Streamflow Reduction Research Articles

Nature-based solutions in mountain catchments reduce impact of anthropogenic climate change on drought streamflow

Quantifying how well Nature-based Solutions can offset anthropogenic climate change impacts is important for adaptation planning, but has rarely been done. Here we show that a widely-applied Nature-based Solution in South Africa – invasive alien tree clearing – reduces the impact of anthropogenic climate change on drought streamflow. Using a multi-model joint-attribution of climate and landscape-vegetation states during the 2015–2017 Cape Town “Day Zero” drought, we find that anthropogenic climate change reduced streamflow by 12–29% relative to a counterfactual world with anthropogenic emissions removed. This impact on streamflow was larger than corresponding reductions in rainfall (7–15%) and reference evapotranspiration (1.7–2%). Clearing invasive alien trees could have ameliorated streamflow reductions by 3–16% points for moderate invasions levels. Preventing further invasive alien tree spread avoided potential additional reductions of 10–27% points. Total clearing could not have offset the anthropogenic climate change impact completely. Invasive alien tree clearing is an important form of catchment restoration for managing changing hydroclimatic risk, but will need to be combined with other adaptation options as climate change accelerates.

Analysis of the Evolution of Climatic and Hydrological Variables in the Tagus River Basin, Spain

During the second half of the 20th century, several Spanish rivers experienced a decrease in the availability of water resources which coincided with an increase in human water demands. This situation is expected to be exacerbated by climate change. This study analyses the evolution of annual streamflow in 16 sub-basins of the Tagus River basin (Spain) during the 1950–2010 period and its relationship with selected variables. Our main objective is to characterize changes in in-stream flows and to identify what factors could have contributed to them. First, we used non-parametric tests to detect trends in the hydro-climatic series. Then, we analyzed changes in the runoff coefficient and applied regression-based techniques to detect anthropic drivers that could have influenced the observed trends. The analysis revealed a general decreasing trend in streamflow and an increasing trend in air temperature, while trends in precipitation are less clear. Residuals from regression models indicate that the evolution of several non-climatic factors is likely to have influenced the decline in streamflow. Our results suggest that the combination of the expansion of forested areas (a 60% increase from 1950 to 2010) and irrigated land (a 400% increase since 1950) could have played an important role in the reduction of streamflow in the Tagus basin.

Impact of Climate Change on Hydrometeorology and Droughts in the Bilate Watershed, Ethiopia

This study aims to assess the potential impacts of climate change on hydrometeorological variables and drought characteristics in the Ethiopian Bilate watershed. Climate projections under two Representative Concentration Pathways (RCP4.5 and RCP8.5) were obtained from the Coordinated Regional Downscaling Experiment (CORDEX) Africa for the near future (2021–2050) and far future (2071–2100) periods. The Soil and Water Assessment Tool (SWAT) model was applied to assess changes in watershed hydrology with the CORDEX-Africa data. The Standardized Precipitation Index (SPI), Streamflow Drought Index (SDI), and Reconnaissance Drought Index (RDI) were calculated to identify the characteristics of meteorological, hydrological, and agricultural droughts, respectively. Due to a significant rise in temperature, evapotranspiration will increase by up to 16.8% by the end of the 21st century. Under the RCP8.5 scenario, the annual average rainfall is estimated to decrease by 38.3% in the far future period, inducing a reduction of streamflow of up to 37.5%. Projections in reduced diurnal temperature range might benefit crop growth but suggest elevated heat stress. Probabilities of drought occurrence are expected to be doubled in the far future period, with increased intensities for all three types of droughts. These projected impacts will exacerbate water scarcity and threaten food securities in the study area. The study findings provide forward-looking quantitative information for water management authorities and decision-makers to develop adaptive measures to cope with the changing climate.

