Mean Annual Flow Research Articles

Quantifying the potential impacts of land-use and climate change on hydropower reliability of Muzizi hydropower plant, Uganda

Abstract Ugandan rivers are being tapped as a resource for the generation of hydropower in addition to other uses. Studies on the reliability of these hydropower plants due to climate and land-use/land cover changes on the hydrology of these rivers are scanty. Therefore, this study aimed to model the impact of the changing climate and land-use/cover on hydropower reliability to aid proper planning and management. The hydropower reliability of Muzizi River catchment was determined from its past (1998–2010) and midcentury (2041–2060) discharge at 30 and 95% exceedance probability under Representative Concentration Pathways (RCPs) of 4.5 and 8.5, respectively. The past and projected hydropower were compared to determine how future climate and land-use changes will impact the discharge and hydropower reliability of Muzizi River catchment. Six LULC scenarios (deforestation, 31–20%; grassland, 19–3%; cropland, 50–77%; water bodies, 0.02–0.01%; settlement, 0.23–0.37%, and Barren land 0.055–0.046% between 2014 and 2060) and three downscaled Regional Climate Model (REMO and RCA4 for precipitation and RACMO22T for temperature from a pool of four CORDEX (Coordinated Regional Climate Downscaling Experiment) Africa RCMs) were examined. A calibrated SWAT simulation model was applied for the midcentury (2041–2060) period, and a potential change in hydropower energy in reference to mean daily flow (designflow ≥ 30% exceedance probability), firm flow (flow ≥ 95% exceedance probability), and mean annual flow was evaluated under the condition of altered runoff under RCP4.5 and RCP8.5 climate change scenarios for an average of REMO and RCA4 RCM. The future land use (2060) was projected using the MOLUSCE (Module for Land Use Change Evaluation) plugin in QGIS using CA-ANN. Three scenarios have been described in this study, including LULC change, climate change, and combined (climate and LULC change). The results suggest that there will be a significant increase in annual hydropower generation capacity (from 386.27 and 488.1 GWh to 867.82 and 862.53 GWh under RCP4.5 and RCP8.5, respectively) for the combined future effect of climate and land-use/cover changes. Energy utilities need to put in place mechanisms to effectively manage, operate, and maintain the hydropower plant amidst climate and land-use change impacts, to ensure reliability at all times.

Dams and Climate Interact to Alter River Flow Regimes Across the United States

AbstractStoring and managing river flows through reservoirs could dampen or increase climate‐induced fluctuations in streamflow, but interactions between the effects of dams and climate are poorly understood. Here, we examined how dam properties control different facets of flow alteration across the coterminous United States (CONUS), and compared alteration trends between dam‐affected and reference stream gages. We quantified departures from the natural flow regime using 730 stations with long‐term daily discharge data. Dam size and purpose explained high variation in flow alteration, and alteration was particularly severe in water‐stressed regions. Importantly, regulation of river flows by dams often dampened climate‐driven alteration (48% of the flow metrics), particularly in watersheds with positive flow trends; while worsening climatic impacts in other cases (44%), or even having dual effects (8%). Our results show that dam and climate impacts on streamflow need to be assessed jointly, and based on a diverse range of flow regime facets (e.g., event magnitude and duration, frequency, and timing) instead of mean annual flows only. By pairing the USGS streamflow records available from upstream and downstream of 209 dams across the CONUS, we advance the notion that dams amplify flow alteration, but also ameliorate some flow alteration metrics. Understanding such potential and limitations is important in light of climate non‐stationarity and a new wave of damming in developing economies, and will be key to further advancing environmental flow science into the future.

Optimizing monthly ecological flow regime by a coupled fuzzy physical habitat simulation–genetic algorithm method

The present study proposes and evaluates a fuzzy hydraulic habitat simulation–genetic algorithm method to optimize environmental flow regime with focus on diversion dam project. Proposed method develops an objective function that minimizes differences between habitat loss and water demand or project loss. Fuzzy physical habitat simulation was used to develop habitat loss function. Moreover, the genetic algorithm was utilized as optimization method. Based on results, minimum available environmental flow in dry seasons was approximately 15% of mean annual flow. However, its maximum would increase to 40% of mean annual flow in wet seasons. Reliability and vulnerability indices for supply of water demand were 80% and 34%, respectively, in the case study. Results of the proposed framework were compared with the Tennant method to demonstrate abilities for optimizing environmental flow. The most important advantage of proposed method is minimization of conflict between stakeholders and environmental advocators. In other words, the proposed method might be able to minimize negotiations to assess environmental flow regime.

