Mean Annual Flow Research Articles

Kyrgyz transboundary rivers’ runoff assessment (Syr-darya and Amu-darya river basins) in climate change scenarios

The research was carried out as part of the preparation “Water Resources” section of the 4th National Communication of the Kyrgyz Republic to the UNFCCC. The article presents extensive information on the current state and changes over a long period of observations of water resources, climate and glaciation in Kyrgyzstan (Syr-Darya and Amu-Darya river basins). The representative rivers were selected as the main objects, which are transboundary between Kyrgyzstan, Uzbekistan, Kazakhstan and Tajikistan. The study aims to assess the inter-annual and intra-annual dynamics of the flow of the rivers for the period from 2020 to 2080, based on the climate projections CMIP5 (RCP4.5 and RCP8.5) and CMIP6 (SSP2-4.5 and SSP5-8.5) (IPCC WG II Report, 2022). The hydrological modeling method in the HBV light and HBV EHT models and the inertial mean annual flow change method were used to estimate changes in water resources. On the rivers of the northern part of the Fergana Valley, the average annual runoff is expected to increase by 1-19%, in the Amudarya river basin (Kyzyl-Suu river) – by 27-64% of the values for 2006-2019. In the Naryn river, water discharge for this period will remain within the current values for 2006-2019. The method of inertial change in the average annual runoff showed an increase in runoff for 2030 and 2040 for all the studied catchment areas by 6-19% of the current values for 2006-2020. The results of the study are intended for decision-makers on the rational use and long-term planning of water resources under climate change.

Integrated assessment of climate change and reservoir operation on flow-regime and fisheries of the Sekong river basin in Lao PDR and Cambodia

This study assesses the cumulative impact of climate change and reservoir operation on flow regime and fisheries in the Sekong River Basin. Ensemble of five selected Regional Climate Models (RCMs) were used to project the future climate under RCP4.5 and RCP8.5 scenarios. The projected future climate was used to simulate the future hydrology using the SWAT model while HEC-ResSim was utilized for reservoir simulation. Finally fish-flow relationship was developed to estimate the fish catch and productivity in future. Upon investigation we found that, Sekong River Basin is likely grow warmer and drier in future under climate change. The basin is expected to face 1.3–3.6 °C rise in mean annual temperature and receive 0–6% less annual rainfall in future. The wet season in the basin is anticipated to be drier (0% to −6%) while the dry season rainfall shows no particular trend (−3%–10%). Such a change in climate is likely to alter the mean annual flow in future between -3 and 5% at Attapeu, -6 to 2% at Ban Veunkhane, Lao PDR, and −7 to 1% at Siempang, Cambodia (basin outlet). Under climate change, we expect decrement in minimum flow but increment in the maximum flow while opposite is anticipated under reservoir operation. Operation of Xekaman 1 and Sekong 4A are likely to increase the minimum flow at river outlet by 32–59% and 13–18% respectively whereas maximum flow is expected to decrease by 28–5%. In addition, climate change is likely to have crucial impact on fisheries with up to 19% and 12% reduction in fish catches and fish productivity respectively. However, reservoirs tend to have negligible impact on fisheries.

The role of Renaissance dam in reducing hydrological extremes in the Upper Blue Nile Basin: Current and future climate scenarios

Climate change poses a great threat to society through its effects on extreme hydrological events (floods and droughts) and the sustainable development. Coping with climate change requires a better awareness of the possible impacts to inform climate adaptation strategies. In this study, we investigated the climate projections from two contrasting GCMs (wet scenario from MIROC5 and dry scenario from CSIRO) and quantified the possible outcomes in terms of hydrological components of the Upper Blue Nile Basin (UBNB). These two climate projection scenarios show that the overall precipitation change in the early (2011–2040), mid (2041–2070), and late century (2071–2100) over the UBNB may vary from –18.3 % to + 13.6 % of the baseline annual precipitation of 1350 mm. The hydrological simulations over the basin from our study showed that evapotranspiration may vary between –13.7 % and + 12.1 % of the average annual total actual ET of 710 mm, and the flow at the basin outlet may range between –40.7 % and + 30.8 % of the mean annual total flow volume of 45 billion cubic meters. As a result, the wet projection exhibited more frequent floods while the dry projection showed severe droughts, specifically in the late century. We also examined the role of the Grand Ethiopian Renaissance Dam (GERD), designed for hydropower generation, in moderating these potential future climate hydrological extremes. Our findings indicate that GERD operations can help reduce downstream flood and drought severity by managing flow releases. Thus, this study illustrates the most possible extreme hydrological events in the UBNB due to climate change and demonstrates the degree to which GERD operations can help reduce the impact of these extremes downstream. The findings and data generated in this study will aid understanding the importance of sustainable water management and reservoir operation that caters to both hydropower generation, and recurring floods and droughts.

