Flood Regime Research Articles

Is Leaf Water-Repellency and Cuticle Roughness Linked to Flooding Regimes in Plants of Coastal Wetlands?

Owing to periodic inundation, wetland plants may have adapted to minimize leaf gas exchange impairments. Increasing the leaf cuticle surface roughness might contribute to buffer flooding stress, since it might increase both leaf water repellency (i.e. leaf hydrophobicity) and the presence of gas films. Thus, a higher leaf hydrophobicity, as well as an increased cuticle roughness, can be expected in species subject to continuous water-logging. We tested this hypothesis by: 1) measuring the contact angle in the liquid-solid interface on leaves of 52 plant species from three coastal wetlands with different diversity and flooding regime in Baja California, Mexico, and 2) by performing scanning electron microscopic analyses in the most hydrophilic and hydrophobic species. Contrary to our expectations, the highest values of leaf water repellency were found in species from non-flooded areas adjacent to the wetlands, whereas neutral leaf water repellency values were found in species subject to flooding, as indicated by the contact angle measurements (77 to 100°). The most water repellent species presented a striking layer of wax on top of the leaf cuticles, while the lest water repellent species were mostly glabrous, which suggest that the augmentation of wax covering in the cuticle might be responding to other environmental factors, such as irradiation.

Possible Increases in Flood Frequency Due to the Loss of Eastern Hemlock in the Northeastern United States: Observational Insights and Predicted Impacts

AbstractTrees exert a fundamental control on the hydrologic cycle, yet previous research is unclear about the nuanced relationship between forest cover and riverine flood frequency. In the Northeastern United States, warming air temperatures have resulted in a decline of Eastern Hemlock (EH), and subsequent increases in observed catchment water yield. We evaluated the possibility of EH loss leading to a changed flooding regime. We first investigated plant hydraulic regulation by root water uptake in EH and American Beech (AB; a candidate successional species) through stable isotope analysis of stream, soil water, and plant xylem water. EH xylem water showed evidence of deeper soil water uptake than AB during both wet and dry seasons, suggesting species succession may be an important mechanism for altering catchment “plant accessible water.” Next, we estimated catchment flood frequency with mechanistic hydrologic simulations for present conditions, and two hypothetical cases where all EH is succeeded by AB. The largest change to catchment extreme discharge after AB succession coincided with fall season tropical moisture export‐derived precipitation. We observed reduced sensitivity under future climatic forcing with an ensemble simulation of five localized constructed analogs downscaled general circulation models. Thus, the influence of forest composition on the flood regime may be most related to the temporal alignment of the synoptic‐scale processes that generate Atlantic Basin tropical cyclones and regional plant phenology of the Northeast United States. Our results provide a justification for using physically based hydrologic models incorporating plant hydraulic regulation when evaluating future flooding frequency.

Species-specific responses of wood growth to flooding and climate in floodplain forests in Central Germany

Abstract: With ongoing climate change, episodes of severe flooding are predicted to become more frequent despite a general trend towards increasing summer drought. We investigated how wood growth of adult trees of two species characteristic of floodplain forests in Central Germany (Fraxinus excelsior L., Quercus robur L.) and two less-typical species (Acer pseudoplatanus L., Carpinus betulus L.) responded to both maximum stream water level and climate, with a special focus on the effects of the extraordinary flood of the Saale River in June 2013 and the extreme drought in summer 1976. Tree-ring widths were measured on wood cores, and standardized ARSTAN tree-ring chronologies were produced. Using variance partitioning as well as linear mixed-effects models, we compared the effects of monthly values for maximum water level, temperature and precipitation sum on tree-ring width. Further, we calculated resistance, resilience and recovery of the tree species to the extreme events of flooding in 2013 and drought in 1976. Wood growth of all species studied, and particularly of F. excelsior, responded positively to the extraordinary flooding in June 2013. However, in the best models for the characteristic floodplain forest species (F. excelsior and Q. robur), mainly precipitation (F. excelsior) or a combination of precipitation and wood growth of the previous year (Q. robur) acted as drivers of wood growth of the current year. In contrast, growth of the less habitat-specific species (A. pseudoplatanus) mainly showed a significant response to the combination of temperature and wood growth of the previous year. C. betulus was the only species studied that benefited from the extreme drought in 1976. However, two years afterwards, only the wood growth of A. pseudoplatanus was still reduced, while F. excelsior and Q. robur fully recovered. In comparison to other regions in Central Europe, the moderate flood regime of the Saale River seems to have the potential to mitigate effects of summer drought in this region, which is one of the driest in Germany. Thus, increased flooding frequency might, to some degree, reduce drought effects brought about by climate change as well.