A Parametric Approach for Determining Fishway Attraction Flow at Hydropower Dams

High discharges at hydropower plants (HPP) may mask fishway attraction flows and, thereby, prevent fishes from locating and using fishways critical for their access to upstream spawning and rearing habitats. Existing methods for determining attraction flows are either based on simple guidelines (e.g., a proportion of HPP discharge) that cannot address the spatial and temporal complexity of tailrace flow patterns or complicated studies (e.g., combinations of detailed hydraulic and biological investigations) that are expensive and time-consuming. To bridge this gap, we present a new, intermediate approach to reliably determine attraction flows for technical fishways at small to medium-sized waterways (mean annual flow up to 400 m3/s). Fundamental to our approach is a design criterion that the attraction flow should maintain its integrity as it propagates downstream from the fishway entrance to beyond the highly turbulent zone characteristic of HPP tailraces to create a discernable migration corridor connecting the fishway entrance to the downstream river. To implement this criterion, we describe a set of equations to calculate the width of the entrance and the corresponding attraction discharge. Input data are usually easy to obtain and include geometrical and hydraulic parameters describing the target HPP and its tailrace. To confirm our approach, we compare model results to four sites at German waterways where the design of attraction flow was obtained by detailed experimental and numerical methods. The comparison shows good agreement supporting our approach as a useful, intermediate alternative for determining attraction flows that bridges the gap between simple guidelines and detailed hydraulic and biological investigations.

Climatic Controls on Mean and Extreme Streamflow Changes Across the Permafrost Region of Canada

Climatic change is affecting streamflow regimes of the permafrost region, altering mean and extreme streamflow conditions. In this study, we analyzed historical trends in annual mean flow (Qmean), minimum flow (Qmin), maximum flow (Qmax) and Qmax timing across 84 hydrometric stations in the permafrost region of Canada. Furthermore, we related streamflow trends with temperature and precipitation trends, and used a multiple linear regression (MLR) framework to evaluate climatic controls on streamflow components. The results revealed spatially varied trends across the region, with significantly increasing (at 10% level) Qmin for 43% of stations as the most prominent trend, and a relatively smaller number of stations with significant Qmean, Qmax and Qmax timing trends. Temperatures over both the cold and warm seasons showed significant warming for >70% of basin areas upstream of the hydrometric stations, while precipitation exhibited increases for >15% of the basins. Comparisons of the 1976 to 2005 basin-averaged climatological means of streamflow variables with precipitation and temperature revealed a positive correlation between Qmean and seasonal precipitation, and a negative correlation between Qmean and seasonal temperature. The basin-averaged streamflow, precipitation and temperature trends showed weak correlations that included a positive correlation between Qmin and October to March precipitation trends, and negative correlations of Qmax timing with October to March and April to September temperature trends. The MLR-based variable importance analysis revealed the dominant controls of precipitation on Qmean and Qmax, and temperature on Qmin. Overall, this study contributes towards an enhanced understanding of ongoing changes in streamflow regimes and their climatic controls across the Canadian permafrost region, which could be generalized for the broader pan-Arctic regions.

Multiple stressors determine river ecological status at the European scale: Towards an integrated understanding of river status deterioration.

The biota of European rivers are affected by a wide range of stressors impairing water quality and hydro-morphology. Only about 40% of Europe's rivers reach 'good ecological status', a target set by the European Water Framework Directive (WFD) and indicated by the biota. It is yet unknown how the different stressors in concert impact ecological status and how the relationship between stressors and status differs between river types. We linked the intensity of seven stressors to recently measured ecological status data for more than 50,000 sub-catchment units (covering almost 80% of Europe's surface area), which were distributed among 12 broad river types. Stressor data were either derived from remote sensing data (extent of urban and agricultural land use in the riparian zone) or modelled (alteration of mean annual flow and of base flow, total phosphorous load, total nitrogen load and mixture toxic pressure, a composite metric for toxic substances), while data on ecological status were taken from national statutory reporting of the second WFD River Basin Management Plans for the years 2010-2015. We used Boosted Regression Trees to link ecological status to stressor intensities. The stressors explained on average 61% of deviance in ecological status for the 12 individual river types, with all seven stressors contributing considerably to this explanation. On average, 39.4% of the deviance was explained by altered hydro-morphology (morphology: 23.2%; hydrology: 16.2%), 34.4% by nutrient enrichment and 26.2% by toxic substances. More than half of the total deviance was explained by stressor interaction, with nutrient enrichment and toxic substances interacting most frequently and strongly. Our results underline that the biota of all European river types are determined by co-occurring and interacting multiple stressors, lending support to the conclusion that fundamental management strategies at the catchment scale are required to reach the ambitious objective of good ecological status of surface waters.

