Seasonal Flow Regimes Research Articles

Agricultural intensification and urban expansion affect the seasonal flow regime in southern Ontario watersheds

AbstractThe mosaic of urban, agricultural and natural covers that typifies most developed landscapes makes it challenging to identify the primary causes of stream flow perturbation in mixed landcover watersheds. This is especially true in southern Ontario, Canada, where approximately 1/3 of the Canadian population lives in urban areas surrounded by agriculture. Whilst previous studies have examined the impacts of urban or agricultural landcover on stream flow separately, they are rarely considered together. Furthermore, major expansions in tile‐drained (TD) cropland in Ontario over the past several decades could affect the flow regime; however, this has never been examined. This study assessed the effect of landcover on flow regime at 19 proximal watersheds that varied in agriculture (0%–87%), natural (2%–97%) and urban landcover (2%–96%) using the Richards–Baker index (RBI), the coefficient of variation (CV) and a Baseflow index (BFI). Urbanized watersheds were consistently the most flashy (highest RBI and CV), regardless of season, whereas agricultural watersheds had moderately flashy conditions that varied between the growing (GS) and non‐growing seasons (NGS). Natural watersheds were the least flashy and had the highest BFI. Watersheds dominated by TD cropland were flashier during the NGS compared with un‐tiled agricultural watersheds. Furthermore, TD‐streams were warmer in the NGS and cooler in the GS, such that the former could affect ice breakup. A 50‐year analysis at three watersheds showed statistically significant increases in flashiness and decreases in BFI at an urbanizing watershed. In contrast, watersheds that remained agricultural or natural underwent small but significant declines in flashiness and increases in baseflow, potentially due to increases in precipitation and forest maturation. Our results suggest that continued expansions of urban and TD cropland may increase NGS flashiness. In contrast, enhanced soil moisture storage provided by TD could decrease the potential for flooding in the GS in southern Ontario.

Greenup Variability Impact on Seasonal Streamflow and Soil Moisture Dynamics in Humid, Temperate Forests

AbstractIn this study, we investigate how seasonal streamflow and soil moisture patterns have responded to variability in vegetation phenology in humid, temperate forested watersheds without significant seasonal snowmelt over the last four decades. We characterize spring streamflow peaks using 50th percentiles of cumulative daily precipitation, streamflow, and soil moisture measurements, and investigate interactions with remotely sensed, greenup anomalies. After removing a dominant precipitation control, 1‐day earlier greenup is usually associated with about 1‐day early spring flow peak at four low‐elevation deciduous catchments using both sequential and multiple linear regressions. This indicates that the strong dependency of seasonal flow regimes on precipitation is mediated by vegetation seasonality, especially by greenup variability. In contrast, we find less significant correlations of the greenup anomalies on flow percentiles from two paired evergreen and two high‐elevation deciduous catchments. At a plot scale, similar correlations were found only at an upslope topographic position, where precipitation also showed tighter coupling with moisture seasonal patterns than downslope. Our study suggests that rainfall‐runoff and rainfall‐soil moisture relations have been closely mediated by vegetation seasonality in deciduous forests, especially by greenup anomalies, but patterned along topoclimate and hillslope gradients. This study emphasizes that it is important to understand phenological responses to ongoing climate change (in both long‐term and interannual variability) for prediction of seasonal flow regimes especially in deciduous forested catchments.

Seasonal and flow regime differences should be considered in explaining zooplankton metacommunity assembly

Environmental and spatial filtering collectively regulate zooplankton communities, while their relative contributions to community structure may change with seasonal variations and flow regime differences. However, there is a lack of research on the combined effect of these two factors. To explore these differences, we studied zooplankton populations as well as environmental and geographical data from different seasons in rivers with variable flows in the Shaying River basin. According to our results, the assembly mechanisms for intermittent river (IR) communities fluctuated more and were more responsive to geographical factors than were the assembly mechanisms of perennial river (PR) communities during the wet and dry seasons. We discovered that the rotifer/protozoan assemblage was more vulnerable to spatial influences, whereas the composition of the crustacean group was heavily impacted by the environment. Our findings contribute to a better understanding of metacommunity assembly mechanisms in highly dynamic ecosystems.