Changes in forest diversity over a chronosequence of fluvial islands

The high environmental heterogeneity of large fluvial systems is reflected by the co-existence of contrasting plant communities and landforms. The main objective of this study was to assess the forest diversity changes in islands of the Middle Paraná River (Argentina) in order to discuss an integrative question: how synchronized are the major changes in the features of islands and forests? Persistence age, elevation and flood regime of 11 main channel islands were determined. Variables related to the vascular plant community and the tree stand structure of forests were also measured in 400 m2 plots. Islands were classified as young or old (YIs or OIs), according to their persistence age, which ranged from two to 108 years. Both island classes differed in their elevation but not in the proportion of low water phase. Only three out of nine tree species were dominant: Tessaria integrifolia and Croton urucurana (restricted to YIs and OIs, respectively), and Salix humboldtiana (distributed in both island classes). Alpha diversity was positively correlated with the age of the YIs and reach the highest value in the oldest island forest. Beta diversity was mainly due to processes of species replacement which differentiate floodplain forests. Gamma diversity reached 101 species, being the perennial herbs a clear majority. The stand structure and the complete floristic composition were significantly different between YIs and OIs, with three and seven indicator species of each island class, respectively. Considering integrative models of succession, our findings suggest that the biogeomorphic phase, recognised by the fluvial biogeomorphic model, prevailed in the whole range of island persistence ages. Therefore, it seems that the increase in forest diversity in a large river is restricted to spatial refugia defined by major hydrogeomorphic shifts.

Change in Spatial Distribution Patterns and Regeneration of Populus euphratica under Different Surface Soil Salinity Conditions

Ecological conservation and restoration have increasingly captured attention worldwide due to the degradation of ecosystems. As one of the most ecologically fragile areas, the Tarim River Basin, of Xinjiang, China, encountered serious decline of desert riparian forests. The Chinese government has implemented the “Ecological Water Conveyance Project” (EWCP) since 2000, protecting and restoring the dominant natural species of the desert riparian forests, i.e., Populus euphratica Oliv. The regenerative effect after the water conveyance was noteworthy. For the purpose of clarifying the mechanism of P. euphratica forest regeneration to find a better prescription for the ecological restoration works in the Tarim River Basin, we investigated the relationship between the distribution of P. euphratica and soil salinity. Experimentally evaluated the effects of surface soil salinity on P. euphratica seed germination and the influence of river flooding on the salinity of surface soils. The results showed that (1) P. euphratica trees mainly spread along the river channel within 2 km; with increasing vertical distance to the channel, the number of trees declined significantly; (2) where the salinity of the surface soil is high, there are less living P. euphratica trees; (3) the germination of P. euphratica seeds decreases with increased soil conductivity; when the soil conductivity was higher than 7 ms/cm, the germination of P. euphratica seeds was severely curtailed. (4) Flooding regimes were a pre-condition of P. euphratica restoration; they had profound effects on improving the germination of the seeds via ameliorating water conditions and reducing salinity. Our results point out that the most efficient ecological prescription for restoring and protecting desert riparian forests is to induce flooding twice yearly during June to August with 10- to 15-day durations each time. Such a plan (especially in the Tarim River Basin) should prioritize the protection of seedlings.