Impacts of Land Use and Land Cover Change on Soil Erosion and Hydrological Responses in Ethiopia

Land use and land cover (LULC) dynamics, in general, and the conversion of the natural vegetation cover into cultivated land, in particular, are major human-induced problems in Ethiopia, which have played a significant role in increasing the rate of soil erosion and altering the hydrological balance in the country. The main aim of this review was to view previous studies in Ethiopia that quantify the change in the rate of soil erosion and hydrological responses as a result of the change in land use and land cover in the country. From the past researches reviewed in this paper, the expansion of cultivated land at the expense of forest land, shrubland, and grassland in Ethiopia has increased the mean rate of soil erosion, sediment yield, surface runoff, mean wet monthly flow, and mean annual stream flow in the last four decades. On the other hand, the change has reduced the dry average monthly flow, groundwater recharge and groundwater flow, and evapotranspiration (ET) in the country. Future research works should pay more attention to the investigation of the impacts of land use and land cover change on groundwater hydrology and the prediction of future soil loss and hydrological imbalance under the changing land use and land cover in the country since little information is available from past researches on these issues. Research works are also required in lowland arid and semiarid areas in Ethiopia to effectively manage soil and water resources in all parts of the country.

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.

Assessment of probability distributions and analysis of the minimum storage draft rate in the equatorial region

Abstract. Rapid urbanization in the state of Selangor, Malaysia, has led to a change in the land use, physical properties of basins, vegetation cover and impermeable surface water. These changes have affected the pattern and processes of the hydrological cycle, resulting in the ability of the basin region to store water supply to decline. Reliability on water supply from river basins depends on their low-flow characteristics. The impacts of minimum storage on hydrological drought are yet to be incorporated and assessed. Thus, this study aims to understand the concept of low-flow drought characteristics and the predictive significance of river storage draft rates in managing sustainable water catchment. In this study, the long-term streamflow data of 40 years from seven stations in Selangor were used, and the streamflow trends were analyzed. Low-flow frequency analysis was derived using the Weibull plotting position and four specific frequency distributions. Maximum likelihood was used to parameterize, while Kolmogorov–Smirnov tests were used to evaluate their fit to the dataset. The mass curve was used to quantify the minimum storage draft rate required to maintain the 50 % mean annual flow for the 10-year recurrence interval of low flow. Next, low-flow river discharges were analyzed using the 7 d mean annual minimum, while the drought event was determined using the 90th percentile (Q90) as the threshold level. The inter-event time and moving average was employed to remove the dependent and minor droughts in determining the drought characteristics. The result of the study shows that the lognormal (2P) distribution was found to be the best fit for low-flow frequency analysis to derive the low-flow return period. This analysis reveals September to December to be a critical period in river water storage to sustain the water availability during low flow in a 10-year occurrence interval. These findings indicated that hydrological droughts have generally become more critical in the availability of rivers to sustain water demand during low flows. These results can help in emphasizing the natural flow of water to provide water supply for continuous use during low flow.

Exploring the Role of Reservoir Storage in Enhancing Resilience to Climate Change in Southern Europe

Recent trends suggest that streamflow discharge is diminishing in many rivers of Southern Europe and that interannual variability is increasing. This threatens to aggravate water scarcity problems that periodically arise in this region, because both effects will deteriorate the performance of reservoirs, decreasing their reliable yield. Reservoir storage is the key infrastructure to overcome variability and to enhance water availability in semiarid climates. This paper presents an analysis of the role of reservoir storage in preserving water availability under climate change scenarios. The study is focused on 16 major Southern European basins. Potential water availability was calculated in these basins under current condition and for 35 different climatic projections for the period 2070–2100. The results show that the expected reduction of water availability is comparable to the decrease of the mean annual flow in basins with large storage capacity. For basins with small storage, the expected reduction of water availability is larger than the reduction of mean annual flow. Additionally, a sensitivity analysis was carried out by replicating the analysis assuming variable reservoir volumes from 25% to 175% of current storage. The results show that increasing storage capacity attenuates the reduction of water availability and reduces its uncertainty under climate change projections. This feature would allow water managers to develop suitable policies to mitigate the impacts of climate change, thus enhancing the resilience of the system.

