Chia-Chi Huang

Streamflow is traditionally regarded as the superposition of binary or a limited number of discrete components. Under this framework, the Baseflow Index (BFI) is a widely used hydrological signature for characterizing subsurface water and catchment behavior. However, streamflow generation is inherently a continuous, spectrum-like delayed response across multiple timescales, rendering such discrete representations a methodological simplification. Building on the Characteristic Delay Curves (CDCs), this study finds that a mixed Weibull cumulative distribution function (CDF) effectively fits CDCs. Drawing on interpretations of the Weibull distribution from Reliability and Survival Analysis, we propose three parameters within the CDCs: the ratio of baseline, the delayed discharge mode (corresponding to the Weibull shape parameter), and the characteristic delayed time (corresponding to the Weibull scale parameter), to characterize streamflow generation mechanisms and catchment behavior. This study examines the relationships and causal structure among these parameters using long-term streamflow records from 60 catchments in Taiwan and 671 catchments in the United Kingdom. The results indicate that characteristic delayed time acts as a common driver of both the ratio of baseline and the delayed discharge mode, while the latter two exhibit weak mutual dependence. Time- and frequency-domain analyses further demonstrate that the proposed parameters better discriminate streamflow dynamic regimes than the BFI. Overall, this study provides a continuous compositional framework for interpreting streamflow hydrographs and establishes a Weibull-based foundation for advancing theories of streamflow generation and aquifer drainage.

Streamflow represents the hydrological output behavior of the catchment system and can elucidate the physical processes of other hydrological variables, such as rainfall–runoff processes and storage–discharge dynamics. Understanding streamflow dynamics not only enhances comprehension of complex hydrological processes and influencing factors, but also aids in estimating potential hydrological conditions in ungauged areas. In this study, we explored the differences in the flow duration curve (FDC) structure of streamflow components, ranging from slow to fast, using multiple hydrograph separation. Additionally, we analyzed the recession index and recession parameters of individual recession segments to characterize the storage-discharge dynamics based on the linear and nonlinear reservoir assumptions, respectively. We applied an analytical probabilistic streamflow model to determine which structure better aligns with the model’s physical basis, assuming streamflow generation from groundwater discharge when a sequence of rainfall events increases soil moisture beyond the retention capacity. It also provides estimations of optimal recession parameters for comparison with individual recession segment results. The recession analyses and multiple streamflow components separation revealed differences in dominant recession index, recession parameters, and streamflow complexity between catchments, highlighting their relationships with catchment characteristics. Recession parameters from FDCs with different components demonstrated the storage–discharge mechanisms associated with changes in streamflow components. The conformity of multiple streamflow component structures to the model’s basic assumptions can be evaluated through the model performance, contributing to an understanding of streamflow component structures in catchments and their relevance to specific streamflow generation mechanisms.

Understanding drought evolution and its driving factors is crucial for effective water resource management and forecasting. This study enhances the analysis of drought probability by constructing bivariate distributions, providing a more realistic perspective than single-characteristic approaches. Additionally, a meteorological drought migration model is established to explore spatiotemporal paths and related characteristics of major drought events in the Choushui River alluvial fan. The results reveal a significant increase in the probability of southward-moving drought events after 1981. Before 1981, drought paths were diverse, while after 1981, these paths became remarkably similar, following a trajectory from north to south. This is primarily attributed to the higher rainfall in the northern region of the Choushui River alluvial fan from February to April, leading to a consistent southward movement of drought centroids. This study proposes that climate change is a primary factor influencing changes in the spatiotemporal paths of drought. It implies that changes in rainfall patterns and climate conditions can be discerned through the meteorological drought migration model. As a result, it provides the potential for simplifying drought-monitoring methods. These research findings provide further insight into the dynamic process of drought in the Choushui River alluvial fan and serve as valuable references for future water resource management.

