Summer Flows Research Articles

Projections of mean and extreme precipitation over China and their resolution dependence in the HighResMIP experiments

Projected near-future changes in mean and extreme precipitation by the end of the mid-21st century over China are investigated using the latest CMIP6 HighResMIP simulations grouped by different categories of horizontal and vertical resolutions. Evaluation of the historical simulations demonstrates improvements in the realism of the simulated precipitation over the different sub-regions with increased model resolutions. More apparent improvements are displayed by models with increased vertical resolution than those with increased horizontal resolutions. The near-future scenario of SSP5–8.5 suggests general increases in mean and extreme precipitation across Northwest and East-Northeast China. Also, models with coarser horizontal resolutions project an intensified precipitation seasonality over China. Models at relatively high vertical resolutions project decreased precipitation and enhanced droughts over Southwest China, while these are less apparent in models with relatively coarse vertical resolutions. All the simulations project a northward shift of summer and winter monsoon flows in East Asia. Meanwhile, a weaker summer monsoon across the tropics with a weakening of the monsoon trough over the South China Sea are projected. This indicates a less favorable environment for precipitation across the southern sub-regions. The complicated responses of precipitation to these environmental mechanisms are partly linked to the presented resolution-dependent uncertainties. Such uncertainties should be recognized and treated with caution in understanding the near-future hydrological changes in China.

Multi-year renewal of green tides: 18 years of algal mat monitoring (2003–2020) on French coastline (Brittany region)

To better understand the green tide phenomenon impacting French coastline and give guidance to stakeholders to elaborate effective mitigation plan, an extensive survey has been deployed on Brittany hot spots. Based on 18 years monitoring database, the objectives of this work were to investigate the inter-and-intra annual evolutions of Ulva beaching, and to assess the parameters driving green tide annual renewal. The yearly cumulated area of Ulva mat on the Brittany coast averaged 2,42 ± 0,84 ha, of which 55 ± 12 % % was reported within Saint Brieuc-Binic Bay, with a maximal beaching generally observed in July. The renewal of green tide at spring time seems to be correlated with the Ulva bloom from the previous autumn period, particularly in bays with low exposure to swell. The residual stock, defined as Ulva fragments maintained in healthy conditions during the winter season, appears as highly dependent on nitrogen summer flows.

The UKSCAPE-G2G river flow and soil moisture datasets: Grid-to-Grid model estimates for the UK for historical and potential future climates

Abstract. Appropriate adaptation planning is contingent upon information about the potential future impacts of climate change, and hydrological impact assessments are of particular importance. The UKSCAPE-G2G datasets were produced, as part of the Natural Environment Research Council (NERC) UK-SCAPE (UK Status, Change and Projections of the Environment) programme, to contribute to this information requirement. They use the Grid-to-Grid (G2G) national-scale hydrological model configured for both Great Britain and Northern Ireland (and the parts of the Republic of Ireland that drain to rivers in NI). Six separate datasets are provided, for two sets of driving data – one observation-based (1980–2011) and one climate-projection-based (1980–2080) – for both river flows and soil moisture on 1 km × 1 km grids across Great Britain and Northern Ireland. The river flow datasets include grids of monthly mean flow, annual maxima of daily mean flow, and annual minima of 7 d mean flow (m3 s−1). The soil moisture datasets are grids of monthly mean soil moisture content (m water / m soil), which should be interpreted as depth-integrated values for the whole soil column. The climate-projection-based datasets are produced using data from the 12-member 12 km regional climate model ensemble of the latest UK climate projections (UKCP18), which uses RCP8.5 emissions. The production of the datasets is described, along with details of the file format and how the data should be used. Example maps are provided, as well as simple UK-wide analyses of the various outputs. These suggest potential future decreases in summer flows, annual minimum 7 d flows, and summer/autumn soil moisture, along with possible future increases in winter flows and annual maximum flows. References are given for published papers providing more detailed spatial analyses, and some further potential uses of the data are suggested. The datasets are listed in Table 1.