Assessment of reservoir performance under climate change: A case study in Shumbrite reservoir, South Gojjam sub-basin, Ethiopia

Climate change has the potential to reduce the water resource availability and hence the amount of storage in reservoirs. In this study, we assessed the impact of climate change on the performance of the Shumbrite reservoir using outputs from the CORDEX-Africa project, MPI Global Circulation Model (GCM) under three Representative Concentration Pathways (i.e., RCP2.6, RCP4.5 and RCP8.5). The trend analysis of future precipitation shows a decrease in the range of 0.57%–4.2%, with a maximum decrease under RCP2.6, and temperature (both maximum and minimum) shows an increasing trend as compared to the baseline period. The HBV model simulated future mean annual flow volume shows a decrease (ranges 0.06% to 1.64%) for RCP2.6 and RCP4.5 and a significant increase of 5.03% for RCP8.5 compared to the baseline period. Mean annual irrigation water demand from CROPWAT shows an increase (0.74% to 8.4%) for all RCPs compared to the baseline period. The time-based reliability for the Shumbrite reservoir has an average value above 80%. The resilience values are below 50% for all RCPs and hence the reservoir will take an extended period to recover from an inability to achieve the target demand volume. The dimensionless vulnerability shows a value below 50% which indicates that the reservoir has no insufficient flow to meet the demand. Generally, the reservoir can be used sustainably with operation rule curves. Generally, this study adds a contribution to the evaluation of small-scale reservoirs under a changing climate and the preparation of guide curves for the respective release.

Assessment of the impact of the Dniester Hydropower Complex on hydrological state of the Dniester River

Operation of the Dniester Hydropower Complex (DHC), build on the middle course of the Dniester River in the middle of the '80s of the last century and extended during last decades, on one hand, produces low-cost energy and contributes to local and regional economical development, but on the other hand, leads to modification of river flow and ecosystems in the downstream, creating a series of dilemmas that are difficult to manage and solve by bazinal countries, Ukraine and the Republic of Moldova. This research aims to assess the changes in flow, phases of the hydrological regime, water temperature and sediments’ regime due to the DHC operation. Main utilized approach was comparative analysis of hydrological time series recorded at the stations situated upstream and downstream of the DHC, for two representative time periods: before and after construction of this hydropower complex. As a result, it was estimated that the mean annual flow downstream the DHC decreased by 9.2%. Seasonal flow changed mainly by significant decrease in February-April (February - 18%, March - 40%, April - 27%), and increase in the autumn months, by 10-14%.Minimum flows upstream of the DHC, increased by 52%, and downstream have doubled, reaching 107m3/s (compared to 51m3/s, before the DHC construction). Maximum annual flow, in the upstream part, in the second period, has slightly increased, while towards the downstream part, there is a reduction of this parameter by about 30%.One of the direct impacts of the DHC operation is hydropeaking effect. Intraday level amplitude downstream of the DHC amounts to 52 cm and the length of the sector that is influenced by this effect is over 100 km. Also, a long river sector is subject to water thermal modifications: when upstream the average annual water temperature has risen by 0.8°C, in the downstream it has diminished by 0.44°C. On a monthly scale, there is a decrease in the water temperature in the spring-summer period, and an increase in the autumn-winter period downstream of the DHC. Sediment transport process was also altered significantly. Due to the DHC operation, suspended sediments decreased by 92-98% downstream of it. The significant decrease in sediment volumes is specific to all months of the year. The reduction of sediment transport has increasedthe transparency of water, which, as a result, influences the development of the aquatic ecosystems.