Finding middle ground: Flow regimes designed for salmon and energy value

In regulated rivers, shaping seasonal flows to recover species at risk depends on understanding when to expect conflicts with competing water users and when their interests are aligned. Multi-objective optimization can be used to reveal such conflicts and commonalities. When species are involved, multi-objective optimization is challenged by the need to simulate complex species responses to flow regimes. Previously, we addressed that challenge by developing a simplified salmon model (Quantus) that defines cohorts of salmon by the river section and time in which they were spawned. Salmon in these space-time cohorts are tracked from the time redds (nests) are constructed until the cohort exits the tributary en route to the ocean. In this study, we modeled seasonal patterns in energy value and developed a Pareto-optimal frontier of seasonal flow patterns to maximize in-river salmon survival and hydropower value. Candidate flow regimes were characterized by two pulse flows varying in magnitude, timing, and duration and constrained by a total annual flow near the historical median. Our analysis revealed times when economic and salmon objectives were aligned and times when they differed. Pulse flows that favored higher energy value were timed to meet demand during extreme temperatures. Both salmon and hydropower objectives produced optimal flow regimes with pulse flows in early summer, but only solutions favoring hydropower value included high flows in mid-winter. Solutions favoring higher age-0 salmon survival provided an extended pulse flow in late winter/early spring, which suggests that access to productive floodplain habitat allowed faster growth and earlier out-migration and reduced the need for higher temperature-moderating flows later in spring. Minimum flows were also higher among solutions favoring salmon over energy. The tools used to produce these results can help to design simplified seasonal flow regimes by revealing compromise solutions that satisfy both fish and energy producers and highlighting when potential conflicts are likely.

Intra-seasonal variability in supraglacial stream sediment on the Greenland Ice Sheet

On the surface of the Greenland Ice Sheet, the presence of low-albedo features greatly contributes to ablation zone meltwater production. Some of the lowest albedo features on the Ice Sheet are water-filled supraglacial stream channels, especially those with abundant deposits of consolidated cryoconite sediment. Because these sediments enhance melting by disproportionately lowering albedo, studying their spatial extent can provide a better understanding of Greenland’s contribution to global sea level rise. However, little is known about the spatial distribution of supraglacial stream sediment, or how it changes in response to seasonal flow regimes. Here, we surveyed a supraglacial stream network in Southwest Greenland, collecting imagery from seven uncrewed aerial vehicle (UAV) flights over the course of 24 days in 2019. Using Structure-from-Motion-generated orthomosaic imagery and digital elevation models (DEMs), we manually digitized the banks of the supraglacial stream channels, classified the areal coverage of sediment deposits, and modeled how the terrain influences the amount of incoming solar radiation at the Ice Sheet surface. We used imagery classified by surface types andin-situspectrometer measurements to determine how changes in sediment cover altered albedo. We found that, within our study area, only 15% of cryoconite sediment was consolidated in cryoconite holes; the remaining 85% was located within supraglacial streams mostly concentrated on daily inundated riverbanks (hereafter termed floodplains). Sediment cover and stream width are highly correlated, suggesting that sediment influx into supraglacial drainage systems widens stream channels or darkens previously widened channels. This reduces albedo in floodplains that already receive greater solar radiation due to their flatness. Additionally, the areal extent of stream sediments increased in August following seasonal peak flow, suggesting that as stream power decreases, more sediment accumulates in supraglacial channels. This negative feedback loop for melting may delay Greenland’s runoff to the latter end of the melt season. This study shows in unprecedented detail where and when sediment is deposited and how these deposits potentially impact the Ice Sheet surface energy balance. These findings may allow for better prediction of how supraglacial floodplains, and the microbiomes they contain, might change in response to increased melting.