Non-Stationary Bayesian Modeling of Annual Maximum Floods in a Changing Environment and Implications for Flood Management in the Kabul River Basin, Pakistan

Recent evidence of regional climate change associated with the intensification of human activities has led hydrologists to study a flood regime in a non-stationarity context. This study utilized a Bayesian framework with informed priors on shape parameter for a generalized extreme value (GEV) model for the estimation of design flood quantiles for “at site analysis” in a changing environment, and discussed its implications for flood management in the Kabul River basin (KRB), Pakistan. Initially, 29 study sites in the KRB were used to evaluate the annual maximum flood regime by applying the Mann–Kendall test. Stationary (without trend) and a non-stationary (with trend) Bayesian models for flood frequency estimation were used, and their results were compared using the corresponding flood frequency curves (FFCs), along with their uncertainty bounds. The results of trend analysis revealed significant positive trends for 27.6% of the gauges, and 10% showed significant negative trends at the significance level of 0.05. In addition to these, 6.9% of the gauges also represented significant positive trends at the significance level of 0.1, while the remaining stations displayed insignificant trends. The non-stationary Bayesian model was found to be reliable for study sites possessing a statistically significant trend at the significance level of 0.05, while the stationary Bayesian model overestimated or underestimated the flood hazard for these sites. Therefore, it is vital to consider the presence of non-stationarity for sustainable flood management under a changing environment in the KRB, which has a rich history of flooding. Furthermore, this study also states a regional shape parameter value of 0.26 for the KRB, which can be further used as an informed prior on shape parameter if the study site under consideration possesses the flood type “flash”. The synchronized appearance of a significant increase and decrease of trends within very close gauge stations is worth paying attention to. The present study, which considers non-stationarity in the flood regime, will provide a reference for hydrologists, water resource managers, planners, and decision makers.

Spatial and seasonal patterns of flood change across Brazil

ABSTRACTBrazil has some of the largest rivers in the world and has the second greatest flood loss potential among the emergent countries. Despite that, flood studies in this area are still scarce. In this paper, we used flood seasonality and trend analysis at the annual and seasonal scales in order to describe flood regimes and changes across the whole of Brazil in the period 1976–2015. We identified a strong seasonality of floods and a well-defined spatio-temporal pattern for flood occurrence. There are positive trends in the frequency and magnitude of floods in the North, South and parts of Southeast Brazil; and negative trends in the North-east and the remainder of Southeast Brazil. Trends in the magnitude (frequency) were predominant in the winter (summer). Overall, floods are becoming more frequent and intense in Brazilian regions characterized by wet conditions, and less frequent and intense in drier regions.

Ice-jam flood regime of the Peace-Athabasca Delta: Update in light of the 2014 event

Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover have been identified as the main agents of flooding and replenishment of the perched basins of Peace–Athabasca Delta, one of the world's largest inland freshwater deltas. The paucity of ice jamming in the lower Peace River following construction of the Bennett Dam (1968) and the potential impacts of climate change have raised concerns regarding habitat degradation. The most recent ice-jam flood, which occurred in 2014, was only the fifth in the post-regulation period and the first since 1997. Despite the sizeable number of years in the regulation period, conventional estimates of flood frequency are shown to be uncertain, but become robust when based on the slope of the cumulative number of floods. The evolution of ice breakup in 2014 was in full accord with previously postulated mechanisms for the flat reach of Peace River, which extends for several hundred kilometres above its mouth. An updated examination of historical hydrometric data at the WSC Peace Point gauge took into account the years 2003–2018, extending the record of the regulation period by 50%. The analysis further confirmed and fine-tuned physically based predictions of the effects of the freezeup level, the breakup flow, the thickness of the winter ice cover, and the accumulated degree-days of thaw on the incidence of ice-jam flooding. Commercially motivated reduction of freezeup flows in recent years likely contributed to the occurrence of the 2014 flood. Total winter snowfall at an index station appears to have reversed a pre-2000 decreasing trend, but its benefit to breakup flow may be moderated by increased incidence of mid-winter thaws.

