Altered Regimes Research Articles

THE INFLUENCE OF THE CHANGES IN WIND VELOCITY ON THE OUTER HEAT EXCHANGE OF THE BUILDINGS

When choosing architectural and planning solutions, such climatic factors as air temperature and humidity, having scalar quantities, as well as solar radiation, wind and precipitation having vector characteristics, must be taken into account. The calculated climatic parameters for the design of building enclosing structures, heat loss calculations and heat supply regulation are provided in the current documentation on norms and standards. The practical exploitation of various buildings demonstrates that in terms of initial climatic data, the choice of design parameters is not always efficiently justified; hence, the influence of the environment on the heating regime of the structures is insufficient in the estimations and sometimes erroneous. The wind is one of such climatic parameters. Its velocity and repeatability impact the heat exchange of the building structure with the environment as well as the alteration in temperature regime. The wind current towards the building creates additional pressure on the facade of the construction from the wind side direction. This leads, firstly, to air infiltration via the enclosing structures, and secondly, to the rise of heat exchange from the outer surface of the wall on the windward side. Based on estimated and analytical research, the values of the change in wind velocity depending on the altitude were analyzed, and its influence on the heat loss during heating of multi-storey buildings was assessed. The alterations in the wind velocity depending on the altitude were analyzed in the conditions of dense (urban) and broad construction. Besides, the authors presented the dependence of the convective component of the heat transfer coefficient of the outer surface of the structure on the values of the wind velocity. Based on the performed and presented calculations, it can be noticed that the heat transfer of the external structure will be much higher for multi-storey buildings than for mid-rise constructions. Thus, the convective component of the heat transfer coefficient of the outer surface rises by 36 % when the wind velocity increases from 5 m/s to 7 m/s. If not taking into consideration this dependence in the design, it can significantly influence the estimation of heat loss and energy efficiency of buildings, especially when it is about the increased percentage of facades glazing. The authors of the article assessed the heat loss for heating the windward and leeward facades at average values of the outside air temperature during the heating season in Ukraine. Hence, for constructions higher than 70 m with a calculated wind velocity of 5 m/s, heat losses increase from 10 % to 19 %. Such great difference in heat loss between the windward and leeward walls of the building requires increased thermal protection from the prevailing winter winds. Therefore, when designing multi-storey buildings, it is necessary to take into account changes in wind velocity according to the altitude. The obtained results can be useful both for choosing architectural and planning solutions, like the materials for external enclosing structures and for the objective assessment of the wind protection degree of individual buildings and territories.

Automatic gap-filling of daily streamflow time series in data-scarce regions using a machine learning algorithm

Complete hydrological time series are crucial for water and energy resources management and modelling in a changing climate. The reliability of the non-parametric stochastic machine learning algorithm MissForest was assessed for gap-filling of daily streamflow time series in a data-scarce region with strong climatic variability. A total of 1,586 reconstructions of streamflows for 1970–2016 were analyzed. Overall, MissForest performed satisfactorily to well, allowing a precise and reliable simulation of the missing data quickly and automatically. MissForest performance increased with the number of predictor records and record length, achieving satisfactory results with 20 or more records having 15 or more years in length. Reconstructed daily streamflow time series of rivers with natural flow regimes were simulated with good performance, which slightly decreased for discharge magnitude alterations by runoff inputs from urbanized areas and water diversion for irrigation. In cases of severe alterations of the flow regime, such as by hydropeaking, MissForest failed at filling daily streamflow series gaps. Reconstructed hydrographs allow analysis of streamflow change and variability and their interactions with key climatic variables.