Assessing the effects of irrigation and hydropower dams on river communities using taxonomic and multiple trait-based approaches

Rivers and streams have suffered multiple transformations to attend the increasing water demands worldwide. Among these, dams and reservoirs cause some of the most severe ecological impacts on rivers, altering the river flow and thermal regimens, nutrient and sediment fluxes, and network connectivity. However, in the context of the different dam purposes and operational schemes, knowledge of the ecological impacts on the riverine biota is still limited. In this study, our main goal was to assess dam-related effects (e.g. hydrological and thermal alteration, water quality changes) on river biological communities and identify key ecological responses associated to flow regulation. To achieve this, diatom, macroinvertebrate, and fish communities were surveyed in control (n = 8) and impacted (n = 11) streams (i.e. downstream of irrigation or hydropower dams) along three consecutive years. The study design aimed at minimising the environmental variability among control and impacts using previously established hydrological classifications. This allowed focusing primarily on the effects of dam operation schemes. In addition to traditional biotic indices based on the composition and structure of these communities, we assessed community-level responses using trait-based analyses with multivariate and fourth-corner analyses. The ecological changes varied with dam purpose and, in general, favoured disturbance-tolerant traits. Common biotic indices did not consistently respond to dam uses; trait-based analyses, in contrast, provided a more detailed picture of the dam-related effects on the studied river communities, with macroinvertebrate traits showing the strongest correlations to dam-related hydrological and physico-chemical variables, followed by diatoms and fish. Changes in the biological communities downstream of irrigation dams were mostly related to the inversion of the seasonal flow regimes (e.g. increases in the summer flows and magnitude of low flow extremes; decreases in the winter flows and in the frequency and magnitude of extreme high flow events) and the reduction of nutrient concentration. These changes favoured planktonic diatoms, macroinvertebrates with short life cycles and small body sizes, and fish feeding on the water column. Hydropower dams elevated significantly the rate of flow change and water temperature, favouring low profile adnate diatoms, multivoltine and passively dispersed macroinvertebrates, and scrapers. The key relationships identified in our study are useful to underpin river biodiversity conservation strategies and to set future research directions aiming at reducing the negative effects of dam operation schemes.

Assessment of upbasin dam impacts on streamflow at Chiang Saen gauging station during the period 1960–2020 in the context of statistical studies

The Langcang-Mekong River Basin is the most important transboundary river basin in Asia. However, over the recent decades, dam construction has had increasingly profound effects on hydrological processes and aquatic, particularly riparian, ecosystems. Understanding these impacts is critical for the foundation of sustainable runoff surface management. In this study, different methods, based on both graphical and statistical techniques, were applied to assess the effects of the dams on annual, seasonal and monthly runoff and to detect hydroclimatic trends in the Upper Mekong Basin during the period 1960–2020. The results reveal two change points with respect to seasonal and annual flow regimes; that is, 2003 for the flood season and annual flows and 2013 for the dry season flow. The duration of the flood season and the volume of annual discharges have both significantly decreased since 2003 and the dry season discharge has significantly increased since 2013 (with both p-values <.05). The quantitative assessment suggests that, due to the effect of dams, the monthly discharges increased by 10–450 m3/s during the dry season (December–May), while the flood season's monthly flows decreased significantly, by 1,028–2,150 m3/s, from July to October at Chiang Saen station. The study of hydrological changes in the Mekong watershed is expected to be a significant contribution towards a better understanding of large watersheds in which the hydrological effects are influenced not only by climate change at large spatial and temporal scales but also by changes in the physical environment due to the construction of dams.

With warming, spring streamflow peaks are more coupled with vegetation green‐up than snowmelt in the northeastern United States