A century of channel change caused by flow augmentation on Sixth Water Creek and Diamond Fork River, Utah, USA

AbstractThe Diamond Fork River in central Utah, USA experienced extreme flow augmentation via transbasin flows. Beginning in 1916, irrigation water was delivered through a tributary, Sixth Water Creek, with daily summer flows regularly exceeding a 500‐year flood at the point of introduction. Flows were dramatically reduced by management action in 2004 but with mandated minimum flows. We examined the geomorphic response of Sixth Water and Diamond Fork using aerial imagery, lidar, topographic cross sections, and sediment transport measurements. River channel response varied with valley confinement and with position in the watershed, which determined the magnitude of augmented flow relative to natural floods and the amount of sediment supply. Confined, steep sections of Sixth Water incised many meters into bedrock, whereas partially confined and unconfined sections developed a braided form even under conditions of general incision. With the removal of flow augmentation, smaller natural floods on Sixth Water are unable to transport bed material and the channel remains static. On the alluvial lower Diamond Fork, a lower slope, upstream sediment supply, and larger natural floods produced a dynamic shifting channel that widened in response to natural floods. After flow augmentation, a coarsened bed is partially mobile and channel narrowing appears to be limited by artificial baseflow, which prevent vegetation establishment in the channel.

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.

Riparian cottonwood mortality following compound impacts from river water withdrawal and hydroclimatic variation

AbstractOriginating in Glacier Park, Montana, the transboundary St. Mary River has provided a focus for international water sharing for a century. It was dammed and diverted in the USA, and more extensively in Alberta, where it supports Canada's centre for agricultural irrigation. Following water withdrawals, the riparian cottonwoods (Populus angustifolia) collapsed, and we assessed the woodland decline downstream from the St. Mary Dam, through eight sets of aerial photographs over seven decades. These revealed 88% woodland loss from 1951 to 1999 (R2 = 0.984), including a steeper decline with the 1980s drought. Following the implementation of an environmental flow regime that increased minimum flows after 1993, the remnant woodlands were stabilized, with a slight recovery by 2022. Analyses of the historical hydrology revealed compound contributions to the woodland mortality. (1) Annual river flows declined with increasing water diversion, and (2) late summer flows were particularly depleted. (3) Dam operations resulted in abrupt stage recessions and irregular spikes. (4) Accompanying climate warming, the spring snowmelt advanced and late summer flows declined. (5) The greatest climatic influence involved multiple‐year clusters with high versus low flows, corresponding with the Pacific decadal oscillation (PDO) from 1920 to 1990 (R2 = 0.365), but less coupled thereafter. This long‐term study demonstrates the coordination between hydroclimatic variation, water management and the fate of riparian woodlands, which reveal ecosystem health. An environmental flow regime provided benefit, and for other regulated rivers in dry ecoregions, we recommend functional flow regimes that provide sufficient minimum flows and avoid abrupt flow recession or irregular spikes.

Absorption Chiller System Driven by the Solar Hybrid System: Case Study in the Algeria Weather Condition

Abstract In this paper, the performances of a novel hybrid solar system using the Al2O3 nanofluid and pure water as a heat transfer fluid to operate a single-effect lithium bromide absorption chiller are investigated. In which the performance of the proposed system using the nanofluid during winter and summer under mixed and forced convection is evaluated. Thus, the performances of the solar collector are investigated experimentally, and the output performances of the water–LiBr absorption chiller system are conducted numerically using matlab platform. The results show that the obtained Reynolds number of the heat transfer fluid is laminar flow in summer with the maximum values of 1700 and 1600 for nanofluid and water, respectively, and the maximum values of 2200 and 2100 for nanofluid and pure water, respectively, in winter. The proposed hybrid system achieves approximately 54% and 36% of maximum thermal efficiency during the winter and the summer, respectively. The obtained performance shows that the absorption cycle at positive evaporation temperatures is very appreciable using the nanofluid as working fluid during both seasons and quite satisfactory using the water during summer and winter seasons. Overall, the proposed system has potential for further development in the solar cooling system.