Implications of Land Use/Land Cover Changes and Climate Change on Black Volta Basin Future Water Resources in Ghana

The Black Volta River basin faces several challenges, which impede the sustainability of its water resources and biodiversity. Climate change coupled with land use/land cover (LULC) change patterns account for most of the observed hydrological changes in the basin. The aim of this study was to assess the impact of changes in the climate and LULC on water resources in the basin, and its effect on the livelihoods of downstream users, particularly regarding water allocations. The water evaluation and planning (WEAP) model was applied to the assessment of runoff and streamflow and the percentage future water demand under climate change scenarios (RCP 2.6 and RCP 8.5), as well as the effects of current and future changes on water supply systems. LULC data from 1990 to 2019 were processed to detect the changes in LULC patterns in the basin. The results showed that from 1990 and 2019, the land use classes of settlements/bare ground, open savannah woodland, croplands, and waterbodies increased by 339.5%, 77.4%, 24.4%, and 607%, respectively. Close savannah woodlands, wetlands, and grasslands all decreased by 97%, 99.8%, and 21.2%, respectively. Overall, there was a significant difference in LULC changes. Hence, measures needed to be put in place to curb the changes, as the observed changes posed a serious challenge to the basin’s water resources. The results from the WEAP simulations also indicated that in the future, changes in discharge would be visible in September with ranges between 0.72 × 106 m3 and 1.9 × 106 m3 for RCP 2.6, and 0.65 × 106 m3 and 2.5 × 106 m3 for RCP 8.5, per month. Although the median values illustrate an increase in water availability from river discharge compared with the reference scenario, the uncertainties in future changes largely exceeded the predicted increases. Annual variability of the mean annual flows is projected to decrease over the period in the Black Volta Basin. Therefore, the outcomes of this study will be useful for different stakeholders within the basin in water resources planning and the formulation of appropriate policies for improving land use planning.

Base flow simulation using a physically based subsurface model – The case of a tropical basin in the Western Ghats, India

A novel framework that integrally uses groundwater and streamflow observations to calibrate a physically based sub surface model to generate base flow on a daily time step is presented in this study. Trend analysis on mean annual base flow time series showed that a declining trend was present in the basin though the upstream humid region was devoid of any trend. GLUE analysis was adopted to the simulation of base flow in the humid region and the most sensitive parameters were identified as the recharge factor (rf) and discharge factor (pd). The model was able to satisfactorily simulate base flow as observed values fell within the 95CI bands for the calibration period. The mean annual BFI values were computed at Muthankera to be 0.236 ± 0.02. The mean seasonal BFI varied from 48 % during post monsoon periods to 21 % during monsoon periods. The base flow simulated by the model was comparable to the output from the Eckhardt filter which has been widely cited to be a suitable baseflow separation method for many river basins. Due to the absence of groundwater level data in the transition region, a combination of model and digital filter was used to calibrate that region. The draft from the groundwater storage computed from the model was found to be highly nonlinear. The rf was seen to be lower and pd to be higher in the transition region than in the humid region. This would imply that in the transition region, the base flow is generated more gradually in comparison to the humid region. This was characteristic of forested basins where the uptake of water by deep rooted trees during evapotranspiration from the unsaturated zone causes delay in the water table response which in turn could result in delay in the base flow generation process. The consequence of the calibration exercise on Kabini basin was establishing that the heterogeneity in terms of precipitation and land use has resulted in the distinct hydrologic responses from the two regions in the basin.

ВЫНОС МИНЕРАЛЬНЫХ КОМПОНЕНТОВ И ОРГАНИЧЕСКОГО ВЕЩЕСТВА ВОДАМИ БОЛОТНОГО МАССИВА КОЛЬСКОГО ПОЛУОСТРОВА

Removal of mineral components and organic matter by the wetland water is evaluated using the observations on the Pulozero oligotrophic bog (the Kola Peninsula). Precipitation is a source of mineral components for bogs, its mean annual mineralization is 26.8 mg/dm3. The average annual inflow of mineral components per bog unit area is 13.0 t/km2. Their mean annual channel flow modulus (14.8 t/km2) is 38% greater than the modulus of their total loss (10.7 t/km2) from the wetland (by infiltration, discharge, and accumulation). It is explained by the partial discharge of groundwater into the stream channel. The accumulation of mineral components within a peat deposit is equal to 2.3 t/km2 per year, or 17.7 % of fallen ones. The concentration of organic matter in the water of a hummock-ridge complex is on average equal to 140.3 mg/dm3, varying from 68.2 to 200.8 mg/dm3. The modulus of organic substance loss within the complex is on average equal to 38.2 t/(km2 year). Its annual channel flow is 12.5 t/(km2 year). The accumulation of organic matter within the peat deposit reaches 25.7 t/(km2 year), or 67.3% of the organic substance mass formed within the bog microlandscape during a year.

