS transport from non-point sources within a catchment.