The variation and attribution analysis of the runoff and sediment in the lower reach of the Yellow River during the past 60 years

Abstract The water and sediment regimes of the Yellow River are the basis of decision-making of major projects of the Yellow River. Based on water and sediment data at the Huayuankou station, Gaocun station, Aishan station, and Lijin station in the lower reach of the Yellow River, the Mann–Kendall test, the T-test for differences, wavelet analysis, slope change ratio method and the double cumulative curve method were applied to analyze the runoff and sediment regime alteration. The results show that the water and sediment of the lower Yellow River have a significant downward trend, and the annual sediment decreases significantly compared with the annual runoff. The annual runoff and sediment of the four hydrological stations changed around the 1980s and 1990s, respectively. The water and sediment of hydrological stations have periodic variations on multiple time-scales, but the variation scales are different. Precipitation, human activities and other factors lead to the decreasing trend of water and sediment in the lower Yellow River, and their contribution rates to the change of water and sediment are also different. Precipitation contributed 0.15%–8.71% and 0.06%–22.32% to the reduction of runoff and sediment load at the hydrological stations, while human activities contributed 91.29%–99.85% and 77.68%–102.21% to the reduction of runoff and sediment load, respectively. Human activity is the main factor in runoff and sediment reduction.

Variability in Population Traits of a Sentinel Iberian Fish in a Highly Modified Mediterranean-Type River

Human pressures on water resources have been suggested as a driver of biological traits that induce changes in native fish populations. This study highlighted the interplay between environmental stress factors, mostly related to flow regulation, and the longitudinal river gradient in biological traits such as the growth, size structure and somatic condition of a sentinel fish, Luciobarbus sclateri. We found an increase in size-related metrics and somatic condition at population levels associated with downstream reaches, although fragmentation and habitat alteration, flow regime alteration and the abundance of non-native fish were also significantly involved in their variability. Age-related parameters and growth were only explained by flow regime alterations and the abundance of non-native fish species. The high plasticity observed in L. sclateri population traits suggests that this is a key factor in the species adaptability to resist in a strongly altered Mediterranean river basin. However, the interplay of multiple stressors plays an important role in fish population dynamics and could induce complex responses that may be essential for long-term monitoring in sentinel species.

Multiple stressors shape invertebrate assemblages and reduce their trophic niche: A case study in a regulated stream