Trends in reconstructed monthly, seasonal and annual flows for Irish catchments (1900–2016)

Trends in reconstructed monthly, seasonal and annual flows for Irish catchments (1900–2016)

Evaluation of Surface Water Resource Availability under the Impact of Climate Change in the Dhidhessa Sub-Basin, Ethiopia

Climate change, with its reaching implications, has become a popular topic in recent years. Among the many aspects of climate change, one of the most pressing concerns has been identified as the impact on the terrestrial water cycle, which has a direct impact on human settlement and ecosystems. The paper begins by reviewing previous studies, and then identifies their flaws and future research directions. The effects of climate change on surface water resources in the Dhidhessa Sub-basin, Abbay Basin, Ethiopia, were studied as practices. For future potential climate change, the results of global climate models (GCMs) and high-resolution regional climate models (RCMs) from multiple climate models were combined with data from Representative Concentration Pathways (RCPs) prepared by the Intergovernmental Panel on Climate Change from the CCAFS Data Distribution Center web page. To evaluate the impacts on water resources, various distributed hydrologic models based on local underlying surfaces were developed. The future potential climate change of the Dhidhessa Sub-basin Province was evaluated by integrating RCP outputs, whereas the climate change of the Dhidhessa River was directly derived from the results of different RCP. Dhidhessa stream flow will decrease in the future compared to the baseline era. The predictions of future discharge (stream flow) were based on climate scenarios data from 1991 to 2020 and for the future with two time windows, 2044 (2030–2059) and 2084 (2070–2099), on a monthly time-step after bias correction to both precipitation and temperature in the future climate described in the under each RCP. According to model results, the quantity of surface water resources in the Dhidhessa river region will decrease over the next 100 years, the percent decrease in mean annual stream flow by 10%, in 2044, and 6.3% in 2084, respectively, making the impact of temperature increase on runoff greater than that of precipitation. The distribution of runoff would be more even across years but more uneven across years in the long-term window, implying a higher possibility of drought and flooding. In general, this study discovered that any effect on this river that results in a decrease in flow will have a direct impact on the area’s ongoing water resource development and socioeconomic development.

Macroinvertebrate habitat requirements in rivers: overestimation of environmental flow calculations in incised rivers

Abstract. Flow variability determines the conditions of river ecosystems and river ecological functioning. The variability of ecological processes in river ecosystems gradually decreases due to river channelization and incision. Prediction of the environmental flow allows us to keep biological diversity and river health developed as a response to the degradation of aquatic ecosystems overexploited by humans. The goal of the study was to test the influence of river incision on environmental flow estimation based on the Biological Monitoring Working Party (BMWP) macroinvertebrate index. A total of 240 macroinvertebrate assemblages of 12 waterbodies differing in bed substrate and amplitude of discharge were surveyed in southern Poland. Variations in the distribution of 151 466 macroinvertebrates belonging to 92 families were analysed. The similarity of benthic macroinvertebrates reflects the typological division of the rivers into three classes: Tatra mountain streams, mountain flysch rivers, and upland carbonate and silicate rivers. As a response variable reflecting the macroinvertebrate distribution in the river, the BMWP_PL index was chosen. The river incision significantly increased the values of e-flow calculations in relation to redeposited channels. The area of optimal habitat for macroinvertebrates decreased with the bed incision intensity. In highly incised rivers, the environmental flow values are close to the mean annual flow, suggesting that a high volume of water is needed to obtain good macroinvertebrate conditions. As a consequence, river downcutting processes and impoverishment of optimal habitats will proceed.

Response of riparian vegetation to short- and long-term hydrologic variation.