Assessment of future eco-hydrological regime and uncertainty under climate changes over an alpine region

The source region of the Yellow River Basin (SYRB) is an alpine region sensitive to climate changes. As global climate change intensifies, it is essential to explore the future hydrological regime alteration and its ecological effects on the basin to provide evidence for water resources management and ecological restoration in the area. This study offers a framework for assessing the future multi-dimensional hydrological regime alteration and its ecological effects. Selected from Global Climate Models (GCMs) that performed well in the Sixth Coupled Model Intercomparison Project (CMIP6), the VIC model was forced by the GCMs after downscaling and bias-correction. Then the hydrological regime alteration and potential ecological effects in the region under SSP126, SSP245, SSP585 future scenarios were systematically assessed by the IHA-RVA system, and the contribution of different uncertainty sources was quantified through the two-way ANOVA. The results indicated that: (1) The temperature and precipitation will increase in the future under all scenarios, while the runoff tends to show a downward trend. By 2015–2100, the flow under SSP126, SSP245 and SSP585 scenarios may vary from −44.7 to 13.0 %, −48.7 to 2.43 %, −53.7 to 4.0 %, respectively, when compared to the reference period (1961–1990). (2) Moderate alterations with RVA = 33 % will be observed in the hydrological regime in the basin under SSP245 and SSP585 scenarios, and low alterations with RVA = 26.7 % under the SSP126 scenario. The spring discharge of the basin will decrease significantly, and the annual extreme flow is projected to decrease, but the frequency of drought and flood events will tend to increase. (3) There are multiple sources of uncertainty in predicting the future hydrological regime in the SYRB. The uncertainty due to the climate model is the dominant factor, followed by the interaction between emission scenarios and the climate model, and the average contribution to uncertainty of them is 69.5 % and 20.8 % respectively. The uncertainty of emission scenarios is mainly reflected in winter flow, while the climate model mainly affects the summer flow and annual maximum flow, and the uncertainty of interaction between them varies in different periods.

Climate Change Impacts on Irish River Flows: High Resolution Scenarios and Comparison with CORDEX and CMIP6 Ensembles

Climate change is likely to impact water quality, resource availability and riverine ecosystems. While large ensembles are available to assess future impacts (e.g., the Coupled Model Intercomparison Projects (CMIP) and/or CORDEX) many countries have developed their own high-resolution ensembles. This poses a selection problem with robust adaptation dependent on plausible ranges of change being adequately quantified. Therefore, it is important to compare projected changes from available ensembles. Here we assess future climate change impacts for 26 Irish catchments. Using a high-resolution national ensemble of climate models projected impacts in mean, low and high flows are assessed and uncertainties in future projections related to catchment size. We then compare future impacts from CORDEX and CMIP6 ensembles for a subset of catchments. Results suggest increases in winter flows (-3.29 to 59.63%), with wide ranges of change simulated for summer (-59.18 to 31.23%), low (-49.30 to 22.37%) and flood (-19.31 to 116.34%) flows across catchments under RCP8.5 by the 2080s. These changes would challenge water management without adaptation. Smaller catchments tend to show the most extreme impacts and widest ranges of change in summer, low and flood flow changes. Both the ensemble mean and range of changes from the national ensemble were more modest and narrower than the CMIP6 and CORDEX ensembles, especially for summer mean and low flows, highlighting the importance of evaluating impacts across ensembles to ensure adaptation decisions are informed by plausible ranges of change.