Few studies have addressed how the diversity of basal resources change with stream regulation and the potential consequences on river biota. We sampled invertebrates above and below a series of dams, over two years, at both downwelling and upwelling zones. In each zone, we recorded the daily temperature and flow variations, estimated the algal development, measured the available resources, and analysed carbon and nitrogen stable isotope compositions of the invertebrate community. The number of hydrological pulses were typically higher below the dams than above the dams especially during high-flow periods whereas the groundwater outlets had minor effects on invertebrate assemblages. Invertebrate abundance, richness and diversity tended to decrease below the dams. Co-inertia analysis showed that flow and temperature variations, and eutrophication explained most of the variance in the invertebrate assemblages, which comprised a higher number of resilient taxa below than above the dams. The proportions of pesticide-sensitive invertebrates were lower below the dams and ovoviviparous and more generalist taxa were prominent. We did not observe the expected CPOM decrease and FPOM increase downstream. Accordingly, the proportions of each functional feeding group were remarkably similar above and below the dams despite the long distance between the sectors (>100 kms). The diversity of basal resources used within assemblages progressively increased downstream above dams. In contrast, the diversity of resources used by organisms below the dams decreased from upstream to downstream suggesting a significant influence of flow regulation on aquatic food webs. Finally, the shorter trophic chains for the invertebrate assemblages below the dams suggests that the effects of stream regulation and eutrophication induced a simplification of food webs. To our knowledge, this study is the first to connect taxonomic and functional trait changes in response to multiple stressors with the associated modifications in isotopic niches within aquatic invertebrate assemblages. ContextUnderstanding how stream regulation and associated anthropogenic pressures act on aquatic assemblages and trophic niches is necessary to guide management actions. GoalWe aimed to investigate the functional responses (traits and trophic niches) of aquatic invertebrate assemblages to stream regulation and eutrophication. MethodsWe used univariate and multivariate analyses to compare the invertebrate assemblages above and below the dams and to assess the contributions of hydrology (including groundwater supplies to the river), temperature and eutrophication to the variability in the composition of invertebrate assemblages. We also considered the relative utilization of a selected set of traits describing invertebrate resilience, resistance and specialization to address the potential functional effects of stream regulation on invertebrate assemblages. Finally, carbon and nitrogen isotope analyses allowed us to characterize the length and width of invertebrate assemblage food webs as related to the availability and diversity of basal resources. ResultsInvertebrate abundance and richness generally decreased below the dams, with the highest impacts on insect taxa. Co-inertia analysis showed that stream regulation and eutrophication were main drivers of the aquatic invertebrate assemblages. The analysis separated the sites above and below the dams according to flow and temperature variation, whereas eutrophication appeared as a secondary stressor that separated the sites within each sector. Furthermore, the series of dams resulted in (i) a higher proportion of resilient (e.g., multivoltine) and resistant (ovoviviparous) taxa and a majority of generalists in assemblages below dams, (ii) an impact on the classical dynamics of CPOM (decrease) and FPOM (increase) sources from upstream to downstream, and (iii) a reduction in the diversity of resource use and in the trophic chain length of invertebrate assemblages below dams. The cooler and less oxygenated upwelling zones had lower invertebrate abundance; however, contrary to our expectation, the variation in the groundwater supply did not affect the composition of epigean invertebrate assemblages. ConclusionThis study provides insights about the impacts of flow regime alteration and eutrophication on food webs that may have been caused by regulation of permanent streams. To our knowledge, this is the first to connect taxonomic and functional trait changes in response to multiple stressors with the associated modifications in energy fluxes in aquatic invertebrate assemblages. This study suggests that bed stability, which is associated with a reduction in channel mobility below the dams and with moderate eutrophication, may provide the shelter and resources that can locally favour invertebrate assemblage dynamics and lessen the effects of flow regulation. In addition, the study suggests that the biological trait-based approach and isotope analysis are complementary approaches for addressing ecosystem functioning. The relative utilization of traits indicates the functional potential of aquatic invertebrate assemblages to face multiple stressors whereas isotope analysis is an expression of the actual effect of the stressors on the trophic structure of aquatic invertebrate assemblages.

Influence of inundation characteristics on the distribution of dryland floodplain vegetation communities

Semi-arid floodplain vegetation is an essential component of floodplain and terrestrial ecosystems due to the wide range of services provided for waterbirds, woodland birds, amphibians and mammals, together with their contribution to natural carbon sequestration. Since overbank flooding has been considered the driving factor of vegetation distribution on floodplains, sustainable water resource management requires a better understanding of the influence of inundation on vegetation distribution. We examined the relationship between the distribution of four flood-dependant vegetation communities and associated long term inundation metrics on the upper Darling River floodplain using electivity analysis and a generalised additive model (GAM) by generating a total of 10,478 individual inundation maps with a high spatial resolution over a period of 29 years. Our results show that the four dominant vegetation communities are situated differently in relation to inundation attributes. The inundation metrics better explained the variation in the distribution of River Red Gum than those of Black box, Coolabah and Lignum communities. We found that the floodplain forests and woodlands in the upper Darling can survive longer periods of drought and shorter inundation duration than previously reported. The results provide practical information for ecosystem management by offering a means of predicting changes in vegetation distribution in relation to alteration in flow regime resulting from water planning arrangements or climate change.

Sustaining environmental flows in water‐deficient rivers via inter‐basin hydropower transfer