AbstractIn forested watersheds, climate change may have substantial implications for both ecological and hydrological processes by altering the snow and vegetation seasonal dynamics. However, to date, there have been few studies which systematically analysed their combined effects on seasonal streamflow behaviour at the watershed scale. Using long‐term remote‐sensed vegetation data and snowpack measurements, we characterized the long‐term patterns in the timings of green‐up/senescence and subsequent growing season length, as well as snowpack development/disappearance and subsequent snowpack duration. We further investigated the correlations of phenological and snowpack seasonal variations with the cumulative percentiles of daily streamflow over two transition periods: vernal (between snowmelt and green‐up) and autumnal (between senescence and snowpack development) windows. We found growing season significantly lengthened by ~30 days over the last four decades at the study watershed, driven by both earlier green‐up and later senescence. The long‐term snowfall significantly decreased by ~2.4 cm, and the snowpack duration has been shortened by ~25 days, mostly driven by earlier snow disappearance. While there were no significant changes in the lengths of vernal and autumnal windows, the normalized streamflow distributions over the vernal window have widened and the low‐frequency peak flow over the autumnal window has shifted earlier over the study period. Interestingly, although the snow disappearance timing has advanced by ~18 days, there was no evidence of changes in the timing of spring peak discharge. Our results suggest that with long‐term decreases in winter snowfall and snowpack duration, the impacts of snowpack dynamics on seasonal streamflow regimes over the vernal and autumnal windows have weakened. Meanwhile, the seasonal streamflow dynamics are more closely mediated by vegetation green‐up with time. This study emphasizes the importance of understanding the phenological responses to climate change for prediction of future seasonal flow regimes in forested watersheds.

Frozen soil change and its impact on hydrological processes in the Qinghai Lake Basin, the Qinghai-Tibetan Plateau, China

Study regionThe Qinghai Lake Basin, Qinghai-Tibetan Plateau. The Qinghai Lake is the largest inland saltwater lake in China. Study focusSignificant increase in runoff into the Qinghai Lake has been reported; however, the relationship between frozen soil changes and runoff remains poorly understood. This study investigated the temporal and spatial variations in frozen soil and associate effects on streamflow and soil moisture in the study region by a distributed eco-hydrological model. New hydrological insightsThe results illustrate that the coverage of permafrost decreased by about 13% from 1971 to 2015, and permafrost degradation mainly occurred in the elevation interval of 3600–4200 m. The maximum frozen depth averaged in the seasonally frozen ground significantly decreased by 0.06 m/10a, while the active layer thickness averaged in the permafrost enhanced by 0.02 m/10a. Permafrost degradation caused enhanced soil liquid water storage and an increase in freezing season runoff. The increase in runoff in the thawing season was dominated by changes in precipitation. The results suggest that frozen soil degradation altered the seasonal flow regime, leading to lags in the monthly runoff peak, and it increased the base flow and reduced the thawing season runoff. This offset of the competing impacts of frozen soil changes in different seasons led to a negative effect on annual runoff. This study provides new understandings of cryospheric hydrological responses to climate change.

Transport and partitioning of metals in river networks of a plain area with sedimentary resuspension and implications for downstream lakes

This study showed that metal transport and partitioning are primarily controlled by suspended solids with seasonal flow regimes in plain river networks with sedimentary resuspension. Eight metal species containing iron (Fe), manganese (Mn), cadmium (Cd), chrome (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn), in multiple phases of sediments, suspended solids (>0.7 μm), colloids (1 nm-0.7 μm) and dissolved phase (<1 nm) were analysed to characterize their temporal-spatial patterns, partitioning and transport on a watershed scale. Metal concentrations were associated with suspended solids in the water column and decreased from low flow to high flow. However, metal partitioning between particulate phase (suspended solids) and dissolvable phases (colloids and dissolved phase) was reversed and increased from low flow to high flow with decreased concentration of total suspended solids and median particle size. Partition coefficients (kp) showed differences among metal species, with higher values for Pb (354.3–649.0 L/g) and Cr (54.2–223.7 L/g) and lower values for Zn (2.5–25.2 L/g) and Cd (17.3–21.0 L/g). Metal concentrations in sediments increased by factors of 1.2–3.0 from upstream to downstream in watersheds impacted by urbanization. The behaviours of metals in rivers provide deeper insight into the ecological risks they pose for downstream lakes, where increased redox potential and organic matter may increase metal mobility due to algal blooms. Areas with heavy pollution of metals and the transport routines of metals in the river networks were also revealed in our research.

