Projected Changes in the Euro-Mediterranean Hydrological Extremes

Hydrological extreme events such as floods and drought are common in Ethiopia which eventually causes environmental hazards. Kulfo River is one of Southern Ethiopian Rift Valley Basin that has experienced flooding for years. Therefore, this study aimed characteristics of hydrological extremes (1985–2014) in the Kulfo River, which is important for effective drought and flood monitoring and early warning systems. The hydrological drought was assessed using the streamflow drought index (SDI). Flood frequency distribution (FFD) software package was deployed to determine the flood frequency curve of the Kulfo River. The goodness-of-fit test results showed that the Generalized Extreme Values (GEV) distribution was found the best-fit probability distribution model in the Kulfo River, while the results of SDI values showed that extreme drought events were observed in 1991, 1992, and 2014 with magnitudes ranging from − 2.04 to − 2.7, − 2.0 to − 2.3, and − 2.10 to − 2.24, respectively, which cause reduction of lake level, lowing of groundwater level, and decreased the amount of river flow. SDI value indicated 6-year drought duration has occurred with the relative frequency of 20% in the 3- and 6-month timescales. The flood frequency results show the lowest probability of having flood magnitude has affected the river morphology. The study provides valuable information for policy and decision makers to implement different adaptation and mitigation measures for extreme hydrological events in the Kulfo River.

There has been a paucity of information on trends in hydrological extremes. Based on daily precipitation and temperature and streamflow records in the arid region, northwest of China, the temporal and spatial characteristics of hydrological extremes were analyzed during 1960–2010. The results showed that: (1) The temperature extremes show patterns consistent with warming, with a large proportion of stations showing statistically significant trends. Warming trends in indices derived from daily minimum temperature are of greater magnitudes than those from maximum temperature. The decreases in frequency for cold extremes mainly occur in summer and autumn, while warm extremes show significant increases in frequency in autumn and winter; (2) Most precipitation indices exhibited increasing trends across the region, and the precipitation increase in the arid region was due to the increase in both precipitation frequency and intensity; (3) There were increasing trends for frequency of extreme hydrological events from 1901 to 2010, particularly post 1970s. Extreme hydrological events occurred mainly during summer. Based on the results obtained in this study, the frequency, magnitude, and intensity of extreme hydrological events are expected to increase over time; (4) Frequency distribution exhibited a general symmetry along the Tianshan Mountains, distributed evenly in Junggar Basin and Tarim Basin. More extremes were observed in the north-west than that in the southeast. The maximum incidence was in west Tianshan Mountains and generally decreased south-eastward.

This study investigates the impact of climate change on river systems within the Eastern Mediterranean Basin (EMB), utilizing the Mann-Kendall (MK) test, enhanced by Sen’s slope estimator (SSE) and Şen’s Innovative Trend Analysis (ITA) methods. The research focuses on hydrological changes,, particularly in streamflow trends, and examines their implications in the context of climate change and anthropogenic activities. The methodology involves a detailed analysis of hydro-meteorological series, including streamflow data from different observation stations. The research enhances the robustness of trend detection by applying advanced methodologies such as trend-preserving pre-whitening, which satisfies the serial independence requirement of the MK test. The study area comprises 10 sub-basins in the EMB, with a particular attention to the Göksu River and its tributaries. The results reveal significant declines in the annual streamflow values at several stations, demonstrating the considerable influence of climate and environmental changes on the basin's hydrology. These findings are further evaluated using ITA graphs, which offer a detailed analysis of the spatial and temporal variability in streamflow patterns. This research contributes to a better understanding of hydrological responses to climatic variability, providing vital information for water resource management and policies in regions undergoing significant environmental changes.