Direct and indirect effects of flood regime on macroinvertebrate assemblages in a floodplain riverscape

AbstractFloods are necessary for the functioning of floodplain ecosystems. Ultimately, flood effects on biota are spatially complex and unfold at various time scales. Yet, biodiversity conservation in regulated floodplains requires a sound quantification of the main pathways how floods affect riverine ecosystems. Here, we investigated the relative effects of flood frequency, magnitude, and time elapsed since the last flood on macroinvertebrate communities of 24 parafluvial habitats in the Maggia river floodplain, Switzerland. Parafluvial habitats were sampled seasonally over 1 year, and a combination of 2D hydrodynamic modelling and piecewise structural equation modelling was used to quantify the direct and indirect effects of local hydrological metrics on macroinvertebrate communities. We found that high flood frequency favoured rheophilic species (EPT taxa) over other groups (Coleoptera, Diptera) and prevented strong competitive interactions and competitive exclusion by maintaining low food resource levels. Importantly, we found that parafluvial habitats exhibited more lentic‐like characteristics as time passed after a flood, which caused EPT taxa to be relatively more abundant and taxa richness higher immediately after flood disturbance. Lastly, overall taxa richness increased over time after a flood and even more so after larger floods. Our results suggest that variation in flood magnitude, frequency, and return period maximizes beta diversity within floodplain riverscapes due to the independent and spatially interactive responses of parafluvial habitats.

Enhancing bird diversity through improved water management in a Viet Nam wetland national park

U Minh Thuong National Park, one of the two largest remaining peat swamp, melaleuca forests in Viet Nam is a very Important Bird and Conservation Area because of its diverse wetland habitats. A devastating forest fire in 2002 resulted in a new regime of fire control based on an imposed permanent flooding regime over large areas of the Park that led to severe wetland and forest degradation. This in turn negatively impacted on bird species diversity and populations. In 2009, the Government of Viet Nam and the park authority adopted an improved water management practice. A survey four years later recorded two new species in the list of 159 bird species for the park. Of these, 25 threatened species were using the wetland park and adjacent Buffer Zone for living and feeding habitat. Based on the largest count occurring during three continuous days, the water bird population in the bird colony in 2013 increased by 33 per cent compared to the 2009 level. The positive response of avifauna to the changes in U Minh Thuong with respect to the hydrology of seasonally flooded melaleuca wetland forest indicates the need to understand how the ‘normal’ wetland processes operate in support of biodiversity conservation before modifying them for fire management.

The growth and nutrient uptake of invasive vines on contrasting riverbank soils

AbstractThis study sought to investigate the research question as to whether the growth and nutrient uptake of two invasive vines, Pueraria lobata and Sicyos angulatus, are affected by the heterogeneity of soil characteristics of two riverbank sites with different flooding regimes. Soil, individual ramets of P. lobata and S. angulatus plants were sampled monthly from quadrats set on homogenous stands from two riparian sites along Tama River, Japan for over a year. Soil nutrients, above‐ground (AGB) and below‐ground biomass (BGB), tissue nutrient, and nonstructural carbohydrate contents were estimated, and resource allocations to different organs were calculated. Flooding frequency directly affected the substrate characteristics of the sites; the frequently flooded site had coarser particle and less nutrient content. There were significant differences between the BGB and the AGB of both P. lobata and S. angulatus between the sites. However, the BGB:AGB ratio of these species were statistically similar regardless of the substrate conditions. Although the biomass of S. angulatus were much reduced in coarse habitat, the total amount of nutrient uptake by P. lobata was not affected by habitat the condition. Concentrations of total nitrogen, total phosphorus, and starch in root tissues of S. angulatus were less in frequently inundated soil. The results of this study suggest that inundation frequency directly affects the substrate condition of a riverbank habitat, which in turn affects plant growth, and invasive plant species growing in such habitat respond differently to substrate condition in terms of growth and nutrient uptake.