Increasing demand for river water now conflicts with an increasing desire to maintain riparian ecosystems. Efficiently managing river flows for riparian vegetation requires an understanding of the time scale of flow effects, but this information is limited by the absence of long-term studies of vegetation change in response to flow variation. To investigate the influence of short- and long-term flow variability and dam operation on riparian vegetation, we determined the occurrence of 107 plant species in 133 permanent plots of known inundating discharge along the Gunnison River in Colorado on five different occasions between 1990 and 2013. Individual species moved up and down the gradient of inundating discharge coincident with increases and decreases in mean annual flow, and the correlations between flow and species occurrence were strongest when flows were weighted by time before vegetation sampling with a median half-life of 1.5 years. Some tall, rhizomatous, perennial species, however, responded to flows on a longer time scale. Logistic regression of species occurrence showed a significant relation with inundation duration for 70 out of 107 species. Plot species richness and total vegetative cover decreased in association with desiccation at low inundation durations and with fluvial disturbance at high inundation durations. Within-plot similarity in species occurrence between years decreased strongly with increasing inundation duration. Moderate inundation durations were dominated by tall, rhizomatous, perennial herbs, including invasive Phalaris arundinacea (reed canary grass). Over the 23-year study period, species richness declined, and the proportion of rhizomatous perennials increased, consistent with the hypothesis that decreases in flow peaks and increases in low flows caused by flow regulation have decreased establishment opportunities for disturbance-dependent species. In summary, annual-scale changes in vegetation were strongly influenced by flow variation, and decadal-scale changes were influenced by decreases in fluvial disturbance from upstream flow regulation beginning decades prior to the onset of this study.

Environmental Flow Assessment of Gorai River in Bangladesh: A comparative analysis of different hydrological methods

The purpose of the study is to assess the Environmental Flow Requirement (EFR) of Gorai River in Bangladesh and to evaluate the change in flow characteristics in recent time compared to past. Daily discharge data of Gorai railway bridge station were collected from Bangladesh Water Development Board and analyzed for two periods. Mean Annual Flow (MAF) in G2 period (2000–20166) is found about 13% lower than G1 period (1984–1999). The Mean Monthly Flow (MMF) in low flow season is increased by 99%, and that of high flow is decreased by 20% in G2 period compared to G1. In this study, EFR was determined considering various methods including Tennant, Tessmann, Variable Monthly Flow, Modified Constant Yield, FDCA Q50-Q90, FDCA Q50-Q75 and Smakhtin Method. The average EFR for low, high and intermediate flows were found as 89, 915 and 273 m3/s, which is 9, 61 and 27% of MAF and 96, 23 and 61% of mean seasonal flow, respectively. The overall annual EFR for the river is found as 295 m3/s or 29% of MAF. It is observed that when the EFRs are expressed as percent of mean seasonal flow, the Low-Flow Requirements (LFR) were found higher than High-Flow Requirements (HFR). However, when the EFRs are expressed as percent of MAF, the LFR is lower than HFR. Among all the EFRs predicted by 8 methods, Smakhtin predicts the smallest HFR (6% of MAF) and FDCA Q50-Q90 have the lowest LFR value (1.2% of MAF). The Tennant method is not found to be capable to capture the temporal change of MMF of different seasons. The Average Annual EFR was found to be reduced by 14% in latter period. A deficient flow situation was observed from December to May. The findings can be used for future reference in management of flows in Gorai river.

Comprehensive low-flow analysis of the Vipava river

The article presents the results of the analysis of low flows at 5 gauging stations on the Vipava River, which has a Dinaric pluvial-nival regime (catchment area of 590 km2). The low-flow statistics show that the gauging station Vipava stands out with the lowest values. Baseflow index (BFI) values are comparable among the considered stations and are around 0.40. Relatively low BFI values indicate low soil permeability. A high similarity between the mean annual minimum 7-day flow (MAM7) and the 95th percentile exceedance discharge (Q95) at all gauging stations indicates a temperate climate. The highest flows values occur in spring and autumn, and the lowest in summer. In wet years there are relatively large fluctuations in flow, while in dry years the flow consists mainly of baseflow. This is also confirmed with the flow duration curves analysis. The seasonality analysis shows a predominant summer regime with low flows.

Variability of Mean Annual Flows in Southern Quebec (Canada)

Snow is the main source of streamflow in temperate regions characterized by very cold and snowy winters. Due to global warming, these regions are experiencing a significant decrease in snowfall. The main objective of this study is to analyze the impacts of snowfall on the spatio-temporal variability of mean annual flows (MAFs) of 17 rivers, grouped into three hydroclimatic regions, from 1930 to 2019 in southern Quebec. In terms of spatial variability, snowfall is the variable most correlated with MAFs (positive correlation), followed by drainage density (positive correlation) and wetland surface areas (negative correlation). Due to the influence of these three factors, MAF values are generally higher in the most agricultural watersheds of the southeastern hydroclimatic region on the south shore than in the less agricultural watersheds of the southwestern hydroclimatic region on the north shore of the St. Lawrence River. As for temporal variability, the four statistical tests applied to the hydrological series detect no significant downward trend in MAFs, despite having reduced snowfall. Instead, they suggest an evolution toward an increase in mean annual flows, as a result of increased rainfall due to the increase in temperature. This evolution is more pronounced on the north shore than on the south shore, likely due to the presence of wetlands and others water bodies, whose runoff water storage capacity does not change over time to be able to store the surplus of the quantity of water brought by the increase in rain.