Spatial Analysis of Base Flow and Stream Flow from the Abbay River Basin after Watershed Management Interventions

Recently, increasing concern in Africa has been registered on the potential impacts on base flow and stream flow due to the implementation of watershed management interventions. This research incorporates spatially monthly geographical hydrological data sets into a developed spreadsheet water balance model to estimate the changes in surface runoff, base flow and stream flow as a result of implementing watershed management interventions in the Abbay River Basin during the period (2010-2018). The model was implemented at sub-catchment level. Considering Year 2005 as a datum for watershed management intervention, results of the modeling and spatial analysis indicated that watershed management interventions relatively reduced surface runoff, increased deep infiltration to groundwater and accordingly increased base flow to the stream. Among the key results in the Abbay Basin that change from Year 2010 to Year 2018 was reduction of surface runoff from the Abbay Basin in the amount of 1,753 million m3/yr. As a result, an increase in annual base flow in the amount of 23 million m3/yr was estimated, leaving a reduction in annual mean flow of the Abbay River in the amount of 1,731 million m3/yr of the Abbay River at El-Diem site. It is expected that the flow of the Abbay River will continue to decrease due to the continuous implementation of the watershed management interventions and agriculture expansion in the Abbay Basin.

The role of tides, river discharge and wind on the residual circulation of Maputo Bay

Lateral variability of residual flow was investigated in Maputo Bay in Mozambique, Africa. The bay is nearly rectangular with 30×30 km, mean depth of 5 m. The bay mouth is ∼18 km wide with mean and maximum depth ∼10 m and 30 m, respectively. Tides ranges between 1 and 3 m at neap and spring tides, respectively. The mean annual river flow is ∼225 m3/s, ranging from ∼150 to ∼400 m3/s during winter and summer, respectively. Southeasterly (SE) wind are the most frequent wind during the year followed by northeasterly (NE) winds. In order to scale the relative importance of tides, wind and buoyancy effects on the residual exchange flows, we designed three-dimensional numerical experiments (Delft3D). Results show strong changes among the scenarios and between neap and spring tide conditions. During spring tide, flows becomes more vertically homogeneous and laterally sheared, modulated by the cross-section shape. Most important exchange flows were during neap tides mainly with scenarios forced by tides. There are some locations prone to entrap sediments, such as the inlet of the Incomati river and to the north of Inhaca Isle.

Coherent Streamflow Variability in Monsoon Asia Over the Past Eight Centuries—Links to Oceanic Drivers

AbstractThe Monsoon Asia region is home to ten of the world's biggest rivers, supporting the lives of 1.7 billion people who rely on streamflow for water, energy, and food. Yet a synthesized understanding of multicentennial streamflow variability for this region is lacking. To fill this gap, we produce the first large scale streamflow reconstruction over Monsoon Asia (62 stations in 16 countries, 813 years of mean annual flow). In making this reconstruction, we develop a novel, automated, climate‐informed, and dynamic reconstruction framework that is skillful over most of the region. We show that streamflow in Monsoon Asia is spatially coherent, owing to common drivers from the Pacific, Indian, and Atlantic Oceans. We also show how these oceanic teleconnections change over space and time. By characterizing past and present hydroclimatic variability, we provide a platform for assessing the impact of future climatic changes and informing water management decisions.

