Forest fires have increased over the last several decades in many regions. Quantifying the general patterns of frequency, areal extent, and seasonality is crucial for understanding fire dynamics. This study aimed to investigate whether the spatial and temporal trends in forest fires have changed across South Korea. The Mann–Kendall test and Sen’s slope estimation were used to analyze the temporal trends in forest fire statistics from 1991 to 2020. The spatial dispersion of fire activity was detected using a standard deviation ellipse and hotspot analysis. An average of 451 fires have occurred annually over the last 30 years, with a yearly increase of 5.82 fires. The burned area in April and May accounted for 80.7% of the annual burned area. The length of the fire season in 2006–2020 was 25 days longer than that in 1991–2005. The risk of large fires is increasing and becoming more concentrated in the northeastern region, such as the Gwangwon and Gyeongsangbuk Provinces of South Korea. Both climate change and forest recovery have led to South Korea becoming more prone to fires. However, forest fires are not burning more intensely nor charring more areas than they did previously. This is probably due to the implementation of surveillance and initial attack systems. Targeted forest fire suppression policies can help to effectively reduce the risk of forest fires in South Korea.

The water retention capacity of forest leaf litter was estimated through lysimeter measurements under field conditions. Six lysimeters were placed in Pinus koraiensis and Quercus acutissima forests and filled with the surrounding leaf litter to represent the effects of litter type on the water retention capacity. Two years of measurements for rainfall and litter weight have been conducted in all lysimeters at 30 min intervals. Field measurements showed that P. koraiensis litter stored more water during rainfall periods than did Q. acutissima litter. As a result, immediately after the cessation of rainfall, 1.82 mm and 3.00 mm of water were retained per unit mass of Q. acutissima and P. koraiensis litter, respectively. Following rainfall, after the gravitational flow had entirely drained, the remaining water adhered to the litter was estimated to be 1.66 ± 1.72 mm and 2.72 ± 2.82 mm per unit mass per rainfall event for Q. acutissima and P. koraiensis litter, respectively. During the study period, approximately 83.7% of incident rainfall drained into the uppermost soil layer below the Q. acutissima litter, whereas 84.5% of rainfall percolated through the P. koraiensis litter. The moisture depletion curves indicated that 50% of the water retained in the Q. acutissima and P. koraiensis litter was lost via evaporation within 27 h and 90 h after the cessation of rainfall, respectively. This study demonstrated the water retention storage of leaf litter and its contribution to the water balance over floor litter according to litter and rainfall characteristics. The results also proved that lysimetry is a reliable method to quantify the variation of litter moisture under natural conditions.

Fires can alter the hydraulic properties of burned soils through the consumption of organic matter on the ground surface. This study examined the effects of rainfall on the presence of soil pore clogging with varying ash layer thickness using laboratory rainfall simulator experiments. The image analysis with resin impregnation showed that rainfall impact caused plugging of soil pores at 22.2% with soil particles and 14.3% with ash particles on near surface soils (0–5 mm below). High rainfall intensities enhanced soil pore clogging by ash particles, particularly at shallow soil depths (0–10 mm). Ash deposits on the soil surface increased the water-absorbing capacity of ash-covered soils compared with that of bare soils. The rainfall simulation experiments also showed that ash cover led to a reduction in soil hydraulic conductivity, owing to the combined effects of surface crust formation and soil pore clogging. The complementary effects of soil pore clogging and water absorption by ash cover could hamper the accurate understanding of the soil hydrologic processes in burned soils.

Fires can burn canopy fuel and replace leafy crowns with charred branches and trunks, thereby affecting hydrological flow and water chemistry. However, little is known about the effects of fire on throughfall volumes and chemical fluxes in temperate forests. Therefore, we aimed to monitor the volume and chemistry of throughfall in pine trees (Pinus densiflora) damaged by the 2022 Gangneung-Donghae Forest fire in the Republic of Korea. Immediately after the forest fire, funnel-type measurements were performed to collect the throughfall beneath five trees at foliage necrosis and crown consumption sites. The amount of water that penetrated in a specified period was continually measured and analyzed in terms of the water quality components. Crown consumption resulted in the passage of more water due to the removal of the tree canopy; however, the ratio of throughfall to total rainfall remained constant as the rainfall amount increased. The throughfall volume was not significantly different owing to the fire damage. The solute concentrations of Ca and TOC at the crown consumption site were higher than those at the foliage necrosis site after the fire; however, no significant difference was observed three months after the fire. In this study, the changes in the amount and water quality of throughfall due to fire were examined over a relatively short period, providing fundamental data for nutrient cycling management of wildfire-damaged soil.

The presence of macroroughness elements directly affects the flow velocity in mountain headwater streams. Hydraulic roughness is the dominant resistance to flow caused by objects protruding into the water, but it is not measurable in the field. This study quantified the reach-average hydraulic roughness based on the channel morphology in two mountain streams. The average flow velocities of the reaches were measured using the dye-tracing method. The magnitude of the hydraulic roughness was derived from the grain size of the streambed materials (D₅₀ and D₈₄) and the cross-sectional/longitudinal bed roughness. The observations for low flows (0.04–0.43 m²/s discharge per unit width) indicated that the longitudinal 90% inter-percentile range ($IP{R}_{90}^L$) seemed to have considerable merit in examining the influences of large roughness elements on flow conveyance. A dimensionless hydraulic geometry relation that can reflect the field measurements over a limited range of hydraulic characteristics was also developed for estimating the reach-average flow velocity in steep and rough streams. Thus, the research framework used appears to provide a reliable method of quantifying reach-average hydraulic roughness from local data in mountain headwater streams.