Abstract. Our study explores the intricate relationships between new water (Fnew), young water (Fyw), and topographic influences within small catchments, utilizing the Height Above the Nearest Drainage (HAND) as a key metric. Analysis revealed that seepage water consistently contains higher fractions of young water than stream water, despite similar contributions of new water from precipitation, highlighting differential hydrological responses. The relationship between Fnew and Fyw exhibited varying slopes for seepage and stream water, suggesting distinct hydrological sources and behaviors, with seepage water reflecting a “flashy” system and stream water indicating a “damped” system. A nonlinear dynamic was observed between HAND and transit time measures, with a notable threshold at approximately 10 m of HAND, beyond which the contribution of young and new water to streamflow declines, indicating a shift in hydrological pathways. This threshold behavior emphasizes the complex influence of topography on water movement and age within catchments. This research bridges gaps in our understanding of catchment hydrology, offering insights into the spatial and temporal dynamics of water movement and the pivotal role of landscape features in shaping hydrological responses.

Streamflow recession, shaped by hydrological processes, runoff dynamics, and catchment storage, is heavily influenced by landscape structure and rainstorm characteristics. However, our understanding of how recession relates to landscape structure and rainstorm characteristics remains inconsistent, with limited research examining their combined impact. This study examines this interplay in shaping recession responses upon 291 sets of recession parameters obtained through the decorrelation process. The data originate from 19 subtropical mountainous rivers and cover events with a wide spectrum of rainfall amounts. Key findings indicate that the recession coefficient (a) increases while the exponent (b) decreases with the L/G ratio (the median of ratios between flow-path length and gradient), suggesting that longer and gentler hillslopes facilitate flow accumulation and aquifer connectivity, ultimately reducing nonlinearity. Additionally, in large catchments, the exponent (b) increases with increasing rainfall due to greater landscape heterogeneity. Conversely, in small catchments, it declines with rainfall, indicating that these catchments have less landscape heterogeneity and thus reduced runoff heterogeneity. Our findings underscore the necessity for further validation of how L/G and drainage area regulate recession responses to varying rainfall levels across diverse regions.

Vegetation growth is sensitive to climatic variations which has a critical implication for hydrological regimes. However, the intertwined associations of climate-phenology-hydrology have rarely been explored in tropical/subtropical regions particularly. In this study, we synthesize hydroclimate records in forested watershed, central Taiwan for last five decades (1975-2020), and the results indicate that the incidences of meteorological and hydrological droughts are becoming prominent after 2001. We further examine the influences of temperature and precipitation on vegetation growth of watershed scale using EVI (enhanced vegetation index) derived from MODIS (Moderate Resolution Imaging Spectroradiometer) at monthly scale, and explore the effects of seasonal precipitation on the variations of landscape phenology and following watershed streamflow between 2001 and 2020. The EVI and temperature shows a linear relationship (R2 = 0.50, p < 0.001) without time-lag effect, whereas EVI and precipitation exhibits a log-linear relationship with two months lag (R2 = 0.40, p < 0.001), showing the accumulative rainfall during relatively dry period (winter-spring) is crucial for vegetation growth. Structural equation modeling reveals that earlier start of growing season (SOS) caused by relatively high spring rainfall (February-March) leads to longer growing season (LOS) and higher P-Q deficit (precipitation minus runoff) during the growing season. Nevertheless, the large amount of precipitation during growing season has no effect on the end of growing season (EOS), LOS and P-Q deficit. Realizing the vegetation growth responding to climatic variations is necessary for current and future hydrologic regime, especially under changing climate.