Seasonal Water Quality and Algal Responses to Monsoon-Mediated Nutrient Enrichment, Flow Regime, Drought, and Flood in a Drinking Water Reservoir.

Freshwater reservoirs are a crucial source of urban drinking water worldwide; thus, long-term evaluations of critical water quality determinants are essential. We conducted this study in a large drinking water reservoir for 11 years (2010–2020). The variabilities of ambient nutrients and total suspended solids (TSS) throughout the seasonal monsoon-mediated flow regime influenced algal chlorophyll (Chl-a) levels. The study determined the role of the monsoon-mediated flow regime on reservoir water chemistry. The reservoir conditions were mesotrophic to eutrophic based on nitrogen (N) and phosphorus (P) concentrations. An occasional total coliform bacteria (TCB) count of 16,000 MPN per 100 mL was recorded in the reservoir, presenting a significant risk of waterborne diseases among children. A Mann–Kendall test identified a consistent increase in water temperature, conductivity, and chemical oxygen demand (COD) over the study period, limiting a sustainable water supply. The drought and flood regime mediated by the monsoon resulted in large heterogeneities in Chl-a, TCB, TSS, and nutrients (N, P), indicating its role as a key regulator of the ecological functioning of the reservoir. The ambient N:P ratio is a reliable predictor of sestonic Chl-a productivity, and the reservoir was P-limited. Total phosphorus (TP) had a strong negative correlation (R2 = 0.59, p < 0.05) with the outflow from the dam, while both the TSS (R2 = 0.50) and Chl-a (R2 = 0.32, p < 0.05) had a strong positive correlation with the outflow. A seasonal trophic state index revealed oligo-mesotrophic conditions, indicating a limited risk of eutrophication and a positive outcome for long-term management. In conclusion, the Asian monsoon largely controlled the flood and drought conditions and manipulated the flow regime. Exceedingly intensive crop farming in the basin may lead to oligotrophic nutrient enrichment. Although the reservoir water quality was good, we strongly recommend stringent action to alleviate sewage, nutrient, and pollutant inflows to the reservoir.

Identification of Seasonal Streamflow Regimes and Streamflow Drivers for Daily and Peak Flows in Alaska

AbstractAlaska is among northern high‐latitude regions where accelerated climate change is expected to impact streamflow properties, including seasonality and primary flow drivers. Evaluating changes to streamflow, including flood characteristics, across this large and diverse environment can be improved by identifying the distribution and influence of flow drivers. Using metrics of mean monthly streamflow data from 253 streamgages, seasonal flow regimes were clustered to guide identification of seasonal flow drivers and form hydrologic groups for identification of peak‐flow populations. Nine seasonally distinct subclasses described variability within three classes dominated by (mostly fall) rainfall, (spring) snowmelt, and (summer) high‐elevation melt. The most glacierized basins exclusively grouped into high‐elevation melt subclasses, and less glacierized basins sometimes exhibited seasonal patterns aligned with rainfall‐dominated and snowmelt‐dominated regimes. Peak‐flow populations varied by subclass from dominant rainfall or dominant snowmelt to mixed rainfall–snowmelt or mixed rainfall, snowmelt, and high‐elevation melt. Within subclasses, rainfall generated higher mean peak flows (relative to mean annual flow) than snowmelt or high‐elevation melt. Seasonal flow regimes showed clear but complex associations with basin characteristics, primarily elevation and winter temperature, and with geographic location. These dependencies provided elevation‐based analogies for changes associated with warming and insights for seasonal flow regime prediction and hydrologic region delineation. These results provide a spatially comprehensive perspective on seasonal streamflow drivers across Alaska from historical data and serve as an important historical basis for analysis.