AbstractIn water‐deficient rivers, environmental flows (e‐flows) are usually sustained via inter‐basin water transfer projects from water‐sufficient rivers, but these projects incur tremendous costs and may lead to many negative ecological effects, such as ecological invasion. This research proposed to transfer hydropower instead of water from water‐sufficient rivers, because hydropower could substitute for water to promote economic development and reduce water withdrawal from water‐deficient rivers (conserved water). In addition, based on the analysis of eco‐hydrological processes, the flow regime alteration plays an important role in restoring riverine ecosystem. With the goal of minimum flow regime alternation, we set up two scenarios to distribute the annual conserved water, and determined the optimal amount of transferred hydropower and the optimal use of conserved water, which could effectively sustain the e‐flows. Accordingly, this paper established a computable general equilibrium model to analyse the substitution of hydropower for water in a water‐deficient river basin, and determined the water withdrawal volume that could be reduced. We adopted a range‐of‐variability approach to measure the degree of flow regime alteration, and optimized the flow regime management scheme. The Luanhe River Basin was adopted as a study case. The results showed that: the water‐hydropower equivalent decreased as the transferred hydropower into the Luanhe River Basin increased; a transferred hydropower amount of 22.46 kWh/s, equivalent to 18.30 m3/s conserved water, was optimal for the river basin; the conserved water should be distributed to the Luanhe River in the proportions of 0.55:0.1:0.35 during the wet, normal and dry seasons, respectively, which is the optimal scheme to sustain the hydrological processes of the river.

Squandering water in drylands: the water-use strategy of the phreatophyte Ziziphus lotus in a groundwater-dependent ecosystem.

Water is the most limiting factor in dryland ecosystems, and plants are adapted to cope with this constraint. Particularly vulnerable are phreatophytic plants from groundwater-dependent ecosystems (GDEs) in regions that have to face water regime alterations due to the impacts of climate and land-use changes. We investigated two aspects related to the water-use strategy of a keystone species that dominates one of the few terrestrial GDEs in European drylands (Ziziphus lotus): where it obtains water and how it regulates its use. We (1) evaluated plants' water sources and use patterns using a multiple-isotope approach (δ2 H, δ18 O, and Δ13 C); (2) assessed the regulation of plant water potential by characterizing the species on an isohydric-anisohydric continuum; and (3) evaluated plants' response to increasing water stress along a depth-to-groundwater (DTGW) gradient by measuring foliar gas exchange and nutrient concentrations. Ziziphus lotus behaves as a facultative or partial phreatophyte with extreme anisohydric stomatal regulation. However, as DTGW increased, Z. lotus (1) reduced the use of groundwater, (2) reduced total water uptake, and (3) limited transpiration water loss while increasing water-use efficiency. We also found a physiological threshold at 14 m depth to groundwater, which could indicate maximum rooting length beyond which optimal plant function could not be sustained. Species such as Z. lotus survive by squandering water in drylands because of a substantial groundwater uptake. However, the identification of DTGW thresholds indicates that drawdowns in groundwater level would jeopardize the functioning of the GDE.

Human stabilization of river flows is linked with fish invasions across the USA

AbstractAimRiver flow regimes are changing globally as a consequence of human use of freshwater resources. Additionally, rivers are among the most invaded ecosystems. Invasion biology predicts that the establishment and spread of non‐native species might be favoured both by new environmental regimes (niche opportunities) and by human‐mediated dispersal (propagule pressure). Here, we expand on past research by asking whether these two mechanisms act in isolation or whether they interact to facilitate the spread of non‐natives.LocationConterminous USA.Time period1987–2016.Major taxa studiedFreshwater fishes.MethodsFirst, we examined the geography of riverine fish invasions across 1,148 watersheds spanning the conterminous USA. We focused on species richness and the life‐history strategies of non‐native (both translocated and exotic) relative to native species pools. Second, we quantified flow regime alteration as the deviation between observed and expected contemporary flow regimes, using a combination of spectral analyses on long‐term discharge data and random forest models. We focused on two biologically relevant facets of the flow regime: flow variability and flow seasonality. We also estimated metrics of propagule pressure: recreational fishing pressure and socioeconomic activity. We then compared the strength of evidence associating riverine fish invasions with flow alteration, propagule pressure, and interactions between the two mechanisms.ResultsWe found that alteration of flow variability and seasonality is widespread across the conterminous U.S. rivers, and has favoured invasions by filtering specific life‐history strategies. Importantly, high levels of flow stabilization and propagule pressure interacted: where co‐occurring, these two drivers were associated with higher fish invadedness levels than expected based on their individual effects.Main conclusionsOur results underscore the need to consider different drivers of invasion concurrently. Otherwise, important synergistic interactions might be missed that could explain (and guide management strategies to mitigate) the high levels of invasion in fresh waters.