An improved method for paleoflood reconstruction and flooding phase identification, applied to the Meuse River in the Netherlands

This study investigates Holocene floodplain evolution and flooding phases as experienced in the Lower Meuse catchment, primarily based on grain-size distributions of channel-fill and floodplain deposits in sediment cores. The presence of post-depositional Fe[sbnd]Mn concretions and resistant organic particulate materials impedes the direct use of grain-size data. By combining end-member modelling results with laboratory observations, we have constructed a Flood Energy Index (FEI), which allows identification of phases of past increased flooding from the grain-size signal. Since concretions and organic-rich sediments regularly occur in floodplain sediments, we emphasize that the quality of a grain-size dataset should be assessed prior to its use for reconstruction of flood events. We suggest that the new approach has potential to become standardized within paleoflood research. The temporal variation of FEI in Meuse sediment cores highlights multi-centennial flooding phases occurring at c. 8500, c. 8000, c. 7600, c. 7000 and c. 5900 cal BP within the fluvio-lacustrine environment (early–middle Holocene). The record of low flood activity in the Subboreal is attributed to a cooler and dryer climate anomaly after the Holocene Climatic Optimum. In the late Holocene, the first flooding phase occurring at c. 2800 cal BP can be linked to the 2.8 ka climate anomaly. During the last two thousand years, the variation of FEI index reveals oscillating flood regimes in the Lower Meuse floodplain. The last three recorded flooding phases most likely coincide with the Roman Period (c. 12 BCE–250 CE), the Medieval Warm Period (c. 950–1400 CE) and the Little Ice Age (c. 1400–850 CE). Despite uncertainty in the age-model, the rapid accumulation rate and amplified flood magnitudes imply increased fluvial instability during the late Holocene, indicating that humans exerted a profound influence on fluvial dynamics in the Meuse. © 2019 Elsevier B.V.

Time series of flood mapping in the Mekong Delta using high resolution satellite images

The Mekong Delta, located in Southern Vietnam, is one of the most affected areas in the world by climate change and sea level rises, especially flooding. Therefore, flood mapping is essential for understanding the flood regime and mitigating its impacts. Remote sensing and GIS can support the accurate and area wide evaluation of floods. In this study, high spatial resolution synthetic aperture radar (SAR) and optical data were used to generate a dense satellite data time series for analysing the spatio-temporal patterns of flooding in the Mekong Delta. To derive water masks, a total of 777 Sentinel-1, 515 Sentinel-2 and 57 Landsat-8 scenes were used to generate cloud free water masks at a 10m spatial resolution in regular 10-day intervals throughout the observation period of hydrological years 2016-2017. The results show a spatial explicit information on the core zones of the seasonal flooding processes for the entire Mekong Delta and their effect of using floodwater for rice cultivation. The outcome maps provide an overall understanding of Mekong Delta flood patterns and many valuable information for policymakers and water resources managers.

Flooding as a cause of ungulate mortality in floodplain forests in Croatia

Floodplain forests with regular flooding regimes are the largest natural retentions areas in Croatia and are important as natural habitats for ungulates. The aim of this study was to determine the scale of mortality caused by flooding within these forests. Over a 10-year period, data on ungulate mortality (red deer, roe deer and wild boar), flood duration and flooded surface area were recorded. The study was conducted in primary (Posavske Sume—RET I) and secondary (Opeke II—RET II) retention areas within Lonjsko Polje Nature Park (Sava River region, Croatia). The longest flood period and the largest flooded surface area were recorded in RET I. Total ungulate mortality was 749 individuals, with 482 individuals in RET I and 267 individuals in RET II, predominantly wild boar. Flood mortality did not differ by gender. The highest mortality of wild boar was recorded for the juvenile and yearling age classes. Low mortality of red and roe deer can be attributed to their body size and ecological niches. Differences in mortality between the primary and secondary retention areas corresponded to differences in flood regimes, flood column heights and micro relief structures. In both retention areas, wild boar mortality and flood duration, i.e. flooded surface area, were positively correlated. Because the growth rate of the analysed ungulate populations was higher than the recorded mortality, no long-term effect of floods is expected on species abundance in these areas.

