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We examined the responses of suspended solids to forest thinning in steep small headwater catchments, PT (0.8 ha) and PR (0.7 ha), that drain a Korean pine (Pinus koraiensis) plantation forest. Based on a paired-catchment design, the relationship between total suspended solids (TSS) and the time differential of water runoff (dQ/dt) indicated a difference in the characteristics of TSS in the rising and falling stages within the initial two years after forest thinning. The relatively high initial TSS responded to the concentration-based first flush criterion in the early stage of the rainfall event concentrated in this initial period after the thinning. The rate of TSS event loads in the PT catchment was 4.3-fold greater than that in the PR catchment within the initial two years after forest thinning. This was induced by the low disturbance of soil surface by forest workers using chainsaws and non-heavy machinery. Three years later, the TSS event loads in the PT catchment appeared to decrease due to trapping and settling by protective vegetation. Therefore, mitigating accelerated TSS events during forest thinning requires appropriate site-specific land preparation, particularly for improving stream water quality in forested catchments. [ABSTRACT FROM AUTHOR]

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 210Pbex, 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 210Pbex 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. [ABSTRACT FROM AUTHOR]

Accurate modeling of flood flow hydrographs for small forested catchments in steep mountainous terrain is challenging because of large errors in the estimation of response time using existing empirical equations. The time of concentration (TC) for a catchment is a widely used time parameter for estimating peak discharges in hydrological designs. In this study, we developed an estimated TC using readily available mountain catchment variables, a small catchment, steep slope, and narrow valley, using empirical equations. For our approach, we used directly measured data from 39 forested catchments (area: 0.02–9.69 km2) during 3648 observed rainfall events over a 10-year observation period. Based on the uncertainties inherent in the empirical equation, the estimated TC values were compared and analyzed through multiple regression and two different modified empirical modelling equations using our observed catchment parameters. The mean TC was significantly correlated with catchment size and stream length but negatively correlated with stream slope (p < 0.01). As a result, the mean TC estimated using the three modelling equations with catchment variables was qualitatively similar and had relative differences ranging from −12.5 to 15.5 min (−49 to 56%). Therefore, the models (particularly modeling equations with multiple regression, a modified empirical formula, and modified SCS Lag) can efficiently determine the TC and can be used in any small forested catchment in steep mountainous terrain. [ABSTRACT FROM AUTHOR]

Event flow characteristics were evaluated based on temperature and level of stream water in 22 forested catchments (area: 13.2–281.4 ha) to investigate sustainable flood management measures. Temperature and stream water levels were during 346 rainfall events in the summer season (July–September) from 2020 to 2022. Rising stream water levels responded to falling stream water temperature between ≤100 and >100 ha forested catchments in two types of time of concentration. Stream water temperature decreased by 3.0 °C when the stream water level increased by up to 0.9 m during rainfall events. Falling stream water temperature at two types of time of concentration was negatively correlated with total precipitation and rising stream water level. Based on the relatively high value of regression and cumulative frequency distribution, the estimated rising stream water level was appropriate in small catchments (≤100 ha) when the stream water temperature decreased, and the stream water level increased during rainfall events. Rising stream water levels and falling stream water temperatures are responses to catchment-scale effects, which are influenced by the nature and rapidity of the hydrological responses. Therefore, the results of the present study indicate that spatial and temporal differences in thermal responses of stream water temperature to water levels were controlled by catchment-scale effects under rapidly changing rainfall. [ABSTRACT FROM AUTHOR]