Connecting changes in Euphrates River flow to hydropattern of the Western Mesopotamian Marshes

The Mesopotamian Marshlands are the largest wetland system in the Middle East. Historically, these marshes served as the floodplains of the Tigris and Euphrates rivers, and they are currently connected to these rivers via surface water feeder canals. Historically, the Mesopotamian marshes received consistent flood pulses during the spring season from March to May. In recent decades, however, several large dams have been constructed in the Tigris and Euphrates basins for irrigation purposes and power generation, severely altering the flow regime, which along with other direct anthropogenic activities, has severely degraded the marsh ecosystem. This work quantifies changes in the riverine flow regime and how they have affected the hydro-pattern of the western Mesopotamian marshes (focusing on the western Al-Hammar marsh) and describes the role of hydrological drivers that are important for marsh restoration. The total area of the Al-Hammar marshes has been reduced from an average of 2800 km2 before 1970 to a minimum of 240 km2 in recent decades, concomitant with reductions in annual average Euphrates River flow (at Hit) from 967 to 602 m3/s and marked flow regime alteration. While climate warming and reduced precipitation were observed in the basin, changes in the fundamental precipitation-flow relationship implicate infrastructural changes (upstream dams) as the primary reason for these changes. This analysis quantified how flow variability under historic and contemporary conditions have affected wetland area and other hydro-pattern characteristics and suggests that at an annual average of least 70 m3/s of water deliveries to the western Mesopotamian marsh are required to restore 1000 km2 of wetland area. Our hope is that this focus on the river-marsh connection will help inform predictive models and scenario analysis for restoration of this unique social-ecological system.

Quantitative assessment of water level regime alterations during 1959–2016 caused by Three Gorges Reservoir in the Dongting Lake, China

Abstract Water level is considered as the key factor affecting the structure and function of lake ecosystems. The Mann-Kendall technique and range of variability approach (RVA) were used to quantitatively evaluate the hydrologic alteration due to Three Gorges Reservoir (TGR) in Dongting Lake. Results indicate the following. (1) The average annual water levels at Chenglingji station showed increasing trends (p < 0.05), while that at Nanzui station showed a decreasing trend (p < 0.05). The turning year occurred in 2003, which reflects the significant effects of the TGR on the water level regime. (2) The highly altered parameters were 1-, 3-, and 7-day minimum water levels both at Chenglingji and Yangliutan, and in October both at Nanzui and Yangliutan, with the degree of hydrologic alteration being larger than 80%. However, 1-, 3-, 7-, and 30-day maximum water levels at three stations had relatively small alteration, with the degree of hydrologic alteration being smaller than 41%. (3) The hydrologic alteration degrees at Chenglingji, Nanzui and Yangliutan station were all moderate, with changes of 50, 46 and 49%, respectively. (4) Water level regimes at Dongting Lake were mainly jointly affected by reservoir operation, land utilization change and river channelization. These changes in water level regimes have a negative impact on aquatic and terrestrial ecosystems. This study provides a scientific reference for the protection of lake ecosystems under hydrologic alteration.

Is It Optimal to Use the Entirety of the Available Flow Records in the Range of Variability Approach?

Reducing the degree of flow regime alteration is a basic principle for biodiversity conservation in rivers. The range of variability approach (RVA) is the most widely used method to assess flow regime alteration. Generally, researchers tend to put all of the available pre-impact and post-impact flow records into the RVA. However, no research has tested whether it is optimal to use the entirety of the available flow records from the perspective of calculation accuracy for the degree of flow regime alteration. In this research, a series of numerical simulations is conducted, demonstrating that the greatest accuracy for flow regime alteration degree assessed by the RVA is achieved when the length of both the pre- and post-impact flow time series is set equal to multiples of periodicity length, and that, when attempting to put the whole available flow record into the RVA, calculation accuracy may be reduced. On the basis of these findings, we further propose revising the traditional RVA procedure by assessing the periodicity of the pre- and post-impact flow time series in advance. If the periodicity of the pre- or post-impact flows is detected, the length of the time series should be set equal to its periodicity.