Response of Fish Assemblages to Habitat Changes and Fishing Activity in a Tributary of the Jinsha River in Southwest China

AbstractReservoir impoundment affects fish habitat condition, in turn likely influencing the spatial and temporal distribution patterns of fish assemblage structure in the impounded area. Fishing activity and instream gravel and sand mining are important anthropogenic factors, and flow discharge, water temperature, and precipitation are important natural environmental variables; both anthropogenic and natural factors likely have impacted fish assemblage structure in impounded and unimpounded waterbodies. We used multivariate analysis to evaluate the combined effects of two adjacent reservoir impoundments in the main stem of a large river (Jinsha River; the upper segments of the Yangtze River), overfishing, instream gravel and sand mining, the natural flow regime, and other environmental variables (water temperature and precipitation) on fish assemblage structure in the Heishui River, a small, free‐flowing tributary upstream of the mainstream reservoirs. Fish assemblage data and abiotic data collected over 10 years (2008, 2010, and 2011–2017) were analyzed. The results showed that fish assemblages of the Heishui River temporally clustered into two groups that were strongly associated with the two downstream cascade reservoirs (Xiangjiaba and Xiluodu reservoirs) in the main‐stem Jinsha River. The relationships between fish assemblage structure and abiotic factors were tested using a distance‐based linear model and were visualized using distance‐based redundancy analysis, showing that the main factors explaining the interannual and seasonal variation in fish assemblage structure were instream gravel and sand mining, mainstream reservoir impoundment, and seasonal flow regime. Among these, instream gravel and sand mining in this tributary played the most significant role in affecting the temporal variation of fish assemblage structure and explained 34.12% of the total variability. Mainstream reservoir impoundment and seasonal flow regime exerted secondary effects on the temporal variation of fish assemblage structure and together explained 13.39% (7.00% and 6.39%, respectively) of the total variability. This study suggests that the relevant administrative agencies should focus more on free‐flowing habitat maintenance in this tributary and the control of instream gravel and sand mining.

Modelling seasonal flow regime and environmental flow in Punarbhaba river of India and Bangladesh

Abstract Investigations of changing the flow regime and flow prediction are of vital societal and hydro-ecological importance in a transboundary river like Punarbhaba river between India and Bangladesh. The present paper investigated flow regime though advance periodicity models (Morlet’s wavelet transformation) at the seasonal scale. Flow prediction using advanced machine learning techniques like Support vector machine (SVM), Artificial Neural Network (ANN), Hybrid wavelet ANN (W-ANN), Random forest(RF) capturing the periodicity, duration, cyclic or semi-cyclic nature of flow wave or wavelet from the historical time series data (1978–2017) is very crucial for environmental flow management and estimating present and future states of environmental flow. Flow alteration modeling (using heat map) and estimation of environmental flow (using duration curve shifting and RVA methods) are another vital objectives of this work to know the present and future hydro-ecological state. The result of periodicity clearly identified two distinct flow regimes before and after 1992-93 triggered by damming over there in 1992. All the prediction models identified the declining trend of flow in all the seasons, however hybrid wavelet ANN model could be treated as the best suited because of its very high-performance level. The degree of hydrological alteration is identified very high in the post-hydrological alteration (PHA) (post-1992) period and it is likely to be intensified predicted period. The estimated environmental flow state in PHA falls under moderate to critically modified states but if alteration goes on in this way ecological deficit will be the obvious result. For the survival of the river estimated environmental flow could be released primarily.

An assessment of the potential impacts of climate change on freshwater habitats and biota of Indiana, USA

Recent climate-driven, physico-chemical changes documented in aquatic systems throughout the world are expected to intensify in the future. Specifically, changes in key environmental attributes of aquatic systems, such as water quantity, clarity, temperatures, ice cover, seasonal flow regimes, external loading, and oxygen content, will undoubtedly have a broad set of direct and indirect ecological consequences. Some anticipated impacts may be similar across different aquatic ecosystems, while others may be system-specific. Here, we review the potential effects of climatic changes for different freshwater habitats within the state of Indiana, USA, a Midwestern state with diverse land and water features. Given this heterogeneity and that the state is among the southernmost states of the US Midwest, evaluation of freshwater habitats of Indiana provides a useful perspective on potential impacts of climate change. In our study, we first review expected or anticipated changes to physico-chemical and habitat conditions in wetlands, lotic systems, small glacial lakes and Lake Michigan. We then highlight anticipated responses of select aquatic biota to these changes. We describe how climatic changes may interact with other anthropogenic stressors affecting freshwater habitats and consider the potential for evolutionary adaptation of freshwater aquatic organisms to mediate any responses. Given anticipated changes, we suggest aquatic ecosystem managers take a precautionary approach broadly applicable in temperate regions to (a) conserve a diversity of aquatic habitats, (b) enhance species diversity and both inter- and intra-population genetic variation, and (c) limit stressors which may exacerbate the risk of decline for aquatic biota.