Quantifying Streamflow Depletion from Groundwater Pumping: A Practical Review of Past and Emerging Approaches for Water Management

AbstractGroundwater pumping can cause reductions in streamflow (“streamflow depletion”) that must be quantified for conjunctive management of groundwater and surface water resources. However, streamflow depletion cannot be measured directly and is challenging to estimate because pumping impacts are masked by streamflow variability due to other factors. Here, we conduct a management‐focused review of analytical, numerical, and statistical models for estimating streamflow depletion and highlight promising emerging approaches. Analytical models are easy to implement, but include many assumptions about the stream and aquifer. Numerical models are widely used for streamflow depletion assessment and can represent many processes affecting streamflow, but have high data, expertise, and computational needs. Statistical approaches are a historically underutilized tool due to difficulty in attributing causality, but emerging causal inference techniques merit future research and development. We propose that streamflow depletion‐related management questions can be divided into three broad categories (attribution, impacts, and mitigation) that influence which methodology is most appropriate. We then develop decision criteria for method selection based on suitability for local conditions and the management goal, actionability with current or obtainable data and resources, transparency with respect to process and uncertainties, and reproducibility.

Climatic and anthropogenic drivers of a drying Himalayan river

Abstract. Streamflow regimes are rapidly changing in many regions of the world. Attribution of these changes to specific hydrological processes and their underlying climatic and anthropogenic drivers is essential to formulate an effective water policy. Traditional approaches to hydrologic attribution rely on the ability to infer hydrological processes through the development of catchment-scale hydrological models. However, such approaches are challenging to implement in practice due to limitations in using models to accurately associate changes in observed outcomes with corresponding drivers. Here we present an alternative approach that leverages the method of multiple hypotheses to attribute changes in streamflow in the Upper Jhelum watershed, an important tributary headwater region of the Indus basin, where a dramatic decline in streamflow since 2000 has yet to be adequately attributed to its corresponding drivers. We generate and empirically evaluate a series of alternative and complementary hypotheses concerning distinct components of the water balance. This process allows a holistic understanding of watershed-scale processes to be developed, even though the catchment-scale water balance remains open. Using remote sensing and secondary data, we explore changes in climate, surface water, and groundwater. The evidence reveals that climate, rather than land use, had a considerably stronger influence on reductions in streamflow, both through reduced precipitation and increased evapotranspiration. Baseflow analyses suggest different mechanisms affecting streamflow decline in upstream and downstream regions, respectively. These findings offer promising avenues for future research in the Upper Jhelum watershed, and an alternative approach to hydrological attribution in data-scarce regions.

Projection of land use to 2030 and its impacts on water availability in a brazilian sub-basin: A LCM and SWAT approach

Changes in land use and land cover (LULC) can result in significant changes in a hydrographic ba- sin flow regime. Future projections about LULC and its interference with water availability help to identify extreme events in advance and help propose appropriate management measures. Thus, this study aimed to make the LULC projection for the year 2030 for the Alto Rio Grande (ARG) sub- basin, located in Southeastern Brazil. This region was chosen because of its intense water resources use and for having recently faced water scarcity as result of prolonged droughts and inadequate water resources management. To identify the LULC trend for the year 2030, the Land Change Modeler (LCM) was used, the map obtained was inserted in the Soil and Water Assessment Tool (SWAT) model previously calibrated and validated for the region’ environmental and climatic conditions. The ARG sub-basin was affected by heavy rains in 2011, which resulted in changes in the landscape due to landslides. This particularity of the region contributed to the projection of LULC for the year 2030 to present an increase in forest and pastures to the agricultural areas detriment. When evaluating the impacts of these changes in water availability, it was observed that the SWAT model presented, for the same rainfall conditions, a reduction in peak streamflows of up to 59% and a reduction in the average monthly flow of up to 63% in 2030 in relation to the LULC observed in 2017. Thus, this study provides an important contribution by identifying a considerable reduction in water availability. These results will help to formulate strategies for water resources management and the adoption of measures to promote water security in the region.

Using soil-moisture drought indices to evaluate key indicators of agricultural drought in semi-arid Mediterranean Southern Africa