The severity and incidence of flash floods are increasing in forested regions, causing significant harm to residents and the environment. Consequently, accurate estimation of flood peaks is crucial. As conventional physically based prediction models reflect the traits of only a small number of areas, applying them in ungauged catchments is challenging. The interrelationship between catchment characteristics and flood features to estimate flood peaks in ungauged areas remains underexplored, and evaluation standards for the appropriate number of flood events to include during data collection to ensure effective flood peak prediction have not been established. Therefore, we developed a machine-learning predictive model for flood peaks in ungauged areas and determined the minimum number of flood events required for effective prediction. We employed rainfall-runoff data and catchment characteristics for estimating flood peaks. The applicability of the machine learning model for ungauged areas was confirmed by the high predictive performance. Even with the addition of rainfall-runoff data from ungauged areas, the predictive performance did not significantly improve when sufficient flood data were used as input data. This criterion could facilitate the determination of the minimum number of flood events for developing adequate flood peak predictive models. [ABSTRACT FROM AUTHOR]

Changes in water quality were examined during selected rainfall events in a headwater stream severely damaged by a forest fire on 21–23 February 2021. Seven water quality parameters were analyzed: pH, electrical conductivity, biochemical oxygen demand, chemical oxygen demand, total suspended solids (TSS), total nitrogen, and total phosphorous. First-flush effect and event mean concentration analyses were conducted in spring (dry season) and summer (wet season) immediately after the forest fire. In spring, the non-biodegradable organic matter concentrations increased along with pollutants related to ash-enhanced soil water repellency in water affected by first-flush effects. In summer, TSS and nutrient concentrations increased along with pollutants related to fire-induced soil surface disturbances after a series of rainfall events. First-flush analyses indicated that cumulative pollutant loads were greater in the spring than in the summer due to a higher concentration of forest fire ash in the early storm runoff immediately after the forest fire. The event mean concentrations revealed that pollutant loads were associated with both forest fire ash and storm events in the immediate post-fire period. Therefore, this study indicated that water quality along a headwater stream is affected by short-term effects of large and intense forest fires and rainfall seasonality. [ABSTRACT FROM AUTHOR]

We examined changes in suspended-sediment yields (SSY) after a 50 % strip thinning in headwater streams draining a Japanese cedar ( Cryptomeria japonica) and cypress ( Chamaecyparis obtusa) plantation forest. We applied a paired-catchment analysis to treated ( K: 17.1 ha) and control ( K: 8.9 ha) catchments. Annual suspended-sediment yield (SSY) in the prethinning period in the K and K catchments was 110.0 and 142.1 kg/ha per year. For the postthinning period, SSY in the K catchment became 5055.6 kg/ha per year, whereas that in the K catchment increased 893.2 kg/ha per year. The paired-catchment analysis revealed that SSY in the K catchment increased 17.0-fold compared with the prethinning period. However, the dominant hysteresis pattern remained clockwise in both pre- and postthinning periods. Sequences of large storm events in the postthinning period elevated SSY in both catchments. Increase in suspended sediment in the K catchment was associated with the combined impacts of thinning and sequences of storm events during the period of thinning operation. [ABSTRACT FROM AUTHOR]

Nutrient concentrations in stream water, rainfall, throughfall, stemflow, surface flow and ground water were compared before, during and after strip thinning (intensive 50%) in plantation forested watersheds in Tochigi, Japan. Influences were evaluated comparing four thinning-applied and two reference basins for 1 year before, 6 months during and 1 year after the thinning. Results show that this strip thinning significantly increased dissolved total phosphorus, total phosphorus and dissolved organic carbon (0.01, 0.04 and 0.53 mg l-1, respectively) during the thinning period and dissolved total nitrogen and total nitrogen (0.34 and 0.46 mg l-1, respectively) after the thinning in stream waters relative to the unthinned basins. The increased phosphorus during thinning indicated ground disturbances by the strip thinning, with a concomitant increase in dissolved organic carbon. Changes in biotic and abiotic conditions resulted in increased nitrogen after the thinning, particularly in the dissolved pool. Changes in hydrological processes due to thinning, for example, a change in flow distributions (less high nutrient stemflow and more low nutrient throughfall) and an increase in water discharge in stream water, possibly weakened the direct influences of thinning on nutrient concentrations. [ABSTRACT FROM AUTHOR]