Upstream forested watersheds supply critical ecosystem services through providing clean freshwater and maintaining stable hydrological conditions. Responses of vegetation phenology to climatic variations have vital implications for hydrological regimes that are region-specific, but the associations of climate-phenology-hydrology have rarely been examined especially in tropical/subtropical regions. In this study, we utilized 46-year (1975–2020) hydroclimate records in forested watershed at central Taiwan, and showed that precipitation and streamflow anomalies and incidences of meteorological and hydrological droughts are becoming prominent after 2001. We further investigated the effects of monthly temperature and precipitation on vegetation growth using monthly EVI (enhanced vegetation index) of a watershed derived from MODIS (Moderate Resolution Imaging Spectroradiometer), and explored the effects of seasonal precipitation on the variations of vegetation phenological and subsequent watershed streamflow between 2001 and 2020. The EVI and temperature showed a linear relationship without time-lag effect (R2 = 0.50, p < 0.001), whereas EVI and precipitation exhibited a log-linear relationship with 2-month lag (R2 = 0.40, p < 0.001), indicating the accumulation of rainfall during relatively dry period was crucial for vegetation growth. Structural equation modeling revealed that earlier start of growing season (SOS) caused by relatively high spring rainfall (February-March) led to longer growing season (LOS) and higher P-Q deficit (precipitation minus runoff) during the growing season. Nevertheless, the large amount of precipitation during growing season has no effect on the end of growing season (EOS), LOS and P-Q deficit. Neither EOS has influence on LOS and P-Q deficit. Understanding the vegetation responses to climatic variations is required for future hydrologic regime projections, especially under changing climate.

Abstract. Streamflow recession discloses hydrological functioning, runoff dynamics, and storage status within catchments. Understanding recession response to landscape structure and rainstorms can be a guidance for assessing streamflow change under climate change. Yet, the documented response direction of recession is inconsistent and diverse. This study tested how landscape structure and rainstorms regulate the response direction. We derived recession rate, a, and nonlinearity, b, from power-law recession (-dQ/dt = aQb) in 19 subtropical catchments with a broad spectrum of 260 rainstorms. Results showed that the recession rate increases with the drainage density and L / G ratio (flow-path length over gradient), indicating that the catchments with the dense network or more short-and-gentle hillslopes would result in high rates. Apart from landscape structure, the rate surprisingly decreases with rainstorm amount. Probably because rainstorm facilitates connectivity in the saturated zones, which might conjoin more water from slow reservoirs and thus water drains slowly. Additionally, the recession nonlinearity increases with spatial heterogeneity (drainage area) but decreases with hillslope hydraulics (drainage density). The swing of response direction, which lies in the predominance between spatial heterogeneity and hillslope hydraulics, needs further clarification, particularly for regional recession assessment under climate changes. Incorrect response direction from landscape structure would lead to considerable bias inference.

The nutrient budget, the difference between the nutrient output via stream and input via precipitation, can provide insights into how environmental processes affect forested ecosystem biogeochemistry. In this study, field measurements of the nutrient budgets—including Na+, Cl−, K+, Mg2+, Ca2+, NO3−, and SO42−—of 19 sites were conducted in Feitsui Reservoir Watershed (FRW) of northeastern Taiwan. A series of power-law regressions were developed to establish the relationship of the nutrient budget to the discharge, nutrient input, agricultural land cover, and slope. The result show that the weekly nutrient budget is significantly affected by agricultural land and input via precipitation (R2 of regression models ≥ 0.90), yet the relationship varies among different nutrient elements. The agricultural land cover is the major factor, while the input via precipitation plays a relatively minor role in the budget of Cl−, Mg2+, Ca2+, and SO42−. These nutrients could be provisioned abundantly from the system, and thus the input via precipitation is not the predominant controlling factor. By contrast, the Na+ and K+ inputs via precipitation are indispensable for accurately estimating the riverine exports. Because weathering is a limited source of K+, the roles of agricultural activities and input via precipitation are likely decisive for transport. Besides, the NO3− budget reveals a strong interplay between the atmospheric input and agricultural land, as expected. Because the nutrient budget model of NO3− is strongly improved, the R2 changes from 0.34 to 0.99 when a larger coefficient in exponent term (10.2) for agricultural land cover (showing that NO3− export is strongly hydrologically controlled) and precipitation input are included. Our analysis is based on one year of data, so extrapolating the result to a long-term period should be done with caution, as there could be substantial inter-annual variation. The nutrient budget approach provides a preliminary assessment to evaluate the impacts of agriculture and atmospheric deposition on nutrient export, which can provide a precursory reference for watershed management for improving water quality and mitigating eutrophication.