Droughts are natural disasters that globally affect large numbers of people each year. While different forms of drought exist, their severity and extent are dependent on critical points of onset. Understanding these onsets is crucial for water, food and energy security, as well as to develop climate change adaptation strategies. This study used the JAMS/J2000 hydrological model to detect agricultural drought using the Soil Moisture Deficit Index (SMDI). The Berg River catchment in Mediterranean South Africa was used as the pilot study area, which experienced a severe drought between 2015 and 2018 and where meteorological drought progressed into agricultural drought that resulted in significant crop reductions and job losses within the agricultural sector. To combat the effects of meteorological shortfalls, water resource management opted to curb agricultural reservoir releases, forcing farmers to rely on groundwater. Modelling results illustrated the importance of detecting headwater stress within the catchment, where in 2015/2017 headwater areas were affected for the first time over the 35-year simulation. Furthermore, regional changes to the groundwater system, during which severe to extremely severe SMDI values (−3 to −4) were simulated, is postulated to be caused by wide-spread groundwater overuse resulting in a 47% reduction in winter (JJA) and a 68% reduction in spring (SON) streamflow. Immediate streamflow reductions were observed, illustrating the low resilience of these systems to meteorological and agricultural droughts, as well as the impacts of water use behavioural changes. By using SMDI in conjunction with a well constrained hydrological model, crucial drought onset triggers can be detected as well as tipping points regarding water use behaviour. As climate change drives an increase in the occurrence of meteorological droughts in many parts of the world, understanding the advance of severe long-term effects is important for the development of effective adaption strategies to promote water, food and energy security.

Streamflow trends during recent decades at some near-natural catchments in Iran

ABSTRACT In this study, we assessed the impacts of climate variations on streamflow in 28 near-natural catchments in Iran. To this end, we analysed the trend of annual streamflow, precipitation, and temperatures using the Mann-Kendall test and Sen’s slope between water years 1982 and 2011. We evaluated the frequency of precipitation in different classes, i.e. below 5, 5–10, 10–15, and above 15 mm/day. Our results indicate a decline in streamflow at 25 catchments, at a rate of −5.66 to −0.19 mm/year. The annual precipitation amount did not decrease significantly, while the frequency of light precipitation events (<5 mm/day) increased. About 60% of the upward trend was significant. Mean temperature increased at all studied catchments, at an average rate of 0.055°C/year. In short, our results indicate that increases in temperature and the frequency of light precipitation events are two leading factors in streamflow reduction across studied catchments.

Large-scale sensitivities of groundwater and surface water to groundwater withdrawal

Abstract. Increasing population, economic growth and changes in diet have dramatically increased the demand for food and water over the last decades. To meet increasing demands, irrigated agriculture has expanded into semi-arid areas with limited precipitation and surface water availability. This has greatly intensified the dependence of irrigated crops on groundwater withdrawal and caused a steady increase in groundwater withdrawal and groundwater depletion. One of the effects of groundwater pumping is the reduction in streamflow through capture of groundwater recharge, with detrimental effects on aquatic ecosystems. The degree to which groundwater withdrawal affects streamflow or groundwater storage depends on the nature of the groundwater–surface water interaction (GWSI). So far, analytical solutions that have been derived to calculate the impact of groundwater on streamflow depletion involve single wells and streams and do not allow the GWSI to shift from connected to disconnected, i.e. from a situation with two-way interaction to one with a one-way interaction between groundwater and surface water. Including this shift and also analysing the effects of many wells requires numerical groundwater models that are expensive to set up. Here, we introduce an analytical framework based on a simple lumped conceptual model that allows us to estimate to what extent groundwater withdrawal affects groundwater heads and streamflow at regional scales. It accounts for a shift in GWSI, calculates at which critical withdrawal rate such a shift is expected, and when it is likely to occur after withdrawal commences. It also provides estimates of streamflow depletion and which part of the groundwater withdrawal comes out of groundwater storage and which parts from a reduction in streamflow. After a local sensitivity analysis, the framework is combined with parameters and inputs from a global hydrological model and subsequently used to provide global maps of critical withdrawal rates and timing, the areas where current withdrawal exceeds critical limits and maps of groundwater and streamflow depletion rates that result from groundwater withdrawal. The resulting global depletion rates are compared with estimates from in situ observations and regional and global groundwater models and satellites. Pairing of the analytical framework with more complex global hydrological models presents a screening tool for fast first-order assessments of regional-scale groundwater sustainability and for supporting hydro-economic models that require simple relationships between groundwater withdrawal rates and the evolution of pumping costs and environmental externalities.