Spatio-temporal dynamics of the Río de la Plata turbidity front; combining remote sensing with in-situ measurements and numerical modeling

The Río de la Plata is a micro-tidal estuary located in Southeast South America. With an annual mean flow of 26,500 m3/s, it receives 160 million tons/yr of suspended sediments. The high content of cohesive fine sediments in the estuary generates high turbidity levels in its inner and intermediate zones, which can be clearly seen in color satellite images. In this work, an image-based algorithm was successfully implemented to remotely detect the turbidity front of the Río de la Plata, based on the top of the atmosphere (TOA) reflectance in the red band of MODIS-Aqua satellite. The algorithm finds the reflectance level that ‘best’ separates two water classes: turbid fluvial and clear ocean waters. The front dynamic was studied combining remotely sensed information and data of river discharge, winds, salinity and sea level time series, in the four-year period 2014–2017. River discharge was identified as the main external forcing, revealing a solid general pattern of behavior: when discharge was high (low) the front tended to be located in the outer (intermediate) zone of the estuary. Sea level seemed to be a secondary forcing, presenting higher correlations along the center of the estuary than near both coasts. Local winds needed to have a relatively persistent (2-day) component in a given direction to affect the location of the front. Additionally, results of an already implemented numerical model of the Río de la Plata were evaluated in terms of spatio-temporal performance, considering turbidity and salinity fronts locations. New strengths and limitations of the model were identified, and an improvement in the parameterization of sediments’ settling velocity was tested. Model results revealed the relative importance of bottom shear stress on the general location of the front, and of salinity on the flocculation process of cohesive sediments. This work provided new insights for the understanding of the Río de la Plata estuarine dynamics through the combination of three complementary tools – remote sensing, in situ data, and numerical modeling, – which may be extended to other systems around the world.

Trends of Hydro-Meteorological Indices in Tendaho Catchment Part of Awash River Basin, Ethiopia

The variability in the intensity and frequency of extreme hydro-meteorological events due to climate change have an enormous impact on managing water resources in developing countries. Frequently it has been recognized as sudden droughts and severe floods. This study analyzed the spatial and temporal trends of five meteorology indices: annual maximum precipitation (AMP), annual precipitation (AP), mean areal precipitation (MAP), annual maximum temperature (AMaT) and annual minimum temperature (AMiT), and three streamflow indices: mean annual flow(MAF), annual maximum flow (AMaxF) and total mean annual flow (TMAF) over the Tendaho Catchment. Mann–Kendall (MK), Spearman Rho (SR), Sen’s slope (SS) test in R-program modifiedmk package, and Sen’s innovative trend test were used to detect trends of 16 meteorological and one streamflow stations from 1979–2017. The result showed that there is statistically significant temporal trends only in AMP, AP, AMaT, and AMiT at 6, 19, 56, and 50% of the stations respectively. The remaining indices have no statistically significant trend in all the stations. It also dictates that the catchment is characterized by a slightly increasing AP and AMP; a slightly decreasing MAP and significant increasing AMaT and AMiT trends. Except for AMaT, there is no particular spatial pattern of AP, MAP, and AMiT on the majority of the stations. Whereas, all streamflow indices showed a slightly decreasing temporal trend at 95% confidence. From this, we can conclude that the decreasing flow trend could be due to the decrement of MAP, an increment of temperature, and construction of reservoirs in the catchment.

Potential for small hydropower development in the Lower Pra River Basin, Ghana

Study region: Lower Pra River (LPR) Basin, Ghana Study focus: This study examines the potential for Small Hydropower Development in a semi-arid environment albeit the lack of data. Geographic and site specific information acquired through geographic and remote sensing techniques are applied in a physically based semi-distributed hydrologic model to simulate discharges for ungauge areas for consideration of hydropower. Flow Duration Curves (FDC) and Power Duration Curves (PDC) have been constructed for suitable sites from which design flow and firm energy is established. New hydrological insights for the region: The final assessment resulted in the identification of ten sites (HP1 – HP10) along the LPR with potential for hydropower generation. The maximum and minimum mean annual flows were 172.9 m3/s and 5.3 m3/s at hydropower sites HP1 and HP2 respectively. HP1 demonstrated the highest hydropower potential with firm power and annual firm energy at 90 % dependability estimated as 12.22 MW and 77.10 GW h with a total annual energy production (AEP) of 183.77 GW h. Hydropower sites HP3 and HP7 also had relatively high hydropower potential with firm power and annual firm energy of 10.0 MW, 47.01 GW h and 9.0 MW, 40.77 GW h respectively with AEP expected to be in the order of 157.67 GW h and 141.91 GW h respectively. A cascade system is proposed for hydropower sites HP1, HP3 and HP7 with estimated combined AEP of 483.4 GW h.

