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In conjunction with global warming, the frequency and intensity of typhoons in the Pearl River Estuary region are increasing annually, and coastal cities are becoming more vulnerable to compound disasters triggered by upstream flooding, typhoons, and astronomical tide surges, seriously threatening the lives and properties of people in the delta region. Quantitative analysis of the degree of variability of each factor associated with a compound flood event and accurate assessment of the magnitude of the compound flood event strongly impact the efficacy of coastal city responses aimed at disaster prevention and damage mitigation. In this study, focused on the Pearl River Estuary area, the tide levels at Sanzao station, wind speeds at Macao station, and daily maximum the maximum daily flow at Sanshui and Makou stations spanning 30 years from 1988 to 2017 were used to characterize the flood magnitude, typhoon magnitude, and astronomical tide magnitude of compound flood events in the Pearl River Estuary area. Data were screened using the annual maximum method. Based on the generalized additive model (GAMLSS) and the time-varying copula function, the encounter combinations of composite flood events of different levels and their main controlling factors were investigated. The results show that: 1) the main factors influencing initiation and magnitude of composite flood events in the Pearl River estuary area are non-consistent sequences that display interdependence. The time-varying copula model constructed in this study is more suitable for studies of composite floods than the traditional constant parameter copula model. Copula models are most applicable to studies of the encountered characteristics of multiple elements contributing to compound flood events and are most in line with actual situations. 2) Compared with the joint return period of traditional constant copula parameters, the period of occurrence of typical compound flood events in the Pearl River Estuary from 1988 to 2017 continuously shortens with time, the probability of occurrence increases, and the probability of future occurrence of extreme compound flood events is strengthening. The trend in which the return period of compound flood events is shortening with higher grades is accelerating. 3) Compound flood events throughout the Pearl River estuary area are more affected by typhoons and astronomical tides and less affected by water flow. When the return period of the three factors is one in 5 years, water flow causes the smallest increase in the co-occurrence return period; i.e., the influence of astronomical tide and typhoon on the scale of upstream flooding, composite flood events of typhoon, and astronomical tide is proportionately larger. Thus, composite flood events in the Pearl River estuary area are mainly impacted by typhoons with higher wind speeds. When the return period of the three factors is one in 10, 20, and 50 years, the increase in the co-occurrence return period caused by flow changes the most; and when the return period of the three factors is one in 10, 20, and 50 years, the acceleration of the co-occurrence return period caused by flow is the largest. When the three factors have a larger return period, astronomical tide exerts proportionately greater influence on the scale of the composite flood event of upstream flood, typhoon, and astronomical tide.

Submarine groundwater discharge (SGD) contributes to the physical and chemical characters of coastal waters by discharging nutrients and contaminants, significantly impacting regional marine ecosystems and contributing to ocean chemical budgets. However, such groundwater discharge varies dramatically across scales and is often not comparable due to different model assumptions and field designs. We used a hydrologic model with integration of fundamental surface and subsurface processes to simulate the coastline level fresh SGD for the Crete Island in the Mediterranean Sea. The modeled hydrological processes suggested that fresh SGD substantially contributes to water flow entering the Mediterranean Sea (2.3 × 108 m3/yr), amounting to 31% of river discharge and 14% of precipitation. Spatially, fresh SGD varied from 2.4 m3/yr/m to 13.4 × 104 m3/yr/m, with an average of 2.6 × 103 m3/yr/m. The local maxima were commonly associated with river mouths reflecting larger hydraulic gradients and higher permeable structures. Temporally, fresh SGD was impacted by episodic precipitation in a delayed and prolonged pattern. We found that fresh SGD variability at the coastline segment level was compared to point measurements and fresh SGD magnitudes summered up to the catchment level were consistent with global products. Our results suggest the coupled surface–subsurface hydrologic modeling approach is a promising strategy to quantify and partition large-scale water budgets down to point observations that typically do not capture the full range of fresh SGD dynamics.