A revised range of variability approach considering the morphological alteration of hydrological indicators

A reasonable assessment of the hydrological regime alteration is a prerequisite for river utilization, restoration, and protection. The hydrological alteration is quantified as changes in different indicators (IHAs) identified by Range of Variability Approach (RVA). However, RVA is unable to reveal the morphological characteristic which demonstrates the overall hydrological regime. This will lead to an incomplete assessment of the hydrological alteration. To supplement the current assessment, this work proposed a revised RVA method in three main steps: (1) to identify the morphological characteristics of each IHA; (2) to quantify the morphological alteration by comparing the Hasse matrices of different time series; (3) to combine the frequency alteration and morphological alteration of IHAs for reflecting overall hydrological alteration. A case study of the upper Yellow River shows that the revised RVA method outperforms RVA in the assessment of the hydrological regime not only because revised RVA captures the hydrological changes of certain IHAs that are not reflected by conventional RVA, but also the alteration identified by revised RVA shows more apparent differences at two stations upstream and downstream the dam than conventional RVA can provide. The revised RVA is more applicable to identify the hydrological alteration due to dam construction which could have negative impacts on river ecosystem since the morphological alteration of time series is considered. As a whole, the new method offers a better understanding of the alteration in hydrological regime, which gives beneficial guidance to river management.

Environmental flows and the mitigation of hydrological alteration downstream from dams: The Spanish case

Dams cause significant alteration in natural flow patterns of rivers throughout the world. As a response, many countries have established environmental flows (e-flows) to reduce hydrological alteration and mitigate ecosystem degradation. This paper assesses hydrological changes downstream from dams in 22 rivers in Spain, 12 of them located in northern wetter regions (Duero and Ebro basins) and the remaining 10 in southern drier regions (Guadalquivir and Júcar basins). First we compared pre-dam and post-dam instream flow patterns, and then we considered a third period starting with the implementation of e-flows. We quantified changes in mean annual flows, monthly flows, and magnitude and timing of extreme flows (1-day maximum, 95th percentile, 1-day minimum and 5th percentile). The analysis aimed at: i) characterizing hydrological alteration in regulated rivers in four river basins in Spain, with a special focus on differences across regions; and (ii) assessing the capacity of the implemented e-flows to mitigate flow regime alteration downstream from dams. All the studied rivers displayed significant changes in magnitude and timing of flows after the dam construction. A quite homogenous trend was observed in the drier basins, where mean annual flows and annual extreme flows decreased significantly. The wetter basins showed no uniform patterns in mean annual flows, while 1-day maximum and 95th percentile flow values decreased. In contrast to drier basins, 1-day minimum and 5th percentile flow values increased markedly. In most cases, the natural Mediterranean annual hydrograph was inverted, with the high-flow period occurring during the low precipitation months (summer) and the low-flow period during the wet season. After e-flows implementation, these patterns of hydrological alteration remained almost unchanged, pointing to a limited capacity of current e-flows to mitigate the hydrological impacts downstream from dams.