Diffuse agricultural pollution is a major driver of nutrient enrichment in European surface waters, and its impacts are expected to intensify as climate change alters the timing, magnitude, and intensity of precipitation events. The Po River basin, like many nutrient hotspots worldwide, exhibits a chronic excess of nitrogen (N) driven by the preferential export of highly mobile nitrate. Extreme precipitation events, however, can also mobilize less mobile nutrient pools through erosion-driven transport. Phosphorus (P), which preferentially accumulates in the solid phase due to its strong affinity for soil particles, is therefore mainly exported during high-flow conditions, leading to marked, event-driven shifts in nutrient stoichiometry. Yet, how these processes vary across basins with contrasting nutrient surpluses, land management, and physical settings remains poorly understood.In this study, we analyzed N and P export dynamics in two agricultural basins of the Po River watershed that experienced similar climatic anomalies but differ strongly in their biogeochemical and physical characteristics. The Chiese basin is characterized by high livestock density and a marked nutrient surplus, whereas the Volano basin exhibits lower livestock pressure and overall nutrient deficit. The two basins further contrast in soil permeability and topography, providing a natural experiment to investigate control mechanisms on nutrient export.Monthly samplings carried out at the basin outlets throughout 2024-2025 were combined with high-frequency autosampler measurements (every 3 hours) during hydrological extremes. Dissolved and particulate phosphorus, including its bioavailable fraction and particulate nitrogen, together with dissolved inorganic nitrogen species (NO3-, NO2-, NH4+), were quantified and linked to continuous discharge records.Both basins displayed elevated nutrient concentrations, with contrasting partitioning between dissolved and particulate forms linked to differences in hydrological functioning. Hydrological extremes exerted divergent controls on nutrient behaviour, resulting in dilution or strong mobilization responses. Event-specific concentration-discharge relationships varied with seasonality and rainfall intensity. Overall, extreme events induced variable but generally P-dominated shifts in nutrient stoichiometry, with short-lived pulses accounting for a substantial fraction of annual export and temporarily altering N:P ratios.

<p>Morocco experiences a semi-arid climate with both Mediterranean and Atlantic  influences, causing a strong variability of rainfall. It has also high mountain ranges from North to South, separating vast regions with contrasted climatic and hydrologic conditions. In this context, Morocco is highly vulnerable to extreme hydrological events, such as floods and extended drought periods, impacting the population and the economic activities. In a global change context, there is a need to investigate whether these hydrological extremes are becoming stronger since an increased vulnerability has been observed in the last decades. Here, we analyzed long-term time series of daily flows from 17 basins located in the North (Mediterranean: Loukkos), in the center (Atlantic: Bourgreg, Oum Errabia) and in the South Western (Tensift, Souss, Massa, Draa) of Morocco. The objective is to evaluate the evolution of floods and low-flows in a regional perspective. For that purpose, statistical models for extreme values allowing non-stationarity are used in combination with trend-detection tests. The results showed increasing trends in maximum annual flows only at two stations in central Morocco, while decreasing trends in the north and south prevail. On the contrary, changes in low-flows and river intermittency are more widespread across the basins with contrasted climatic conditions.</p>

Study area – Vranov reservoir on the river Dyje lies in the South Moravian region in the Czech Republic. A multipurpose reservoir, Vranov, is a significant hydraulic structure whose primary purpose is water retention, including managing flood conditions. The reservoir itself and its surroundings are intensively used for recreation, and a hydroelectric power station has also been built there. In 1982, the reservoir began to be used as a source of drinking water. The objective of this study is to elaborate hydrological data – daily total precipitation and daily surface runoff – from 1981 to 2019, evaluate the influence of the Vranov water work, focusing on the hydrological extremes, and evaluate individual functions of this reservoir. The results showed an increase in the number of days of hydrological drought. The influence of the dam as a flood protection element is significant – in summer 2006, a peak of flow rate value of 551 m3 s−1 was transformed to 234 m3 s−1. The reservoir itself and its catchment area have multipurpose usage. Problems may arise in connection with extreme hydrological events when the reservoir cannot perform all its functions at the same time as it does at normal water levels and runoffs. Whether in times of flood or drought, it is necessary to resort to emergency manipulation of the reservoir and choose which of the purposes of the reservoir will be primary at that time.

This book focuses on climate change and hydrological extremes, i.e. droughts and floods, which are globally important natural hazards with associated costly impacts on society and the environment. Floods and droughts result from the superposition of different processes at various space and time scales: physical processes in the atmosphere, catchments, the river systems, and anthropogenic activities. However, the characteristics of hydrological extremes have been altered due to climate change and variability, such that approaches for their detection, attribution, and the frequency of occurrence need to be revisited as they are no longer stationary processes. For more accurate estimation of hydrological extremes under nonstationary and uncertain conditions, there is a need for holistic assessments. Time-frequency analysis, hydrological modeling, physical-cause analysis, multivariate statistical analysis, and uncertainty analysis are powerful tools for detecting, attributing, and making frequency analysis of nonstationary hydrological extremes in a changing climate. Both nonstationarity and uncertainty of frequency analysis of extreme hydrological events should be integrated to reveal the possible operational alternatives to the assumption of stationarity in hydrological extremes frequency analysis. In Focus–a book series that showcases the latest accomplishments in water research. Each book focuses on a specialist area with papers from top experts in the field. It aims to be a vehicle for in-depth understanding and inspire further conversations in the sector.