Owing to active orogenic movement and the monsoon climate, rainfall-induced landslide disasters often occur in Taiwan. Hence, hillslope hydrology and stability have received considerable research attention. However, it remains difficult to accurately estimate the duration and consequences of hillslope instability induced by hillslope hydrology. Research on hillslope hydrology and stability is complicated by spatial heterogeneity, hydrological processes operating at various scales, spatiotemporal evolution, and geomorphological properties. Recent advances in critical zone science have provided an approach to extend geoscience studies. The “deep coupling” concept is essential for integrating physical, chemical, and biological processes on various spatiotemporal scales and for providing a macro and unified framework for evaluating internal properties and processes. Critical zone science and hillslope hydrology and stability both depend on interdisciplinary perspectives and approaches, monitoring strategies, and model analysis of integrating and coupling processes. They both share the characteristics of spatial heterogeneity, continuous evolution, and relevance to ecosystem services. To address the challenges related to hillslope hydrology and stability in Taiwan, we reviewed the progress in, relevance between, and common challenges to hillslope hydrology, stability, and critical zone science. We then presented a process-based integrated monitoring strategy, an interdisciplinary perspective, and a coupling analysis framework and model. The aim of this study was to promote the advancement of research on hillslope stability and hydrology in Taiwan.

Baseflow is among the most important components of streamflow. It is the main source of streamflow from groundwater systems in the dry season and also plays an important role as a water resource in the ecological environment and for human activities. In recent years, because of climate change, the number of dry season days in Taiwan has increased, and the wet season has been delayed, resulting in fewer rainy days and increased precipitation intensity. In addition, the spatial distribution of rainfall is uneven, and rivers are short and fast-flowing. Taiwan has become a country with abundant rainfall but insufficient water resources; therefore, the assessment of baseflow is important. This study selected eight basins with distinct wet and dry seasons in southern Taiwan as the study area. The baseflow characteristics and their relationships with climate features were assessed using time series analysis, baseflow attribution analysis, and wavelet analysis. The results showed that baseflow has an increasing trend; both precipitation and evapotranspiration have a significant positive correlation with baseflow, and the impact of precipitation is greater than that of evaporation. Sensitivity analysis showed that baseflow increases with increasing evaporation and precipitation; this behavior is related to the concentration of precipitation and evaporation in the wet season. Baseflow attribution analysis showed that the contribution of climate change to baseflow (75.0%) was larger than that of human activities (−2.9%), indicating that climate change was the main factor in the increase in baseflow. Wavelet analysis showed that both the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) are correlated with baseflow, where the PDO is more strongly correlated than the ENSO. The main timescales of the ENSO and PDO are a 4–8-year band and an 8-year band, respectively. The ENSO may have a timescale above the 8-year band, and the PDO exhibits periodic correlation changes at a 1-year band.

This study used streamflow recession analysis to assess changes in groundwater–streamflow relationships using streamflow data. The groundwater response to climate and Normalized Difference Vegetation Index (NDVI) changes was considered as a variable in the recession analysis. The results could thus be compared with the integrated impact to assess the consistency of these effects on the catchment discharge processes. The results showed that the recession characteristics are linked to regional topography and streamflow condition. Variation of storage–discharge sensitivities in most catchments was not linked to the variation of groundwater due to the lower capacity of groundwater contribution. The quantified impacts aided in assessing the relationship between climate, land cover, groundwater storage and discharge. This study provides a simple explanation of the effects of environmental coevolution on storage–discharge processes.

Climate factors and human activities are the leading causes of changes in the hydrological cycle. In addition to being an important part of the hydrological cycle, runoff is also an important indicator for assessing the amount of available water. Therefore, it is necessary to explore the reasons that have caused changes in runoff. In this study, the causes of runoff changes in the alluvial fan of the Choshui River from 1980–2018 are explored. Two simple methods, including a decomposition method based on the Budyko structure and a method based on climate elasticity, for which the necessary data are easy to obtain, are used to quantify the impact of climate factors and human activities on runoff changes. The results show that the runoff in the long term shows a significant transition point in 2003, where climate factors have contributed more than 90% of the change, while the influence of human activities on the changes in runoff appears to be relatively small. Moreover, the Budyko method and the Normalized Difference Vegetation Index (NDVI) show that the vegetation cover has decreased. In addition to providing a simple method to assess the causes of changes in runoff, this study also analyzes the causes of changes in the runoff of the alluvial fan of the Choshui River to provide a reference for water resource policy and land use management.