Factors Affecting Spatiotemporal Variation in Survival of Endangered Winter‐Run Chinook Salmon Out‐migrating from the Sacramento River

AbstractAmong four extant and declining runs of Chinook Salmon Oncorhynchus tshawytscha in California’s Central Valley, none has declined as precipitously as the Sacramento River winter run. Migratory winter‐run Chinook Salmon employ a life history strategy to reside and feed in stopover habitats on their way from freshwaters to the ocean. Migratory winter run, on their way from freshwaters to the ocean, employ a life history strategy to reside and feed in stopover habitats that have been affected by anthropogenic disturbance. Using acoustic telemetry, we examined conditions that influenced reach‐specific movement and survival of out‐migrating juveniles during a prolonged, multi‐year drought from 2013 to 2016, followed by one of the wettest years on record (2017). We modeled how time‐varying individual riverine covariates and reach‐specific habitat features influenced smolt survival. Model selection favored a model with mean annual flow, intra‐annual deviations from the mean flow at the reach scale, reach‐specific channel characteristics, and travel time. Mean annual flow had the strongest positive effect on survival. A negative interaction between mean annual flow and intra‐annual reach flow indicated that within‐year deviations at the reach scale from annual mean flow had larger effects on survival in low‐flow years. These factors resulted in higher survival during years with pulse flows or high flows. Changes in movement behavior in response to small‐scale changes in velocity were negatively associated with survival. Covariates of revetment and wooded bank habitat were positively associated with survival, but the effect of these fixed habitat features changed depending on whether they were situated in the upper or lower part of the river. Fish exhibited density‐dependent stopover behavior, with slowed downstream migration in the upper river in the wet years and extending to the lower river in the most critically dry year. This paper contributes two key findings for natural resource managers interested in flow management and targeted habitat restoration. The first is new insight into how the magnitude of pulse flows in dry and wet years affects survival of winter‐run fish. The second is that density dependence influences where stopover habitat is used. Despite this, we identified an area of the river where fish consistently exhibited stopover behavior in all years.

Impact of long-term climate change on flow regime in a Mediterranean basin

Study regionCatchment in Southern Italy Study focusThe flow regime of intermittent rivers is very sensitive to anthropogenic pressures and climate change (CC). This study investigated the long-term impact of CC on the flow regime of an intermittent river system, the Celone River (S-E, Italy), under the Mediterranean climate. The trends in precipitation and temperature measured over the past century were investigated, and the flow regime was characterized by using 27 indicators of hydrological alteration. Climate projections for the future (2030–2059) were used to predict potential CC impact on the flow regime. The Soil and Water Assessment Tool (SWAT) model was run to simulate hydrology for the current and future scenarios. New hydrological insights for the regionA significant increase in temperature and a reduction in the number of days with temperatures below 0 °C was recorded from 1958 to 2012. For the future, climate model projections predicted an increase in the daily temperature and a decrease in rainfall (4–9%) as compared with the baseline (1980–2009). As a result, the model predicted reductions in the mean annual flow by up to 21% and 39% compared to the baseline and the historical period (1965–1996), respectively. It also predicted a reduction in the maximum annual flow recorded over 90 consecutive days up to 18%, and an extension of the no-flowing phase by up to 12 days.

Future hydropower operations in the Zambezi River basin: Climate impacts and adaptation capacity