Quantitative Estimation on Contribution of Climate Changes and Watershed Characteristic Changes to Decreasing Streamflow in a Typical Basin of Yellow River

Climate changes and underlying surface of the watershed have affected the evolution of streamflow to a different degree. It is of great significance to quantitatively evaluate main drivers of streamflow change for development, utilization, and planning management of water resources. In this study, the Huangshui River basin, a typical basin of the Qinghai-Tibetan Plateau, China, was chosen as the research area. Huangshui is the largest first-class tributary in the upstream of the Yellow River. Based on the Budyko hypothesis, streamflow and meteorological datasets from 1958 to 2017 were used to quantitatively assess the relative contributions of changes in climate and watershed characteristics to streamflow change in research area. The results show that the streamflow of Huangshui River basin shows an insignificant decreasing trend; the sensitivity coefficients of streamflow to precipitation, potential evapotranspiration, and watershed characteristic parameter are 0.5502, −0.1055, and −183.2007, respectively. That is, 1 unit increase in precipitation would induce an increase of 0.5502 units in streamflow, and 1 unit increase in potential evapotranspiration would induce a decrease of 0.1055 units in streamflow, and an increase of 1 unit in the watershed characteristic coefficient would induce a decrease of 183.2007 units in streamflow. The streamflow decreased by 20.48 mm (13.59%) during the change period (1994–2017) compared with that during the reference period (1958–1993), which can be attributed to watershed characteristic changes (accounting for 73.64%) and climate change (accounting for 24.48%). Watershed characteristic changes exert a dominant influence upon the reduction of streamflow in the Huangshui River basin.

Impacts of Climate and Anthropogenic Activities on Streamflow Regimes in the Beiluo River, China

Quantitatively assessing the characteristics of river streamflow variation and conducting research on attribution identification are the basis for formulating climate-change response strategies and rational use of water resources. Based on the daily streamflow data of the Zhuangtou Hydrological Station in 1970–2018, this paper analyzes the streamflow changes in the Beiluo River Basin and studies the impact of climate change and anthropogenic activities on the streamflow in this basin. A non-parametric Mann–Kendall test and Pettitt’s test were used to determine the trend and detect abrupt changes of streamflow and baseflow. The method based on precipitation and potential evapotranspiration, as well as the double-mass curve of precipitation–streamflow, was established to evaluate the impact of climate change and non-climate factors on annual streamflow. The results reveal a statistically significant downward trend (p = 0.01) in both annual streamflow and baseflow, with the abrupt point year in 1994 and 1988, respectively. When comparing to a modest declining trend in annual average precipitation, we see that the temperature showed a significant upward trend (p = 0.01), whose abrupt point year was 1996. Under the policy of returning farmland to forest, land-use analysis shows that the area of farmland had decreased by 222.4 km2, of which 31.4% was mainly converted into the forestland. By the end of 2015, the area of forestland had increased by 123.4 km2, which has largely caused streamflow decrease. For the method based on precipitation and potential evapotranspiration, climate change contributed 43.7% of the annual streamflow change, and human activities (mainly refers to LUCC) contributed 56.3%. For the DMC of precipitation–streamflow, the precipitation contributed 9.4%, and non-precipitation factors (mainly refers to human activities) contributed 90.6%, and human activities played a more vital part in driving streamflow reduction in different decades, with a contribution rate of more than 70%. This study is of great practical significance to the planning, management, development and utilization of water resources in basins.

Influences of natural salinity sources and human actions on the Shapour River salinity during the recent streamflow reduction period.

The Shapour River, with a catchment area of 4254 km2, is a major river system in southern Iran. While the upstream river flow (the upper Shapour River) is fresh, it becomes extremely salinized at the downstream confluence of the Shekastian saline tributary and the entering nearby Boushigan saline spring. Then, the river passes via the Khesht plain and finally discharges into Raeisali-Delvari storage dam which went into operation in 2009. Over the 2006-2019 period, reduced precipitation and over-utilization of freshwater resources resulted in ~ 72% streamflow reduction in the Shapour River. Due to not using the saline waters for irrigation, drinking, and industrial purposes, the ratio of saline-outflow of Shekastian tributary and Boushigan spring to fresh-outflow of upper Shapour River increased by ~ 3 times; consequently, river salinity fluctuation domain at the Khesht plain inlet dramatically increased from 2.1-4.0 dS m-1 to 3.7-26.0 dS m-1. It resulted in major economic damages to the agricultural sector of middle Shapour River. On the seasonal timescale, consecutive processes of salt accumulation during irrigation season of the Khesht plain date orchards and then salt drainage during the rainy season have adjusted salinity fluctuation domain from 3.7-26.0 dS m-1 at the plain inlet to 5.2-8.9 dS m-1 at the plain outlet. In the lower Shapour River, storage/mixing of fresh/saline inflow waters in the Raeisali-Delvari reservoir has adjusted strong river salinity fluctuation domain from 0.9-10.7 dS m-1 at the reservoir inlet to 3.6-5.5 dS m-1 at the reservoir outlet. The success of the Raeisali-Delvari reservoir for salinity adjustment is due to its suitable location on the Shapour River, by being situated downstream of all main river tributaries with natural saline/fresh sources of water.