Stratification and its consequences in two constructed urban stormwater wetlands

Stratification in constructed urban stormwater wetlands is one of the fundamental physical processes that affect hydrodynamics, transport and fate of stormwater pollutants. Adverse effects of stratification include decreasing pollutant retention capacity, causing the water at lower depths to become anoxic, degrading water quality and increasing stress on the downstream aquatic communities. The current study reports on a comprehensive field monitoring program of stratification and hydrodynamics in two ice-free seasons (May – October) in two constructed urban stormwater wetlands in Calgary, Canada, with different inlet, outlet, morphometric and vegetation designs. Despite their small sizes of 0.5 and 1.2 ha and shallow water depths of 0.8 m, stratification was strong and persistent in the wetlands. The response of stratification and mixing to atmospheric forcings (e.g., air temperature, atmospheric instability, rainfall depth, wind speed) and the impact of design characteristics (inlet/outlet design, water depth, surface area and aquatic vegetation) were examined and discussed. Thermal stratification, defined as a vertical temperature gradient >1 °C/m, was found to be significantly higher (up to ten times) near the inlets and last longer (up to twice) than in the main cells and the outlet basins due to the relatively cold summer inflows. The wetland with twice the permanent water volume and surface area and half the length-to-width ratio had denser submerged aquatic vegetation, higher (by up to 2 °C) water temperature and more severe (up to eight times) thermal stratification. Strong densimetric stratification and low wind stress on the water surface caused hypoxic conditions near the bed, potentially adversely affecting water quality and downstream aquatic communities.

Validation of Nadir SWH and Its Variance Characteristics from CFOSAT in China’s Offshore Waters

The offshore waters of China are a typical monsoon−affected area where the significant wave height (SWH) is strongly influenced by the different seasonal mean flow in winter and summer. However, limited in situ validations of the SWH have been performed on the China–France Oceanography Satellite (CFOSAT) in these waters. This study focused on validating CFOSAT nadir SWH data with SWH data from in situ buoy observations for China’s offshore waters and the Haiyang−2B (HY−2B) satellite, from July 2019 to December 2021. The validation against the buoy data showed that the relative absolute error has a seasonal cycle, varying in a narrow range near 35%. The RMSE of the CFOSAT nadir SWH was 0.29 m when compared against in situ observations, and CFOSAT was found to be more likely to overestimate the SWH under calm sea conditions. The sea−surface winds play a key role in calm sea conditions. The spatial distributions of the CFOSAT and HY−2B seasonal SWHs were similar, with a two−year mean SWH−field correlation coefficient of 0.98. Moreover, the coherence between the two satellites’ SWH variance increased with SWH magnitude. Our study indicates that, in such typical monsoon−influenced waters, attention should be given to the influence of sea conditions on the accuracy of CFOSAT SWH, particularly in studies that combine data from multiple, long−duration space−based sensors.

A mixed distribution approach for low-flow frequency analysis – Part 1: Concept, performance, and effect of seasonality

Abstract. In seasonal climates with a warm and a cold season, low flows are generated by different processes so that the annual extreme series will be a mixture of summer and winter low-flow events. This leads to a violation of the homogeneity assumption for all statistics derived from the annual series and gives rise to inaccurate conclusions. In this first part of a two-paper series, a mixed distribution approach to perform frequency analysis in catchments with mixed low-flow regimes is proposed. We formulate the theoretical basis of the mixed distribution approach for the lower extremes based on annual minima series. The main strength of the model is that it allows the user to estimate return periods of summer low flows, winter low flows, and annual return periods in a theoretically sound and consistent way. Using archetypal examples, we show how the model behaves for a range of low-flow regimes, from distinct winter and summer regimes to mixed regimes where seasonal occurrence in summer and winter is equally likely. The examples show in a qualitative way the loss in accuracy one has to expect with conventional extreme value statistics performed with the annual extremes series. The model is then applied to a comprehensive Austrian data set to quantify the expected gain of using the mixed distribution approach compared to conventional frequency analysis. Results indicate that the gain of using a mixed distribution approach is indeed large. On average, the relative deviation is 21 %, 39 %, and 63 % when estimating the low flow with a 20-, 50-, and 100-year return period. For the 100-year event, 75 % of stations show a performance gain of >10 %, 41 % of stations > 50 %, and 25 % of stations > 80.6 %. This points to a broad relevance of the approach that goes beyond highly mixed seasonal regimes to include the strongly seasonal ones. We finally correlate the performance gain with seasonality indices in order to show the expected gain conditional to the strength of seasonality expressed by the ratio of average summer and winter low flow seasonality ratio (SR). For the 100-year event, the expected gain is about 70 % for SR=1.0, 20 % for SR=1.5, and 10 % for SR=2.0. The performance gain is further allocated to the spatial patterns of SR in the study area. The results suggest that the mixed estimator is relevant not only for mountain forelands but to a much wider range of catchment typologies. The mixed distribution approach provides one consistent approach for summer, winter, and annual probabilities and should be used by default in seasonal climates with a cold winter season where summer and winter low flows can occur.