The complex interaction of climate change and human activities has led to significant changes in hydrological patterns, thus affecting the catchment hydrological processes around the world. This has resulted in increased water resource problems and conflicts between social development and environmental sustainability. Moreover, the groundwater resource is less concerned than others due to the challenges posed by groundwater and aquifer monitoring. Recession analysis is typically used to explore the groundwater-streamflow relationship with easily accessible streamflow data. The environmental impacts can be considered as a variable into the recession analysis to assess the changes in dynamic groundwater storage and storage-discharge relationships. Seventeen gauge stations in Southern Taiwan were selected as a case study to elucidate the spatial and temporal results under a comprehensive impact. In addition, the quantified environmental impacts and changes in vegetation coverage were compared to assess whether these changes were consistent and their effect on the catchment drainage processes. The results showed that the regional differences in low-flow recession characteristics and the dynamic storage indicated the local differences in the aquifer properties. Decreasing dynamic storage and increasing storage-discharge sensitivities in most catchments indicated a consistent change with quantified environmental impacts. This demonstrated that the environmental change led to more groundwater loss and a lower streamflow with a reduced flow variation. This study attempted to explore the storage-discharge dynamics caused by the overall environmental impact, and the quantified impact can help us to realize whether the groundwater storage and the susceptibility to baseflow increase or decrease in the catchment drainage behaviors. This provides a simple way to explain comprehensive effect on storage-discharge processes under the environmental co-evolution.

Streamflow recession usually exists between rainfall events, and the recession can be expressed as the power relationship between the streamflow and the recession rate. Under the assumption that groundwater drainage from the aquifer to the river channel, this method can inversely evaluate the basin-scale groundwater storage using observed streamflow. It has often been used to explore regional groundwater storage conditions and storage-discharge relationships, and even to estimate hydrogeological parameters. However, the groundwater storage generating the streamflow is only a component of the dynamic storage that includes streamflow and evapotranspiration dynamic, or other forms of mobile groundwater. In order to understand the mechanism of the aquifer dynamic storage responses under environmental changes (included climate change, human activities, etc.), this study used the analytic streamflow distribution model to optimize the estimation of recession parameters during the dry and wet seasons, and explored the parameters change over time and the relationship with seasonal evapotranspiration and basin wetness conditions. Combining the water balance methods, we also quantified the dynamic storage relevant to groundwater drainage and vegetation available water (i.e. storage insensitive to streamflow), respectively, to explore the hydrological response mechanism of aquifer dynamic storage in two seasons. The results showed that the difference in recession parameters in the dry and wet seasons is related to the basin wetness condition, and the evapotranspiration effect is relatively limited. In addition, the parameters change over time also indicated that the environmental change has gradually changed the streamflow recession mechanism. By comparing the response of dynamic storage components to rainfall events and evapotranspiration, this study also demonstrated that the variability in different forms of dynamic storages during each season, which is helpful for understanding the store and loss process of groundwater storage at the basin scale and improving the possibility for predicting the different environmental impacts on groundwater storage.

Sensitivity analysis of the hydrological behaviour of basins has mainly focused on the correlation between streamflow and climate, ignoring the uncertainty of future climate and not utilizing complex hydrological models. However, groundwater storage is affected by climatic change and human activities. The streamflow of many basins is primarily sourced from the natural discharge of aquifers in upstream regions. The correlation between streamflow and groundwater storage has not been thoroughly discussed. In this study, the storage–discharge sensitivity of 22 basins in Taiwan was investigated by means of daily streamflow and rainfall data obtained over more than 30 years. The relationship between storage and discharge variance was evaluated using low‐flow recession analysis and a water balance equation that ignores the influence of rainfall and evapotranspiration. Based on the obtained storage–discharge sensitivity, this study explored whether the water storage and discharge behaviour of the studied basins is susceptible to climate change or human activities and discusses the regional differences in storage–discharge sensitivity. The results showed that the average storage–discharge sensitivities were 0.056 and 0.162 mm⁻¹ in the northern and southern regions of Taiwan, respectively. In the central and eastern regions, the values were both 0.020 mm⁻¹. The storage–discharge sensitivity was very high in the southern region. The regional differences in storage–discharge sensitivity with similar climate conditions are primarily due to differences in aquifer properties. Based on the recession curve, other factors responsible for these differences include land utilization, land coverage, and rainfall patterns during dry and wet seasons. These factors lead to differences in groundwater recharge and thus to regional differences in storage–discharge sensitivity.