Increasing anthropogenic nitrogen (N) emission via different pathways has shown prominent impact on aquatic ecosystems for decades, but the effects of interaction among climate-, landscape- and human-associated variables on riverine DIN (dissolved inorganic nitrogen, mainly NO3− and NH4+) export are unclear. In this study, the data of 43 watersheds with a wide range of climate-, landscape- and human-associated gradients across Taiwan were evaluated with partial redundancy analysis (pRDA) to examine their interactive controls on riverine DIN export. Results show that the annual riverine DIN export in Taiwan is approximately 3100 kg-N km−2 yr−1, spanning from 230 kg-N km−2 yr−1 in less disturbed watersheds (eastern and central Taiwan) to 10,000 kg-N km−2 yr−1 in watersheds with intensive human intervention (southwestern and northern Taiwan). NO3− is generally the single dominant form of DIN, while NH4+ renders significance in disturbed watersheds. Nearly all environmental variables display a positive correlation with DIN export, except for landscape setting variables (e.g., slope, area, channel length), which show a negative relationship. In terms of seasonal pattern, climate and human-landscape variables are related to NO3− export independently in the wet season, yet in the dry season climate-human variables jointly dominate NO3− export. Meanwhile, human-landscape (LH) variables (λ1 of LH > 0.60) control NH4+ exports in both seasons, and human-associated (H) variables (λ1 of H = 0.13) have a minor effect on NH4+ exports in dry season. Precisely, the contribution of controlling variables on DIN export vary with species and seasons, indicating water quality management could be time-dependent, which should be taken into consideration for designing mitigation strategies.

Event water transit time estimation has rarely been done for violent rainstorms (e.g., typhoons) in steep and fractured mountainous catchments where the range of transit time, potential controlling factors, and the validity of time-invariant parametrization are unclear. Characterized by steep landscape and torrential typhoon rainfall, Taiwan provides great opportunities for inquiring into the above questions. In this study, the hydrometrics and δ18O in rainwater and streamwater were sampled with a ~3-h interval for six typhoon events in two mesoscale catchments. The TRANSEP (transfer function hydrograph separation) model and global sensitivity analysis were applied for estimating mean transit time (MTTew) and fraction (Few) of event water and identifying the chronosequent parameter sensitivity. Results showed that the MTTew and Few varied from 2.0 to 11.0 h and from 0.2 to 0.8, respectively. Our MTTew in the mesoscale catchments is comparable with that in microscale catchments, showing a fast rainfall-runoff transfer in our steep catchments. The average rainfall intensity is a predominant indicator, which negatively affects the MTTew and positively affects the Few, likely activating preferential flow-paths and quickly transferring event water to the stream. Sensitivity analysis among inter- and intra-events demonstrates that parameter sensitivity is event-dependent and time-variant. A quick and massive subsurface flow without distinct mixing with groundwater would be triggered during large rainstorms, suggesting that time-variant parameterization should be particularly considered when estimating the MTTew in steep and fractured catchments at rainstorm scale.

typhoon rainstorms in fractured mountainous catchments: Implications for time-variant parameterization” by Lee et al., the authors aim to quantify temporal variability of transit times and event water fractions and to identify controls thereon in a Typhoon-dominated region. They do so by calibrating the TRANSEP model to individual events and then comparing the model parameters and outputs under different conditions. The overall objective of the study is, in principle, worthwhile and the presented data set very interesting. I nevertheless struggle to get enthusiastic about the analysis for several reasons:

A transfer-function hydrograph separation model is used to examine event water fraction and transit time of typhoon rainstorms in steep catchments in Taiwan. Few studies of isotope hydrograph separation that involve event water fractions and transit times in high rainfall, subtropical, and steep catchments have been reported in the literature. In addition to a novel catchment setting, a time-variable sensitivity analysis is used to infer processes and controls on event water fractions and characteristics of event water transit time distributions. The results are placed in context with data generated from literature values and show that rainfall intensity influences event water fractions