Impacts of Water Level Fluctuations on Soil Aggregate Stability in the Three Gorges Reservoir, China

Aggregate is the basic unit of soil structure, which is crucial to the sustainability of soil system functions such as structural stability and Fertility Maintenance. Three Gorges Dam (TGD) has extensively led to a dramatic hydrological regime alteration, which may consequently affect various soil physical properties. The aim of this study was to investigate the long-run temporal variation of soil aggregate stability as induced by water-level fluctuations in the riparian zone of the Three Gorges Reservoir (TGR). Sampling plots were established along different elevations considering the interval of 5 m, starting from 150 m to 175 m. A Laser Diffraction based analysis that allows the measurement of soil aggregate stability after the removal of soil organic matter helped to particularly study the effect of external factors on soil aggregate stability of the study area. In addition, wet-sieving method considering the effect of chemical binding agents was used to quantify aggregate stability. The present results indicated a significant increase of Mean Volume Diameter, MVD (p < 0.05) within the study period. Continuous drying-wetting cycles mended soil aggregate stability with a 14.25% increase of the MVD from 2012 to 2016. In the Water-Level Fluctuation Zone (WLFZ), the lower land has predominantly contributed to the increase of soil aggregate stability compared to upper land, with an increase of 62.19% and 37.81% for MVD, 60.88% and 39.12% for D10, 95.34% and 4.66% for D90 at lower and upper elevations, respectively. Sediment deposition below 165 m has precluded a direct effect of water stress on soil aggregates, which certainly declined soil disaggregation. The removal of SOM while analyzing aggregate stability by LD may explain the contradiction between the resulted MVD, and the MWD and GMD. The increase of MWD and GMD was mainly attributed to the increase of SOM with r2 = 0.89 (p < 0.01) and r2 = 0.90 (p < 0.01), while the increase of MVD was highly predicted by the decrease of SOM with r2 = 0.88 (p < 0.01). Since this study presents a remarkable change of soil in the riparian area due to dry-wet cycles, our results may help to deeply understand the soil ecology and environmental changes in the WLFZ.

Alterations in the Water-Level Regime of Tonle Sap Lake

The Tonle Sap Lake (TSL), located in southwestern Mekong Basin, is the largest freshwater lake in Southeast Asia. The water-level regime of the TSL has been changing over the last few decades due to the rapid development of water infrastructures and climate change in the Mekong Basin. This paper makes efforts to quantify the water-level regime alterations of the TSL by analyzing the pre- and post-1991 historical data of daily water levels from Kompong Luong Station. The method of modified indicators of hydrologic alteration was utilized, and the TSL’s hydrological behavior was explored further by focusing on the redistribution of hydrology and unfavorable disturbances. The results reveal that the coefficients of variation of most indicators in recent years increase compared with the preimpact years from 1960 to 1990. This demonstrates that the Mekong Basin and TSL are experiencing an increasingly unstable and varied hydrological cycle, which is more susceptible to interference with increased frequency and degree. Moreover, the system may have suffered more unfavorable human interference in dry years and in the dry season. It is expected that the findings in this paper will contribute to the protective development and adaptive management of the Mekong Basin and the TSL in the future.

Long-term alterations of flow regimes of the Mekong River and adaptation strategies for the Vietnamese Mekong Delta

Study regionThe Mekong basin, where climate change and anthropogenic interventions (e.g., dams, sand mining, and sluice gates) have intensified in the recent decades affecting the pristine flow regime and salinity intrusion. Study focusThis paper aims at quantifying the flow regime alterations in the entire Mekong from 1980 to 2015 and linking with the controlling drivers of alterations. In this regard, various indicators, analytical methods, and a semi two-dimensional hydrodynamic and advection-dispersion model were used. New hydrological insights for the regionThe flow regime alterations in the high-dam development period (2009–2015) are more pronounced than in the low-dam development period (1993–2008), compared to the no-dam development period (1980–1992), based on most of the indicators analyzed. In the high-dam development period all existing dams with large reservoir capacity seemed to have cumulatively reduced the flood pulses and frequency and increased the low-flow discharge along the entire Mekong through reservoir operations, exceeding climate change effect. In the recent years the water levels in the low-flow season in the Vietnamese Mekong Delta (VMD) have decreased, possibly because of increased riverbed incision caused by reduced sediment supply and increased sand mining. The reduced water levels together with the increased number of the sluice gates constructed seemed to have increased salinity intrusion in the VMD which may be partly reduced by early emergency water release from upstream dams.