Impact of climate change on hydro-meteorological trigger conditions for debris flows in Austria

Debris-flow activity is expected to change in a future climate. In this study we connect a susceptibility model for debris-flows on a regional scale with climate projections until 2100. We use this to assess changes of hydro-meteorological trigger conditions for debris flows in six regions in the Austrian Alps. We find limited changes on an annual basis, but distinct changes when separating between hydro-meteorological trigger types and regions. While regions in the east and in the south of Austria may experience less days susceptible to debris flows in summer, there is a general trend of increasing susceptibility earlier in the year for both, rainfall-related and snow-related trigger conditions. The outcomes of this study serve as a basis for the development of adaption strategies for future risk management from this debris-flow hazard.

A century of variations in extreme flow across Finnish rivers

River flow in cold climates is known to be one of the hydrological systems most affected by climate change, playing a central role in the sustainability of downstream socio-ecological systems. Numerous studies on the temporal and spatial variations of streamflow characteristics have been done, and a comprehensive study on the variation of hydrologic extremes is becoming increasingly important. This study evaluated the long-running changes in the magnitude, time, and inter-annual variability of hydrologic extremes, including high and low flow in 16 major Finnish rivers. We applied four new hydrologic extreme indices for summer–winter low flow ratio, spring-absolute high flow ratio, time-to-peak index, and increasing rate index during the snowmelt period to analyze the spatiotemporal variations of extreme streamflow from 1911 to 2020. The most detected trends in flow regimes have started in the last six decades and become more severe from 1991 to 2020, which is likely to be dominated by anthropogenic global warming. The results also indicated that alteration of low pulses in most rivers was associated with an increase (decrease) in winter (summer) flows, suggesting the annual minimum flow in summer frequently contradicts natural hydrologic regimes in Arctic rivers. Southern Finland has experienced higher variations in extreme hydrology over the last century. A new low flow regime was detected for southern rivers, characterized by frequent annual minimum flow in summer instead of winter. Moreover, the annual maximum flow before/after spring dictated a new high-flow regime characterized by frequent double peak flows in this region.

Probabilistic-forecasting method for determination of low flow discharge of Pivdennyi Buh, Black Sea area, and Lower Dnieper rivers

The aim of the study is to substantiate the probabilistic-forecasting method for forecasting the low flow discharge and its implementation for the Pivdennyi Buh, Black Sea area and Lower Dnieper rivers, taking into account climatic patterns of precipitation distribution and establishing probabilistic characteristics of low flow discharge in multi-year period. The study area is in a zone of significant risk due to the shortage of water resources, the formation of extremely low runoff in the dry flow period, which requires its definition and forecasting. The methodological basis of forecasts is to solve the equation in determining the components of the low flow of rivers by constructing regional dependences for forecasting the average decade summer-autumn low flow from previous water discharge (in runoff modules), establishing their probabilities water discharge for a number of intakes. Results. The paper substantiates the probabilistic-forecasting method for forecasting low flow discharge (in summer, autumn and winter periods) based on the construction of regional dependences of average decadal runoff modules on previous water discharge for groups of basins of studied rivers taking into account climatic dependences of precipitation distribution in the territory and the establishment of probabilistic characteristics of the low flow water discharge in a multi-year period. The methodology of territorial short-term forecasts of average decade water discharge of low flow of summer, autumn and winter river runoff is assessed as satisfactory with a margin of error of 70 % to 97 %, with a number of members of more than 500 points. To determine the cumulative probability of the forecast values of the average decade water discharge of the summer, autumn and winter dry weather flow, the empirical distribution of the average monthly water discharge in the limited seasons, which are generalized in the basins of the studied rivers of Ukraine, is established. Scientific novelty. For the first time for the zone of insufficient natural water content of rivers the method of territorial forecasts of low flow discharge, determination of their probability of occurrence in a multi-year period and cartographic representation of prognostic values have been developed and practically implemented. The practical importance is the use of forecast modules maps of low flow for spatial monitoring and assessment of low water levels on rivers in the whole region, including ungauged rivers, and their probabilistic characteristics – to assess the possible occurrence of low runoff, even when it reaches values close to the environmental runoff, which are critical for the functioning of the river ecosystem.