Abstract. Transit time with its indicative significance in regulating rainfall-runoff mechanism is a key factor for understanding many biogeochemical processes, but is rarely investigated in steep and fractured mountainous catchments. Mountainous catchments in Taiwan are characterized by active endogenic tectonics and exogenic typhoons and thus provide opportunities to explore the hydrodynamic systems over time. In this study, the hydrometrics and δ18O in rain and stream water were sampled by ~ 3-hour interval for six typhoon events in two mesoscale catchments. The TRANSEP (transfer function hydrograph separation model) and global sensitivity analysis was applied for estimating mean transit time (MTTew) and fraction (Few) of event water and identifying the chronosequent parameter sensitivity. Results show that TRANSEP could satisfactorily simulate the streamflow and δ18O change with the efficiency coefficients of from 0.85 to 0.97 and from 0.61 to 0.99, respectively. The MTTew and Few varied from 2 to 11 h and from 0.2 to 0.8, respectively. Our MTTew in the meso-scale catchments is similar with that in micro-scale catchments, showing a fast transfer in our steep catchments. The mean rainfall intensity which negatively controls on the MTTew and positively on the Few is a predominant indicator which likely activates preferential flow paths and quickly transfers event water to the stream. Sensitivity analysis among inter- and intra-events suggested that parameter sensitivity is event-depend and time-variant, affirming a nonlinear behavior in event water transfer function and time-variant parameterization should be particularly considered when estimating the MTTew in steep and fractured catchments.

Little is known about differences in species diversity among ecological communities subject to different levels of human-caused habitat transformation and how this disturbance contributes to diversity through symbiotic dependencies with the environment in freshwater ecosystems. We estimated α and β diversities of benthic macroinvertebrates and relationships between diversity and environmental variables in Ado River (natural) and Yasu River (intermediately disturbed) watersheds, Japan. Alpha diversity was consistently slightly higher in the natural river watershed than in the intermediately disturbed one, but the spatial distribution was not equivalent. The opposite pattern was found for β diversity. Significant differences in environmental variables existed between the two river watersheds, with especially high chlorophyll-a concentrations detected in the intermediately disturbed watershed. Alpha diversity was not correlated with specific environmental variables, whereas water temperature and chlorophyll-a concentrations were the two most significant environmental variables influencing β diversity across sites in the two watersheds. These results suggest that diversity patterns in freshwater benthic macroinvertebrates are differentially influenced by levels of human-caused habitat transformation, especially that intermediately disturbed habitats may benefit species turnover, and further understanding how they relate to environmental variables is essential for protecting local to regional diversity and can provide useful information for conservation planning to maximise biodiversity at the watershed scale.

Typhoons episodically trigger landslides and export large quantities of particulate and dissolved organic carbon (POC and DOC) to the ocean from high-standing islands of the western North Pacific. On landslide scars, new soils are forming and organic carbon (OC) re-accumulates; however, these young soils may remain vulnerable to recurrent erosion. Here, we examined soil aggregate breakdown and concomitant release of OC from soils on recovering landslide scars in Taiwan. We calculated the kinetic energies of falling raindrops during two past typhoon events that hit Taiwan, i.e. Morakot (2009) and Fanapi (2010), at 137 kJ m(-2) and 21 kJ m(-2), respectively. Soils from a chronosequence of recovering landslide scars (6 to 41 yrs) in the Central Mountain Range of Taiwan were subjected to ultrasonic vibrations simulating those typhoon energies. The younger soils of the chronosequence (< 12 yrs) displayed little aggregate stability. Our results show that it may take several decades before soil aggregate stability significantly improves to withstand energy inputs corresponding to more frequent, moderate typhoons such as Fanapi; however, even the soils from an undisturbed reference site showed almost complete aggregate breakdown when subjected to the simulated energy of a rare but severe typhoon such as Morakot. With increasing recovery age and vegetation change towards forest cover, OC was increasingly accumulated in the soil aggregates and efficiently protected against disruption by lower simulated typhoon energy; however, upon increased aggregate breakdown induced by high simulated typhoon energy, significantly more DOC and POC was released. In light of increasing frequency of typhoon events with global climate change, active reforestation can stabilize soil structure and thus enhance protection against recurrent erosion of the newly forming soils.