Is multicomponent environmental flow management always better than the fixed minimum flows for a river ecosystem?

AbstractEnvironmental flows (e‐flows) management is a basic task in reservoir operation. A simple approach for managing e‐flows is to maintain a fixed minimum flow, which releases at least a constant amount of water to sustain daily streamflow. Then e‐flows management becomes increasingly more sophisticated. Newer approaches include multicomponent e‐flow management, which provides different flows during different periods. This management approach produces a better flow regime than the fixed minimum flows because it more closely resembles the natural flow regime. However, in addition to e‐flow management strategies, flow regimes are also influenced by reservoir capacity, reservoir inflow, and water supply. The effects of these three variables on flow regime alteration have been rarely studied. To compare the effects of different reservoir capacities, reservoir inflows, and water supply quantities on flow regime alteration, we developed a hypothetical reservoir system for use in numerical experiments. In addition to the fixed minimum flows, we tested the effects of three other widely used e‐flow management strategies on flow regime alteration: the fraction of inflow, flow components, and four‐period release approaches. The range of variability approach is adopted to evaluate the flow regime alteration. The results show that the flow regime alteration decreases with increasing reservoir inflows and increases with increasing water supply quantity for all of the four e‐flow management strategies. We found that for certain values of reservoir capacity, reservoir inflow sequence, or water supply, the simple fixed minimum flows approach may produce the same results as the more sophisticated approaches.

Morphometric properties of alternate bars and water discharge: a laboratory investigation

Abstract. The formation of alternate bars in straightened river reaches represents a fundamental process of river morphodynamics that has received great attention in the last decades. It is well-established that migrating alternate bars arise from an autogenic instability mechanism occurring when the channel width-to-depth ratio is sufficiently large. While several empirical and theoretical relations are available for predicting how bar height and length depend on the key dimensionless parameters, there is a lack of direct, quantitative information about the dependence of bar properties on flow discharge. We performed a series of experiments in a long, mobile-bed flume with fixed and straight banks at different discharges. The self-formed bed topography was surveyed, different metrics were analyzed to obtain quantitative information about bar height and shape, and results were interpreted in the light of existing theoretical models. The analysis reveals that the shape of alternate bars highly depends on their formative discharge, with remarkable variations in the harmonic composition and a strong decreasing trend of the skewness of the bed elevation. Similarly, the height of alternate bars clearly decreases with the water discharge, in quantitative agreement with theoretical predictions. However, the disappearance of bars when discharge exceeds a critical threshold is not as sharp as expected due to the formation of so-called “diagonal bars”. This work provides basic information for modeling and interpreting short-term morphological variations during individual flood events and long-term trajectories due to alterations of the hydrological regime.

The natural flow regime: A master variable for maintaining river ecosystem health

AbstractRiver health is sustained by time‐based variation in their flows, and the maintenance of natural flow regime is essential for keeping the rivers healthy. However, the dynamism in natural flow regime now stands altered by changing climate, and the omnipresent regulation of river flows throughout the world has severely impacted the river health. It is well documented that harnessing and altering the flow of the rivers and streams comes at a huge cost. Numerous rivers in the world have stopped supporting socially and economically important native species or sustain vibrant ecosystems that offer valuable goods and services. The alteration of the natural flow regime has led to the collapse of many healthy and resilient river ecosystems throughout the world. Therefore, to ensure appropriate naturalized flow regimes for riverine organisms and a sustainable nexus between energy demand, water requirements and climate, more understanding is required to study the consequences of alteration in natural flow regime triggered by climate change and anthropogenic interventions. Moreover, conservation and management practices must be firmly based on scientific principles to restore the integrity of river ecosystems. The current approaches often fail to take into consideration a basic fundamental principle that the integrity of the river ecosystem is largely determined by the natural dynamic character of their flow regimes. In this synthesis, we try to explain how the maintenance of the natural flow regime is an essential requirement for maintaining river ecosystem health.