Changes in streamflow regimes and their responses to different soil and water conservation measures in the Loess Plateau watersheds, China

AbstractInvestigating the changes in streamflow regimes in response to various influencing factors contributes to our understanding of the mechanisms of hydrological processes in different watersheds and to water resource management strategies. This study examined streamflow regime changes by applying the indicators of hydrologic alteration method and eco‐flow metrics to daily runoff data (1965–2016) from the Sandu, Hulu and Dali Rivers on the Chinese Loess Plateau, and then determined their responses to terracing, afforestation and damming. The Budyko water balance equation and the double mass curve method were used to separate the impacts of climate change and human activities on the mean discharge changes. The results showed that the terraced and dammed watersheds exhibited significant decreases in annual runoff. All hydrologic metrics indicated that the highest degree of hydrologic alteration was in the Sandu River watershed (terraced), where the monthly and extreme flows reduced significantly. In contrast, the annual eco‐deficit increased significantly, indicating the highest reduction in streamflow among the three watersheds. The regulation of dams and reservoirs in the Dali River watershed has altered the flow regime, and obvious decreases in the maximum flow and slight increases in the minimum flow and baseflow indices were observed. In the Hulu River watershed (afforested), the monthly flow and extreme flows decreased slightly and were categorized as low‐degree alteration, indicating that the long‐term delayed effects of afforestation on hydrological processes. The magnitude of the eco‐flow metrics varied with the alteration of annual precipitation. Climate change contributed 67.47% to the runoff reduction in the Hulu River watershed, while human activities played predominant roles in reducing runoff in the Sandu and Dali River watersheds. The findings revealed distinct patterns and causes of streamflow regime alteration due to different conservation measures, emphasizing the need to optimize the spatial allocation of measures to control soil erosion and utilize water resources on the Loess Plateau.

Increased Vegetation in Mountainous Headwaters Amplifies Water Stress During Dry Periods

AbstractThe dynamics of blue and green water partitioning under vegetation and climate change, as well as their different interactions during wet and dry periods, are poorly understood in the literature. We analyzed the impact of vegetation changes on blue water generation in a central Spanish Pyrenees basin undergoing intense afforestation. We found that vegetation change is a key driver of large decreases in blue water availability. The effect of vegetation increase is amplified during dry years, and mainly during the dry season, with streamflow reductions of more than 50%. This pattern can be attributed primarily to increased plant water consumption. Our findings highlight the importance of vegetation changes in reinforcing the decrease in water resource availability. With aridity expected to rise in southern Europe over the next few decades, interactions between climate and land management practices appear to be amplifying future hydrological drought risk in the region.

Structural changes to forests during regeneration affect water flux partitioning, water ages and hydrological connectivity: Insights from tracer-aided ecohydrological modelling

Abstract. Increasing rates of biodiversity loss are adding momentum to efforts seeking to restore or rewild degraded landscapes. Here, we investigated the effects of natural forest regeneration on water flux partitioning, water ages and hydrological connectivity, using the tracer-aided ecohydrological model EcH2O-iso. The model was calibrated using ∼ 3.5 years of diverse ecohydrological and isotope data available for a catchment in the Scottish Highlands, an area where impetus for native pinewood regeneration is growing. We then simulated two land cover change scenarios that incorporated forests at early (dense thicket) and late (old open forest) stages of regeneration, respectively. Changes to forest structure (proportional vegetation cover, vegetation heights and leaf area index of pine trees) were modelled for each stage. The scenarios were then compared to a present-day baseline simulation. Establishment of thicket forest had substantial ecohydrological consequences for the catchment. Specifically, increased losses to transpiration and, in particular, interception evaporation drove reductions in below-canopy fluxes (soil evaporation, groundwater (GW) recharge and streamflow) and generally slower rates of water turnover. The greatest reductions in streamflow and connectivity were simulated for summer baseflows and small to moderate events during summer and the autumn/winter rewetting period. This resulted from the effect of local changes to flux partitioning in regenerating areas on the hillslopes extending to the wider catchment by reducing downslope GW subsidies that help sustain summer baseflows and saturation in the valley bottom. Meanwhile, higher flows were relatively less affected, especially in winter. Despite the generally drier state of the catchment, simulated water ages suggested that the increased transpiration demands of the thicket forest could be satisfied by moisture carried over from previous seasons. The more open nature of the old forest generally resulted in water fluxes, water ages and connectivity returning towards baseline conditions. Our work implies that the ecohydrological consequences of natural forest regeneration depend on the structural characteristics of the forest at different stages of development. Consequently, future land cover change investigations need to move beyond consideration of simple forest vs. non-forest scenarios to inform sustainable landscape restoration efforts.