Devastating typhoons that induce enormous losses to various sectors of the economy underline the importance of an improved understanding of the regional hazard-to-loss relationship. This study utilizes the up-to-date loss data of typhoons in Taiwan from 2006 to 2015 to analyze the interannual variations in the annual aggregate losses (AALs) and develop a loss prediction model for the major administrative divisions. Return period analysis applied to the AALs identifies western-to-southwestern Taiwan as the high-risk region, among which Chiayi and Pingtung exhibit the highest 10-year AALs over 100 million. The gamma hurdle model (GHM) is adopted for loss prediction for its ability to stepwise model the loss occurrence and amount, leading to straightforward discussion regarding the explanatory power and statistical significance of meteorological predictors in their marginal and joint space. In the first part of the GHM, maximum daily rainfall and maximum gust wind are selected as the two most significant meteorological predictors for the logistic regression model of the loss occurrence, showing a remarkable model accuracy of \({\sim 0.9}\). In the second part of the GHM, maximum sustained wind is added to the gamma generalized linear model of the loss amount, generating the cross-validated Nash–Sutcliffe efficiency (mean absolute error) values higher (lower) than 0.6 (3 million) for several southwestern cities. Event assessment for Typhoons Soudelor (2015) and Morakot (2009) further demonstrates the utility of the GHM and illustrates the essential for accounting for the combination effect of rainfall and wind on loss estimation.

Over the last few decades, there have been startling advances in our understanding of climate system and in modelling techniques. However, the skill of seasonal climate prediction is still not enough to meet the various needs from industrial and public sectors. Therefore, there are tremendous on-going efforts to improve the skill of climate prediction in the seasonal to interannual time scales. Since seasonal to interannual climate variabilities in Korea and East Asia are influenced by many internal and external factors including East Asian monsoon, tropical ocean variability, and other atmospheric low-frequency variabilities, comprehensive understanding of these factors are essential for skillful seasonal climate prediction for Korea and East Asia. Also, there are newly suggested external factors providing additional prediction skill like soil moisture, snow, Arctic sea ice, and stratospheric variability, and techniques to realize skills from underlying potential predictability. In this review paper, we describe current status of seasonal climate prediction and future prospect for improving climate prediction over Korea and East Asia.

As a unique biomarker of terrigenous organic matter (OM), lignin has provided valuable information for tracing the sources of OM in land to ocean transfer. Oceanian small mountainous rivers (SMRs) are characterized by extremely high erosional rate and quick change in microclimate within watershed, which may potentially affect the distribution of soil OC and lignin concentrations and compositions. Bulk OC% and lignin were determined on surface soils and soil profiles from a Taiwanese SMR (Jhuoshuei River) and nearby region along a large altitudinal gradient (3–3176 m) to investigate the influence of microclimate on soil OC and lignin. Both surface soils OC% and lignin increased in higher altitude, suggesting higher preservation of OM in the cold region. Variations in lignin vegetation indices (S/V and C/V) in surface soils generally reflect the vegetation change in this river basin, and were more affected by precipitation seasonality than mean annual precipitation. Lignin concentration decreased with depth, along with a decrease in S/V and C/V and an increase in degradation indices ((Ad/Al)v and DHBA/V), reflecting a decreased input and/or biodegradation of lignin in subsoils. Our survey on soil lignin in Taiwan SMR provided the basis for utilizing lignin to trace the source of OC in land to ocean transfer as well as paleo-climate and paleo-vegetation reconstruction study in Taiwan SMRs.