How do inorganic nitrogen processing pathways change quantitatively at daily, seasonal, and multiannual scales in a large agricultural stream?

Abstract. Large agricultural streams receive excessive inputs of nitrogen. However, quantifying the role of these streams in nitrogen processing remains limited because continuous direct measurements of the interacting and highly time-varying nitrogen processing pathways in larger streams and rivers are very complex. Therefore, we employed a monitoring-driven modelling approach with high-frequency in situ data and the river water quality model Water Quality Analysis Simulation Program (WASP) 7.5.2 in the 27.4 km reach of the sixth-order agricultural stream called Lower Bode (central Germany) for a 5-year period (2014–2018). Paired high-frequency sensor data (15 min interval) of discharge, nitrate, dissolved oxygen, and chlorophyll a at upstream and downstream stations were used as model boundaries and for setting model constraints. The WASP model simulated 15 min intervals of discharge, nitrate, and dissolved oxygen with Nash–Sutcliffe efficiency values higher than 0.9 for calibration and validation, enabling the calculation of gross and net dissolved inorganic nitrogen uptake and pathway rates on a daily, seasonal, and multiannual scale. Results showed daily net uptake rate of dissolved inorganic nitrogen ranged from −17.4 to 553.9 mgNm-2d-1. The highest daily net uptake could reach almost 30 % of the total input loading, which occurred at extreme low flow in summer 2018. The growing season (spring and summer) accounted for 91 % of the average net annual uptake of dissolved inorganic nitrogen in the measured period. In spring, both the DIN gross and net uptake were dominated by the phytoplankton uptake pathway. In summer, benthic algae assimilation dominated the gross uptake of dissolved inorganic nitrogen. Conversely, the reach became a net source of dissolved inorganic nitrogen with negative daily net uptake values in autumn and winter, mainly because the release from benthic algae surpassed uptake processes. Over the 5 years, average gross and net uptake rates of dissolved inorganic nitrogen were 124.1 and 56.8 mgNm-2d-1, which accounted for only 2.7 % and 1.2 % of the total loadings in the Lower Bode, respectively. The 5-year average gross DIN uptake decreased from assimilation by benthic algae through assimilation by phytoplankton to denitrification. Our study highlights the value of combining river water quality modelling with high-frequency data to obtain a reliable budget of instream dissolved inorganic nitrogen processing which facilitates our ability to manage nitrogen in aquatic systems. This study provides a methodology that can be applied to any large stream to quantify nitrogen processing pathway dynamics and complete our understanding of nitrogen cycling.

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.

Seasonally distinct runoff–recharge partitioning in an alpine tundra catchment

AbstractHydrological processes within the alpine tundra of the Taiga Cordillera ecozone in northwestern Canada are poorly understood, yet these areas receive more precipitation per unit area than lowlands and sustain late summer and winter flow in large river systems when contributions from other areas are reduced. The objective of this study was to quantify the spatial and temporal variability in streamflow and groundwater recharge within an alpine tundra basin with discontinuous permafrost and explore the potential impacts of climate change on the timing and intensity of these hydrological processes. Hydrometric and remote sensing methods were used to complete a water balance assessment of the study basin and compare spatial and seasonal differences in inputs, outputs, runoff ratio, and runoff–recharge partitioning during the 2019 open water season. During the freshet, the basin received large daily melt volumes and responded with highly efficient runoff. Evapotranspiration became the primary means of water loss in the early summer but declined as the summer progressed. During the summer, groundwater discharge exceeded precipitation inputs and sustained headwater subbasin streamflow. Groundwater recharge occurred primarily via glaciofluvial upland infiltration during the freshet and channel bed infiltration during the summer. The partitioning of basin outputs between runoff and groundwater recharge was highly seasonal, with a freshet ratio favoring runoff (0.83) while the early and late summer favored recharge (0.28 and 0.17, respectively). As climate change continues, higher air temperatures and greater precipitation are expected for the study basin. Longer open water seasons and declining permafrost extent within the study basin will result in a greater proportion of input water routed to storage and/or groundwater recharge instead of runoff. Shrubification and treeline expansion may also increase evaporative losses from alpine tundra areas, reducing both rapid runoff and delayed aquifer recharge contributions important for larger rivers at lower elevation.