High-resolution records documenting the 4.2 ka event (from 4200 to 3900 cal yr BP) are scarce. Archaeological and paleoenvironmental data from continental archives suggest that this event is coincident with a megadrought in the eastern Mediterranean basin. Here, we present an unprecedented high-resolution alkenone-based sea surface temperature (SST) record from the northwestern Mediterranean Sea showing a stepwise cooling of ca. 2 °C between 4400 and 3800 yr BP, which shares similarities with speleothem records from central Europe and the Mediterranean region. Our data suggest a reorganization of the North Atlantic atmospheric circulation during this event.

Response of hydrological drought to meteorological drought in the eastern Mediterranean Basin of Turkey

Chapter 16 - Management of extreme hydrological events

This study examines the 1911–2010 variability and trends in annual streamflow at 139 sites across the Fraser River Basin (FRB) of British Columbia (BC), Canada. The Fraser River is the largest Canadian waterway flowing to the Pacific Ocean and is one of the world’s greatest salmon rivers. Our analyses reveal high runoff rates and low interannual variability in alpine and coastal rivers, and low runoff rates and high interannual variability in most streams in BC’s interior. The interannual variability in streamflow is also low in rivers such as the Adams, Chilko, Quesnel and Stuart where the principal salmon runs of the Fraser River occur. A trend analysis shows a spatially coherent signal with increasing interannual variability in streamflow across the FRB in recent decades, most notably in spring and summer. The upward trend in the coefficient of variation in annual runoff coincides with a period of near-normal annual runoff for the Fraser River at Hope. The interannual variability in streamflow is greater in regulated rather than natural systems; however, it is unclear whether it is predominantly flow regulation that leads to these observed differences. Environmental changes such as rising air temperatures, more frequent polarity changes in large-scale climate teleconnections such as El Niño-Southern Oscillation and Pacific Decadal Oscillation, and retreating glaciers may be contributing to the greater range in annual runoff fluctuations across the FRB. This has implications for ecological processes throughout the basin, for example affecting migrating and spawning salmon, a keystone species vital to First Nations communities as well as to commercial and recreational fisheries. To exemplify this linkage between variable flows and biological responses, the unusual FRB runoff anomalies observed in 2010 are discussed in the context of that year’s sockeye salmon run. As the climate continues to warm, greater variability in annual streamflow, and hence in hydrological extremes, may influence ecological processes and human usage throughout the FRB in the 21st century.

In the year 2013, the Northwestern Himalaya witnessed an extreme hydrological event that severely affected the upper Ganga Basin. The unexpected high sediment mobilisation in the basin affected the human population, infrastructure, and ecology in several reaches of the Ganga river and its tributaries. In an exceedingly affected reach, the Kedarnath valley, a considerable sediment volume (~ 26243 x 104 m3) was mobilised during the 2013 extreme hydrological event. In the Srinagar valley, up to 50 x 104 m3 of legacy sediment was washed away during the same event. This sediment mobilisation generated sediment pulses in the affected region, which is assessed through remote sensing, field observations and existing literature, in the Kedarnath and Srinagar valleys. Results show that the evolution of translative, dispersive, and their combined sediment pulse pattern in both selected reaches of the upper Ganga Basin. The mountainous region of the upper Ganga basin has very few monitoring stations observing sediment movement, and data from such stations are mostly unavailable due to which our understanding of the sediment movement in the Himalaya remains very poor. Therefore, it is suggested that there is a need for detailed studies of the sediment transport mechanism in the Himalayan river basins especially during such extreme hydrological events when large volume of sediment is mobilised.