Fluvial sediment export from small mountainous rivers in Oceania has global biogeochemical significance affecting the turnover rate and export of terrestrial carbon, which might be speeding up at the recognized conditions of increased rainfall intensity. In this study, the historical runoff and sediment export from 16 major rivers in Taiwan are investigated and separated into an early stage (1970-1989) and a recent stage (1990-2010) to illustrate the changes of both runoff and sediment export. The mean daily sediment export from Taiwan Island in the recent stage significantly increased by >80% with subtle increase in daily runoff, indicating more sediment being delivered to the ocean per unit of runoff in the recent stage. The medians of the runoff depth and sediment yield extremes (99.0-99.9 percentiles) among the 16 rivers increased by 6.5%-37% and 62%-94%, respectively, reflecting the disproportionately magnified response of sediment export to the increased runoff. Taiwan is facing increasing event rainfall intensity which has resulted in chain reactions on magnified runoff and sediment export responses. As the globe is warming, rainfall extremes, which are proved to be temperature-dependent, very likely intensify runoff and trigger more sediment associated hazards. Such impacts might occur globally because significant increases of high-intensity precipitation have been observed not only in Taiwan but over most land areas of the globe.

Runoff response to rainfall intensification under global warming is crucial, but is poorly discussed due to the limited data length and human alteration. Historical rainfall and runoff records in pristine catchments in Taiwan were investigated through trend analysis and cross temperature difference analysis. Trend analysis showed that both rainfall and runoff in the 99.9-percentile have been significantly increasing in terms of frequency and intensity over the past four decades. Cross temperature difference analysis quantified that the rainfall and runoff extremes (including the 99.0–99.9-percentiles) may increase by 69.5% and 99.8%, respectively, under a future scenario of 1 °C increase in temperature. This increase in intensity resembles the increase in intensity observed between 1971–1990 and 1991–2010. The amplified runoff response can be related to the limited catchment storage capacity being preoccupied by rainfall extremes. The quantified temperature effect on rainfall and runoff intensification can be a strong basis for designing scenarios, confirming and fusing GCMs’ results. In addition, the runoff amplification should be a warning for other regions with significant rainfall intensification. Appropriate strategies are indispensable and urgently needed to maintain and protect the development of societies.

28 The accuracy of streamflow composition simulated by different models has been rarely discussed. In 29 this study, total 23 flood events covering full rainfall spectrum were simulated by using HBV and 30 TOPMODEL. Simulated streamflow compositions were compared with hydrograph decomposed by 31 independent geochemical data via end-member mixing analysis (EMMA). Results showed that both 32 models gave satisfactory streamflow simulation in terms of the Nash efficiency coefficient, correlation 33 coefficient, and discharge volume. However, the modeled interflow and base flow behaved differently 34 with the changing storm intensity and duration. The HBV simulated base flow considerably increased 35 as the storm duration prolonged; by contrast, the TOP-derived base flow remained stable. On the other 36 hand, HBV prefers generating less interflow to percolate more to the base flow for fitting the stream 37 flow. Accordingly, HBV is more suitable for thin soil layer. We suggested that a proper model 38 selection should take the implicit environmental background into account for simulating reliable 39 streamflow composition. Compared with the EMMA-derived flows, both models showed a significant 40 time lag (2-4 hr). If EMMA-derived hydrograph is real, the modeled base flow responses are required 41 to speed up. Our model’s intercomparison against independent validation by geochemical data is a 42 good means of studying the model behaviors. The selection of a more appropriate hydrological model 43 should consider the characterization of the model structure and the watershed characteristics. 44 45 46

Typhoon rainfall characteristics over a mesoscale mountainous watershed (drainage area of 620 km2) located in eastern Taiwan were analyzed to fill the gaps in our knowledge concerning the linkage between typhoon track, rainfall patterns, and flood peak time. This study used spatially high‐resolution radar‐derived rainfall estimates from 38 storm events (∼2800 h) to investigate this linkage. The effect of spatial rainfall patterns on the timing of flood peak for the selected events was examined with the aid of a diffusive wave model. The results show that the typhoon rainfall was spatially aggregated and that the relative variations in the rainfall became smaller at higher rainfall rates. The maximum hourly rainfall was approximately twice the areal mean rainfall. Three major rainfall types were identified statistically, and different typhoon tracks appeared to have preferable rainfall types. This finding is presumably due to the interaction of the typhoon circulation and precipitation with the mountainous landscape. Flood lead times were derived for the different rainfall types, and it was found that differences in their lead times could be as large as ∼3 h over the studied mesoscale watershed. It is recommended that this empirical approach be incorporated into flood forecasting and warning systems.