Simple Summary: Fish passage facilities are crucial for restoring river connectivity and protecting ecosystems. This study quantitatively assessed the upstream migration of fish under various environmental conditions. In the Heishuihe River downstream of the Songxin Dam, 19 fish species were monitored, with 15 species reaching the fishway entrance and 12 successfully passing through. The entrance attraction and passage rates of the vertical-slot fishway at Songxin were 15.7% and 40.42%, respectively. The results indicate that May is the optimal period for fish migration, with better performance observed during nighttime than daytime. Optimal entrance attraction occurred at a flow rate of 6–7 m3/s and a temperature of 19–20 °C, while the best passage efficiency was achieved at a flow rate of 0–0.5 m3/s and a temperature of 17–20 °C. A multifactorial Cox regression model identified diurnal rhythms, release location, temperature, and flow rate as primary hindering factors, while body length and water level were found to be promoting factors. A nomogram was developed to predict the success rates of fishway attraction and passage based on these key factors. This study provides theoretical and data support for optimizing fishway operations and offers scientific insights into ecological restoration. Fish passage facilities are essential for restoring river connectivity and protecting ecosystems, effectively balancing economic and ecological benefits. Systematic and comprehensive monitoring, assessment, and optimized management are therefore crucial. This study quantitatively evaluated the entire upstream migration process of fish from the downstream river to the entrance and exit of the fishway and investigated the upstream movement patterns of fish under various environmental factors. A total of 19 fish species were monitored in the Heishuihe River downstream of the dam, with 15 species reaching the fishway entrance and 12 species successfully passing through it. The entrance attraction and passage rates of the vertical-slot fishway at the Songxin hydropower station were 15.7% and 40.42%, respectively. The best upstream performance was observed in May, with fish demonstrating better upstream timing and speed during nighttime compared to daytime. Specifically, the highest entrance attraction efficiency was recorded at a flow rate of 6–7 m3/s and a temperature of 19–20 °C, while the optimal passage efficiency was observed at a flow rate of 0–0.5 m3/s and a temperature of 17–20 °C. Additionally, a multifactorial Cox proportional hazards regression model was constructed to identify key factors influencing the probability of fishway entrance attraction and successful passage. The model elucidated the impact patterns of these key factors on fish upstream migration, ultimately generating an alignment diagram for prediction and control. This study provides a theoretical foundation and data support for developing optimized operational schedules for fishways. The findings offer a more comprehensive and systematic approach for monitoring and evaluating fish passage facilities, serving as a scientific basis for ecological restoration and fish conservation in this region and similar areas. [ABSTRACT FROM AUTHOR]

Understanding the transition from meteorological to hydrological drought is essential for accurately predicting hydrological droughts. However, the factors driving this transition are intricate, and a comprehensive understanding of how direct human activities influence this shift in drought is lacking. In this study, we initially explored the spatiotemporal correlation between the occurrence of meteorological and hydrological droughts. Subsequently, we formulated multiple hydrological replenishment scenarios using the soil and water assessment tool (SWAT) model to assess the environmental impact of the transition from meteorological to hydrological droughts. The Xijiang River Basin (XRB), the primary tributary of the Pearl River basin, was selected as the study area. Our results identified 91 meteorological droughts in the XRB, and only 66 hydrological droughts from 1978 to 2018. The transition rate from meteorological to hydrological drought demonstrated large spatial variability, with a basin‐average rate of 56% and the lowest transition rate of 45% in the headstream The transition from meteorological to hydrological drought was mostly rapid between November and December (~2 months), but can be prolonged during spring (about 3–5 months) and winter (around 7–9 months). Additionally, analysis of regeneration scenarios indicated that human activity has mitigated drought severity over recent decades. The primary driver affecting drought duration and frequency during the transition from meteorological to hydrological droughts shows conspicuous spatial disparities in densely populated areas. Although human activity significantly contributes to drought duration and severity compared to climate change during the transition in the headstream, its effects are more pronounced downstream in terms of drought duration and frequency. One plausible explanation is that increased water consumption downstream has considerably prolonged drought progression, whereas water management has counteractive effects on drought progression due to climate change in the headstream. Our findings offer valuable insights into the transition process from meteorological to hydrological droughts in the presence of extensive human activities. [ABSTRACT FROM AUTHOR]