Formation damage and cleanup in the vicinity of flooding wells: Multi-fluid suspension flow model and calibration on lab data

Injection of water into a rock formation through the flooding wells is a common practice to maintain reservoir pressure and increase ultimate recovery of hydrocarbons. While flowing through the well from surface to the rock formation, injected water typically transports fine particles, which are introduced either externally (untreated water) or internally (e.g., particles detached from the inner surface of a pipeline due to erosion and abrasion as well as mineral particles detached from pore surface). In the framework of the two-fluid approach, we formulate mathematical model for filtration of a particle-laden suspension in the vicinity of flooding wells. Two mechanisms affecting transport of non-colloidal fines are considered: trapping and mobilization. Trapped particles reduce permeability and porosity of the rock, which leads to the injectivity decline of the well. The model contains minimal set of tuning parameters describing particle trapping and mobilization rates. This is achieved by critical overview of existing models. We calibrate suspension filtration model against existing experimental (laboratory) data on fines mobilization and trapping in porous media. A parametric study of different flooding scenarios is carried out numerically. In particular, we investigate the effectiveness of periodic flooding regime, when the periods of injection are followed by flowback in order to wash out trapped fines from the near-wellbore zone and restore well injectivity. Implications to formation damage and cleanup phenomena peculiar to hydraulic fracturing and drilling are discussed. Practical recommendations are given.

Effects of recent morphodynamic evolution on flood regimes in the Pearl River Delta

Both the river network and the regions outside the estuary mouths in the Pearl River Delta (PRD) of China experienced significant changes from 1999 to 2014. A validated hydrodynamic model across the entire PRD and adjacent regions outside the estuary mouths is employed to simulate both present (circa 2014) and past conditions (circa 1999). The total net water flux of the PRD decreased. The flow division of the West River is increasing, with values of 3.63% and 4.66% for the Makou and Denglongshan sections, respectively. The flood flow division of the North River is correspondingly decreasing. The value of the flood levels significantly decreased (more than 2 m) in the upper portion of the PRD, moderately decreased in the middle of the PRD (more than 1.1 m) and slightly decreased in the bottom part of the PRD (less than 0.22 m). In addition, the effects of morphodynamic evolution in different regions (i.e., the river network, coastline and bathymetry changes outside the estuary mouth) on floods are quantified. The results indicate that the decreased net water flux was caused by the increased channel’s capacity and the gentler water-level profile from the downcutting riverbed of the river network. The uneven morphodynamic evolution of the riverbed of the river network was primarily responsible for changes in the flood flow division in the PRD, and morphological evolution outside the estuary mouth was primarily responsible for reallocation within the outlets. The downcutting riverbed in the river network was primarily responsible for the lower flood levels in the upper and middle portion of the PRD. Reclamation seemed to have barely affected the flood level. The deepening bathymetry outside the estuary mouth was mainly responsible for the decrease in the flood level in the bottom portion of the PRD. The downcutting riverbed may decrease the stability of the riverbank, increasing the flood risk. The morphodynamic evolution of both the river network and the regions outside the estuary mouth should be considered to avoid unwanted side effects when designing local projects and flood mitigation strategies for the PRD.

Local flooding history affects plant recruitment in riparian zones

AbstractAimsMany rivers across the globe are severely impacted by changed flooding regimes, resulting in drastic shifts in vegetation, but the processes driving the exchange of flood‐sensitive and flood‐tolerant species are understood less. We studied the role of long‐term and recent flooding histories for riparian plant recruitment in response to various changes in flooding regime.LocationVindel River catchment (Northern Sweden).MethodsWe experimentally changed long‐term flooding regimes by transplanting turfs between high and low elevations in 2000 and in 2014 (n = 8 per treatment). We sowed seeds of five riparian species in both transplanted turfs and non‐transplanted controls and counted seedling numbers over two growing seasons. Further, we inventoried natural seedling frequencies in 190 plots in 19 reaches in 2013 and 2014, and related natural seedling numbers to plot flooding history in the period 2012–2014.ResultsWe observed effects of long‐term flooding history in the second year of the transplantation study (2015), but not in the first year. In 2015, turfs transplanted to locations with less flooding resulted in higher plant recruitment while transplantation to sites with more frequent flooding reduced recruitment compared to the controls. Since these differences were only found in recently transplanted turfs and not in older turfs, the legacy effect of long‐term flooding history can be transient. In the field seedling survey, similar differences were found between flooding‐history categories in 2013, but not in 2014, when the moisture conditions of the most recent year determined flooding. Further, lowest seedling numbers were observed when the previous flooding occurred in winter, and higher seedling numbers when floods occurred in spring or not at all.ConclusionsBoth long‐term and recent flooding histories can affect plant recruitment, and their influence should be taken into account when designing restoration projects.