Detecting Changes in River Flow Caused by Wildfires, Storms, Urbanization, Regulation, and Climate Across Sweden

AbstractChanges in river flow may appear from shifts in land cover, constructions in the river channel, and climatic change, but currently there is a lack of understanding of the relative importance of these drivers. Therefore, we collected gauged river flow time series from 1961 to 2018 from across Sweden for 34 disturbed catchments to quantify how the various types of disturbances have affected river flow. We used trend analysis and the differences in observations versus hydrological modeling to explore the effects on river flow from (1) land cover changes from wildfires, storms, and urbanization; (2) dam constructions with regulations for hydropower production; and (3) climate‐change impact in otherwise undisturbed catchments. A mini model ensemble, consisting of three versions of the S‐HYPE model, was used, and the three models gave similar results. We searched for changes in annual and daily stream flow, seasonal flow regime, and flow duration curves. The results show that regulation of river flow has the largest impact, reducing spring floods with up to 100% and increasing winter flow by several orders of magnitude, with substantial effects transmitted far downstream. Climate changed the total river flow up to 20%. Tree removal by wildfires and storms has minor impacts at medium and large scales. Urbanization, on the contrary, showed a 20% increase in high flows also at medium scales. This study emphasizes the benefits of combining observed time series with numerical modeling to exclude the effect of varying weather conditions, when quantifying the effects of various drivers on long‐term streamflow shifts.

Integrated modeling to assess flow changes due to future dam development and operation in Stung Sen River of Tonle Sap Lake Basin, Cambodia

Abstract In response to rapid development and high demand for electricity, Cambodia has planned to build more hydropower dams. As a result, there are significant concerns for the changes in seasonal flow regimes which will result in degrading fisheries and biodiversity downstream. In this paper, we assess how a multipurpose dam affects downstream flows. To predict the magnitude of flow changes, flows which were simulated using SWAT (Soil and Water Assessment Tool) and other necessary data were computed into HEC-ResSim (Reservoir Simulation) models to simulate regulated flow from the dam. Downstream flows under three different operation scenarios (full-level, low-level, and seasonal variation) were modeled to compare with the baseline flows. For low-level and seasonal variation operation scenarios of Stung Sen dam, daily and seasonal flows saw a significant change. There would be an average increase of 42% in dry season flow and an average decrease of 46% in wet season flow, with the corresponding standard deviation of 22% and 19%, respectively, at the outlet of Stung Sen Basin, resulting from an operation to maximize energy production. The natural river flow in this river would be significantly changed due to this dam construction.

Preliminary evidence of spawning phenologies of freshwater fish in a wet–dry tropical river: the importance of both wet and dry seasons

Wet–dry tropical rivers are characterised by highly predictable, yet highly variable, seasonal flow regimes. The wet season is often regarded as an important period of ecosystem productivity, dispersal and connectivity, and also for freshwater-fish spawning and recruitment. However, few studies have examined fish spawning across hydrological seasons in these rivers. We conducted a pilot study to determine (1) the temporal occurrence (and hence spawning period), and (2) the suitability of standard sampling methods of young fish in the Daly River, Northern Territory, Australia. Fish spawned throughout the year, with spawning phenologies varying substantially among species. The highest diversity and abundance of young fish occurred during the wet season, although early life stages of a high number of species were also present in the dry-season and transition periods. A high number of species spawned all year round, whereas other species had very discrete spawning periods. Three of the four sampling methods tested were successful in catching early life stages and should be employed in future studies. The present study highlighted that all hydrological seasons in the wet–dry tropics are important for fish spawning, and has important implications for future research on the drivers of spawning patterns, and for predicting the effects of flow modifications on freshwater fishes of the wet–dry tropics.