The mechanics of diurnal thermal stratification in river pools: Implications for water management and species conservation

AbstractWe examined conditions that form or prevent thermal stratification in river pools using field measurements, statistical modelling, and three‐dimensional (3D) computational fluid dynamics (CFD) modelling. Our motivation is to identify variables that control stratification for exploitation to enhance or prevent thermal gradients as needed to benefit species in rivers. One study pool (UT) is located above water storage reservoirs and receives natural flows, and the other pool (PT) is regulated and receives unnaturally high, cold water in summer; both pools are on the Trinity River, California. Thermal stratification formed in UT pool in spring at a critical flow of 1.01 m3/s, peaked at 8.1°C in summer, and exhibited diurnal formation and destruction under sub‐critical flows until fall. At PT pool, the 14.2 m3/s baseflow caused mixing that prevented stratification and formed a spatially homogenous thermal environment. Statistical analyses indicated the daily range in air and inlet water temperature at UT pool best correlated with the occurrence and strength of stratification but were progressively irrelevant as flows increased above the critical value. The validated CFD model was used to reproduce the diurnal stratification and explain the processes involved. The CFD model correctly predicted the observed critical flow and replicated the dynamics of stratification at UT pool and isohyets observed at PT pool. The CFD results also confirmed that low flows are the main variable for stratification to form, and the daily range in inlet water temperature drives the strength of the thermal gradient. The model estimated a critical discharge at PT pool of 2.0 m3/s, twice that for UT pool owing to its 2.6‐times larger area, suggesting critical flows scale with pool size. Results showed that releasing critical and lower flows in summer on regulated streams may conserve water and provide thermal gradients that benefit poikilothermic species; alternately, higher than critical flows can prevent stratification where needed to improve water quality.

Effect of the continental shelf in seasonal oceanographic conditions at the Northern edge of the Eastern Tropical Pacific

The continental shelf in front of Nayarit is located in the northern limit of the tropical Eastern Pacific, characterized by constituting a convergence zone of different masses of water, forming an area of significant hydrographic variability. Based on satellite remote sensing data and reanalysis between 2003 and 2019 of sea surface temperature (SST), wind stress, Ekman velocity, and geostrophic velocity, the contribution of the seasonal cycle to the variability of the study area were analyzed through climatological means, Hovmöhler diagrams, and Empirical Orthogonal Functions. The results show that in the zone in front of Matanchén Bay (21.5 °N) and the adjacent continental shelf, there is a core of warm surface water. The distribution of the SST is explained by the seasonal pattern of meridional/zonal variability in the thermal gradient, where the EOFs show the influence of the annual scale in both modes, with the only difference being that the first describes the meridional gradient as the dominant signal (66.2%), and the second shows the zonal behavior of the thermal gradient (16.6%). The summer weakening of the wind stress and Ekman speed is the product of the irregular shape of the coastline, the extension of the continental shelf, and the divergence of the North American monsoon around 21°N, whereas during the rest of the year an intensification of these parameters prevailed in front of Cabo Corrientes. On the other hand, the intense geostrophic flow in summer does not contribute to the increase in SST on the continental shelf because it diverges around 22.5°N. Likewise, during the winter, the formation of a cyclonic geostrophic gyre located inside the continental shelf, between the coast and the Marías Islands, stands out.