Effects of flooding regime and meteorological variability on the removal efficiency of treatment wetlands under a Mediterranean climate

The modeling of free-water surface constructed wetlands (FWS-CWs) provides an improved understanding of their processes and constitutes a useful tool for the design and management of these systems. In this work, a dynamic simulation model for FWS-CWs was developed and used to simulate the operation of a FWS-CW proposed for improving the treatment of sewage effluents entering the Tablas de Daimiel National Park in central Spain. The process-based model simulates carbon, nitrogen and phosphorus dynamics, including key hydrological processes for wetlands under a fluctuating Mediterranean semiarid climate. The model allows for the simulation of the operation of FWS-CWs with variable flooding regimes, relating the surface water level to the flooded area and the water outflow. Simulations of the proposed FWS-CW under different water management schemes and scenarios were run, and the consequences of those management strategies on the treatment efficiency were analyzed. Under the Mediterranean climate and geology of the study area, namely, high water losses through evapotranspiration and infiltration, the decrease in nutrient concentrations was higher when the flooded area was reduced in summer than when a constant flooded area was maintained. Moreover, the meteorological variability introduced in different scenarios produced different results in terms of water outflow, but differences in terms of nutrient concentrations were not significant. The ability of the model to simulate different hydrological scenarios and their consequences on water quality makes it a useful decision-support tool.

Alternative transient states and slow plant community responses after changed flooding regimes.

Climate change will have large consequences for flooding frequencies in freshwater systems. In interaction with anthropogenic activities (flow regulation, channel restoration and catchment land‐use) this will both increase flooding and drought across the world. Like in many other ecosystems facing changed environmental conditions, it remains difficult to predict the rate and trajectory of vegetation responses to changed conditions. Given that critical ecosystem services (e.g. bank stabilization, carbon subsidies to aquatic communities or water purification) depend on riparian vegetation composition, it is important to understand how and how fast riparian vegetation responds to changing flooding regimes. We studied vegetation changes over 19 growing seasons in turfs that were transplanted in a full‐factorial design between three riparian elevations with different flooding frequencies. We found that (a) some transplanted communities may have developed into an alternative stable state and were still different from the target community, and (b) pathways of vegetation change were highly directional but alternative trajectories did occur, (c) changes were rather linear but faster when flooding frequencies increased than when they decreased, and (d) we observed fastest changes in turfs when proxies for mortality and colonization were highest. These results provide rare examples of alternative transient trajectories and stable states under field conditions, which is an important step towards understanding their drivers and their frequency in a changing world.

A stochastic event-based approach for flood estimation in catchments with mixed rainfall and snowmelt flood regimes

Abstract. The estimation of extreme floods is associated with high uncertainty, in part due to the limited length of streamflow records. Traditionally, statistical flood frequency analysis and an event-based model (PQRUT) using a single design storm have been applied in Norway. We here propose a stochastic PQRUT model, as an extension of the standard application of the event-based PQRUT model, by considering different combinations of initial conditions, rainfall and snowmelt, from which a distribution of flood peaks can be constructed. The stochastic PQRUT was applied for 20 small- and medium-sized catchments in Norway and the results give good fits to observed peak-over-threshold (POT) series. A sensitivity analysis of the method indicates (a) that the soil saturation level is less important than the rainfall input and the parameters of the PQRUT model for flood peaks with return periods higher than 100 years and (b) that excluding the snow routine can change the seasonality of the flood peaks. Estimates for the 100- and 1000-year return level based on the stochastic PQRUT model are compared with results for (a) statistical frequency analysis and (b) a standard implementation of the event-based PQRUT method. The differences in flood estimates between the stochastic PQRUT and the statistical flood frequency analysis are within 50 % in most catchments. However, the differences between the stochastic PQRUT and the standard implementation of the PQRUT model are much higher, especially in catchments with a snowmelt flood regime.