Similarity in seasonal flow regimes, not regional environmental classifications explain synchrony in brown trout population dynamics in France

Abstract Investigating the variables driving synchrony in populations and the spatial scales at which they operate is worthwhile for both a basic understanding of population dynamics and for management. Various environmental classifications have been developed to group sites according to predefined similarities in abiotic variables (with spatial and/or temporal components) and to explain population dynamics in running waters. These classifications may be useful to managers and decision makers, as studies have shown relationships between time‐dependent population variability and environmental classifications on different spatial scales. We investigated whether synchrony among brown trout (Salmo trutta) populations could be explained by three regional (hydro‐ecoregions, hydrological units, and natural flow regime classification) and one local environmental classification and their associated variables. We first analysed whether densities of two size classes (0+ and &gt;0+ fish) varied over a large spatial scale (i.e. 112 sites spread throughout France sampled for &gt;11 years). We then determined whether the degree of synchrony matched the spatial patterns defined by the environmental classifications. Finally, we investigated whether variables such as river distance between sites, hydrology, and temperature were associated with patterns of synchrony. Overall, relationships observed between synchronies in environmental variables and trout population densities were weak. Synchrony was best explained by the classification based on hydrological units, defined as discrete physical units that provided boundaries with real biological significance for river assemblages (12% for 0+ density and 9% for &gt;0+ density and total densities). However, when considering environmental variables alone, synchrony in trout populations was mainly associated (from 11% to 23%) with seasonal flow variables such as high flows during reproductive and emergence periods. Temperature and river distance did not explain synchrony between trout populations. High flows during emergence appear to be a key driver of brown trout population dynamics. Floods during the reproductive period could explain synchrony among brown trout populations, although the effect was weaker than that of high flows during the emergence period. We concluded that existing physical, morphological, and biogeographical classifications at regional spatial scales were not sufficiently integrative to account for environmental drivers (such as flow variables) that were most associated with the spatiotemporal patterns of synchrony in trout populations. Flow explained synchrony better than classifications did. Further investigations should focus on the spatial patterns of flow variations during the emergence and reproductive periods to derive geographical areas that best capture synchrony patterns in trout dynamics. This approach could be extended to other freshwater fish species for which climatic drivers are important, and may help to explain the distributional changes of exotic species, the propensity of populations to fluctuate synchronously and the scale of synchrony patterns under different scenarios of climate change, or the threat of extinction of native species if the Moran effect is high.

Effects of 21st century climate change on seasonal flow regimes and hydrologic extremes over the Midwest and Great Lakes region of the US

Analyzing future changes in hydrologic extremes such as floods, low flows, and soil moisture extremes is important because many impacts on ecosystems and human systems occur during extreme events. To quantify changes in hydrologic extremes, this study conducts hydrologic modeling experiments over 20 Midwestern watersheds using the Variable Infiltration Capacity (VIC) model forced by historical observed datasets and future projections from statistically downscaled Global Climate Model (GCMs) simulations. Our results show that peak daily streamflow at the 100-yr reoccurrence interval will increase (+10–30%) in most watersheds by 2080s due to significant increases in precipitation (P) and increasing P as rainfall during winter and spring seasons. The simulations also show strong shifts towards earlier peak flow timing (up to a month), especially in strongly snowmelt-dominated watersheds. These effects are linked to strong decreasing trends in maximum Snow Water Equivalent (SWE) with warming, which are simulated over essentially the entire domain. Projected changes in 7-day extreme low flows are smaller in magnitude (−10–+10%) with somewhat larger decreases simulated at the end of century; however, the timing of extreme low flows is projected to shift from winter/spring to summer and fall in strongly snowmelt-dominated watersheds in the northernmost parts of the domain. Extreme low soil moisture increases over most of the domain in the future projections up to the 2050s, but by the 2080s there are more widespread decreases in extreme low soil moisture, especially in the northernmost parts of the domain.