AbstractSouthern Africa could experience major hydrological changes in coming decades, bringing drastic changes to hydropower. Focusing on four large hydropower dams in the Kafue and Zambezi Rivers (Itezhi‐Tezhi, Kafue Gorge Upper, Kariba, and Cahora Bassa), accounting for over 90% of the installed capacity in the basin, this study analyzed how these dams have altered river flows in the past, and how hydropower generation could be affected by future climate change and water use scenarios based on dynamically downscaled simulations from the Fifth Coupled Model Intercomparison Project for a Representative Concentration Pathway of 8.5 W/m2. Future climate change could affect reservoir levels by decreasing water inflows and by increasing evaporation. Results showed a decrease in median annual water levels for all reservoirs, with largest changes expected at Itezhi‐Tezhi (−1.3 to −3.3 m), followed by Cahora Bassa (−0.8 to −2.8 m), Kariba (−0.2 to −0.9), and Kafue Gorge (0.1–0.2 m). Flows downstream of dams show a consistent negative trend, with mean annual flow changes of −30% to −49% at the furthest downstream gauge. The combined reduction in water levels and flows could result in decreases on average annual hydropower generation and increases in interannual variability; annual generation could decrease by −61% to −28% at Kariba, by −51% to −21% at Itezhi‐Tezhi, by −7% to −39% at Kafue Gorge, and −27% to −9% at Cahora Bassa. Reservoir operation optimization could marginally offset the projected changes in hydropower generation, with largest gains for Kariba (+6%) and Cahora Bassa (+10%). Overall, opportunities to offset losses via operations would probably decrease with the expected drier climate and escalating reduction in river discharge and water availability. Given these negative projections, it is becoming critical to adapt and coordinate reservoir operations to best cope with the expected challenging future in this important river basin.

Wild trout and hydrologic response to restoration of incised streams with improved water use for ranching and fisheries

AbstractIn the face of climate change and dwindling water supplies, this long‐term monitoring study examines the effectiveness of restoring two incised and degraded streams (Crystal Creek and Black Slough) to benefit livestock ranching and wild trout populations in a spring creek and wetland complex, central Idaho, USA. By raising the incised streambeds and connecting to historical floodplains, the restoration aimed to raise groundwater levels to create subsurface saturation conditions to sustain grazing forage and hay production without flood irrigation; diverted flows from the Big Wood River previously used for flood irrigation were instead routed through the restored systems. The restoration used natural channel design methods to reconstruct 5.2 km of Black Slough and 8.4 km of Crystal Creek to emulate a downstream reference reach (Willow Creek); within the relic incised channels, off‐channel oxbow ponds and wetlands were created while filling other segments. Additionally, screened gravels were placed in the reconstructed channels. We monitored before–after restoration changes in channel morphology, groundwater levels, wetlands, instream flows, spawning substrates and redds, fish populations, and invertebrates for the restored creeks and reference reach over 10+ years. Following restoration, average groundwater levels increased by 0.35 m, mean annual flows at Willow Creek (receiving stream) increased from 0.64 to 1.11 m3/s, and average days per year that baseflows were below 0.28 m3/s decreased from 44.5 to 5.4 despite a decrease in average annual precipitation (16.97 to 11.53 cm). Moreover, residual pool depths increased (<0.18 to >0.90 m), spawning substrate sizes increased (0.1 to 19.0 mm), and egg‐to‐fry trout survival rates increased from <20% to >90%. Six years after restoration, fish diversity increased from three to seven species, matching reference conditions. We also found dramatic increases in Brown Trout and Rainbow Trout redd counts (7 to 161 and 17 to 143, respectively) and relative abundance (0.013 to 1.166 trout/m and 0.029 to 0.222 trout/m, respectively). Invertebrate indices of restoration effectiveness were positive for both restored streams but negative for the reference reach. Overall, this study exemplifies how restoration reduced consumptive water use, created sustainable streams with hydrological connections and ponds and wetlands, and improved trout populations.

Improving hydrological climate impact assessments using multirealizations from a global climate model

AbstractMany flood risk assessments account for uncertainties in future anthropogenic emissions by considering multiple representative concentration pathways (RCPs). The imperfect knowledge and representation of the climate system is considered with multiple global climate models (GCMs). Yet, uncertainty introduced by incomplete representation of natural variability is also relevant but not always accounted for. A set of realizations provides improved insights in natural variability presented by the GCM. This study explores the potential of using a set of realizations from a single GCM‐RCP combination instead of single realizations. We use (subsets of) 16 realizations from EC‐Earth for RCP8.5 and focus on three locations along the Rhine. We use a single GCM‐RCP combination to avoid the interference of additional sources of uncertainty. We find that projected changes in future river flows highly depend on the realization chosen. Individual ensemble members provide different changes for annual mean flow, extreme flows, and regime shift. By increasing the number of realizations and combining their annual maxima in extreme value analysis, future projections of flow extremes converge. We conclude that a single ensemble realization gives overconfident and possibly erroneous projections. In climate science, this is well studied; however, in flood risk assessments, it is still often neglected.