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

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

Transformation of climate and its sway on hydro-climatology using representative concentration pathways (RCP) in Geba catchment of Ethiopia

Frequent change in climate and its inconsistent characteristic across Ethiopia at the behest in regional scale encourages researchers in suitable planning of the strategies for adaptive capacity. Geba catchment in this region not only put influence by the hydro-climatic conditions but also extends to assess the ongoing impacts. Regional Climate model (RCM) outputs using representative concentration pathways (RCP) including climate change scenarios of precipitation, temperature, and stream flow for three span of time firstly 1976–2005, second 2021–2050, and the last 2051–2080 has been evaluated. Non-linear power transformation for precipitation and linear shifting and scaling for temperature climate datasets was applied by extrapolating existing climate dataset. Pragmatic use of HEC-HMS, the hydrological model was pledged to generate current and future climatic condition in stream flow of Geba catchment. In an average, the respective performance of the model reveals a Nash Sutcliffe Efficiency (NSE) of 0.83 and 0.80 during calibration and validation. The trends of projected climate variables have shown positive change for temperature and potential evapotranspiration whereas stream flow showed a declining trend with manifestation in precipitation. In comparison to the base period, the average maximum temperature may rise by 1.25 °C, 2.01 °C for RCP4.5, 1.67 °C, and 3.39 °C for RCP8.5 for a period 2021–2050 and 2051–2080 respectively. For the same period, an increase in minimum temperature was recorded under RCP4.5 and RCP8.5. The annual precipitation may decrease by 2.94% and 3.32% for RCP4.5 and 2.01% and 3.1% for RCP8.5 for 2021–2050 and 2051–2080 respectively. Besides, annual evapotranspiration may increase up to 2.6% and 4.3% for RCP4.5 and 2.5% and 3.0% for RCP8.5. Overall, the mean annual stream flow indicates a decreasing order from 0.93 to 1.67% for RCP4.5 and by 1.3 to 1.76% for RCP8.5. This provides a detail analysis that climate change affects the water balance of Geba catchment.

Regionalization of mean annual flow for ungauged catchments in case of Abbay River Basin, Ethiopia

Streamflow measurement is one of the features of Ethiopia and other developing countries suffer to estimate. Climate variable (rainfall) and physiographic variables (catchment area, land slope, and elevation) were selected to develop a regression model equation that can be used to estimate the mean annual flow for ungauged catchments in the Abbay river basin. 70% of 27 hydrometric stations were used to train the regression model, and the remaining 30% were used for validation. Furthermore, statistical tests were used for selecting the best trustworthy model. The result shows that both catchment area (A) as the only predictor with both coefficients of determination (R2) and Nash–Sutcliffe efficiency (NSE) value of 0.93 and catchment area (A), mean annual rainfall (MAR), and the average elevation (AE) as predictors with both coefficients of determination (R2) and Nash–Sutcliffe efficiency (NSE) value of 0.96 regression equations were the trustworthy models. Moreover, the validation analysis was performed for nine stations distributed in the study area and resulting from a statistical test and graphical visualization the second regression model equation was a trustworthy model. The model parameters (catchment characteristics) are easily accessible for practitioners that want to estimate the mean annual flow in the study area.

Environmental Flows Assessment in Nepal: The Case of Kaligandaki River

Environmental flow assessments (e-flows) are relatively new practices, especially in developing countries such as Nepal. This study presents a comprehensive analysis of the influence of hydrologically based e-flow methods in the natural flow regime. The study used different hydrological-based methods, namely, the Global Environmental Flow Calculator, the Tennant method, the flow duration curve method, the dynamic method, the mean annual flow method, and the annual distribution method to allocate e-flows in the Kaligandaki River. The most common practice for setting e-flows consists of allocating a specific percentage of mean annual flow or portion of flow derived from specific percentiles of the flow duration curve. However, e-flow releases should mimic the river’s intra-annual variability to meet the specific ecological function at different river trophic levels and in different periods over a year covering biotas life stages. The suitability of the methods was analyzed using the Indicators of Hydrological Alterations and e-flows components. The annual distribution method and the 30%Q-D (30% of daily discharge) methods showed a low alteration at the five global indexes for each group of Indicators of Hydrological Alterations and e-flows components, which allowed us to conclude that these methods are superior to the other methods. Hence, the study results concluded that 30%Q-D and annual distribution methods are more suitable for the e-flows implementation to meet the riverine ecosystem’s annual dynamic demand to maintain the river’s health. This case study can be used as a guideline to allocate e-flows in the Kaligandaki River, particularly for small hydropower plants.