<p>Hydrological extremes can be particularly impactful in populated areas where they are modulated by human intervention such as reservoir regulation. However, little is known about how reservoir regulation affects hydrological extremes, particularly at a regional scale. Here, I present a large data set of natural and regulated catchment pairs in the United States and assess how reservoir regulation affects local and regional drought and flood characteristics. I show that (1) reservoir regulation affects drought and flood hazard at a local scale by reducing severity (i.e. intensity/magnitude and deficit/volume) but increasing duration; (2) regulation affects regional hazard by reducing spatial flood connectedness in winter and by increasing spatial drought connectedness in summer; (3) the local alleviation effect is only weakly affected by reservoir purpose for both droughts and floods. I conclude that both local and regional flood and drought characteristics are substantially modulated by reservoir regulation, an aspect that should neither be neglected in hazard nor climate impact assessments.</p>

In the context of rapid global population growth and extensive economic development, urbanization is expanding rapidly. The expansion of urbanization brings about increasingly complex challenges for cities, and flooding is one of the disasters faced. Climate change has led to a significant increase in extreme hydrological events, particularly a sharp rise in rainfall intensity, further elevating the risk of flooding in low-lying urban areas. The study area is located in Taipei City, characterized by low-lying terrain surrounded by mountains, and is influenced by subtropical climate. The frequent occurrence of heavy rainfall during the monsoon season and typhoons contributes to frequent flooding events, with the additional impact of climate change increasing the risk of intense rainfall. Therefore, the real-time prediction of regional flooding and its application in urban management becomes an imperative task, aiding in early warning, effective flood risk response, and ensuring sustainable urban development. This study utilizes the Double-Encoder Transformer model for real-time flood forecasting leveraging dual-encoder architecture to process and analyze diverse data types relevant predicting floods. One encoder could be dedicated to interpreting meteorological data, such as rainfall spatial distribution. This encoder focuses on extracting and understanding the complex patterns in weather-related data, which are crucial for predicting the likelihood of flooding. The second encoder, on the other hand, could handle geographical and environmental data, including terrain topology, and land use patterns. This encoder is adept at understanding how environmental factors contribute to flood risk in specific areas. By concurrently processing these two streams of information, the Double-Encoder Transformer can create a more comprehensive prediction model. It can identify correlations between meteorological conditions and environmental responses, leading to more accurate and timely flood forecasts. This approach enhances the model's ability to predict not only when and where floods might occur but also their potential severity, aiding in disaster preparedness and resource allocation. Overall, the application of the Double-Encoder Transformer in flood forecasting represents a significant advancement in disaster management, leveraging AI's power to integrate and analyze complex, multi-faceted data for better, more informed decision-making in critical situations.

Abstract. Future changes in river runoff will impact many sectors such as agriculture, energy production, or ecosystems. Here, we study changes in the seasonality, frequency, and magnitude of moderate low and high flows and their time of emergence. The time of emergence indicates the timing of significant changes in the flow magnitudes. Daily runoff is simulated for 93 Swiss catchments for the period 1981–2099 under Representative Concentration Pathway 8.5 with 20 climate model chains from the most recent transient Swiss Climate Change Scenarios. In the present climate, annual low flows typically occur in the summer half-year in lower-lying catchments (<1500 m a.s.l.) and in the winter half-year in Alpine catchments (>1500 m a.s.l.). By the end of the 21st century, annual low flows are projected to occur in late summer and early autumn in most catchments. This indicates that decreasing precipitation and increasing evapotranspiration in summer and autumn exceed the water contributions from other processes such as snowmelt and glacier melt. In lower-lying catchments, the frequency of annual low flows increases, but their magnitude decreases and becomes more severe. In Alpine catchments, annual low flows occur less often and their magnitude increases. The magnitude of seasonal low flows is projected to decrease in the summer half-year in most catchments and to increase in the winter half-year in Alpine catchments. Early time of emergence is found for annual low flows in Alpine catchments in the 21st century due to early changes in low flows in the winter half-year. In lower-lying catchments, significant changes in low flows emerge later in the century. Annual high flows occur today in lower-lying catchments in the winter half-year and in Alpine catchments in the summer half-year. Climate change will change this seasonality mainly in Alpine catchments with a shift towards earlier seasonality in summer due to the reduced contribution of snowmelt and glacier melt in summer. Annual high flows tend to occur more frequent, and their magnitude increases in most catchments except some Alpine catchments. The magnitude of seasonal high flows in most catchments is projected to increase in the winter half-year and to decrease in the summer half-year. However, the climate model agreement on the sign of change in moderate high flows is weak.