The sensitivity of runoff generation to spatial snowpack uniformity in an alpine watershed: Green Lakes Valley, Niwot Ridge Long‐Term Ecological Research station

AbstractSeasonal water storage in high‐elevation alpine catchments are critical sources of water for mountainous regions like the western U.S. The spatial distribution of snow in these topographically complex catchments is primarily governed by orography, solar radiation, and wind redistribution. While the effect of solar shading is relatively consistent from year‐to‐year, the redistribution of snow due to wind is more variable – capable of producing snowpacks that have varying degrees of uniformity across these hydrologically‐important catchments. A reasonable hypothesis is that a warmer climate will cause snowfall to become more dense (i.e. wetter and heavier), possibly leading to less wind redistribution and thus produce a more uniformly distributed snowpack across the landscape. In this study, we investigate the role of increasingly uniform spatial snowpack distributions on streamflow generation in the Green Lakes Valley Niwot Ridge Long Term Ecological Research station, within the headwaters of the Boulder Creek watershed in Colorado. A set of idealized hydrologic simulation experiments driven by reconstructed snowpacks spanning 2001–2014 show that more a more uniform spatial snowpack distribution leads to an earlier melt‐out of 31 days on average and tends to produce less total streamflow, with maximum decreases as large as 7.5%. Isolating the role of snowpack heterogeneity from melt‐season precipitation, we find that snowpack uniformity reduces total streamflow by as much as 13.2%. Reductions in streamflow are largely explained by greater exposure to solar radiation in the uniformly distributed case relative to a more heterogeneous snowpack, with this exposure driving shifts towards earlier snowmelt and changes in soil water storage. Overall, we find that the runoff efficiency from shallower snowpacks is more sensitive to the effects of uniformity than deeper snowpacks, which has potential implications for a warming climate where shallower snowpacks and enhanced sensitivities may be present.

Late twentieth century rapid increase in high Asian seasonal snow and glacier-derived streamflow tracked by tree rings of the upper Indus River basin

Given the reported increasing trends in high Asian streamflow and rapidly increasing water demand in the Indian subcontinent, it is necessary to understand the long‐term changes and mechanisms of snow- and glacier-melt-driven streamflow in this area. Thus, we have developed a June–July streamflow reconstruction for the upper Indus River watershed located in northern Pakistan. This reconstruction used a temperature-sensitive tree-ring width chronology of Pinus wallichiana, and explained 40.9% of the actual June–July streamflow variance during the common period 1970–2008. The high level of streamflow (1990–2017) exceeds that of any other time and is concurrent with the impact of recent climate warming that has resulted in accelerated glacier retreats across high Asia. The streamflow reconstruction indicated a pronounced reduction in streamflow in the upper Indus River basin during solar minima (Maunder, Dalton, and Damon). Shorter periods (years) of low streamflow in the reconstruction corresponded to major volcanic eruptions. Extreme low and high streamflows were also linked with sea surface temperature. The streamflow reconstruction also provides a long-term context for recent high Asian streamflow variability resulting from seasonal snow and glaciers that is critically needed for water resources management and assessment.

Climate vs. Human Impact: Quantitative and Qualitative Assessment of Streamflow Variation

This paper presents a novel framework comprising analytical, hydrological, and remote sensing techniques to separate the impacts of climate variation and regional human activities on streamflow changes in the Karkheh River basin (KRB) of western Iran. To investigate the type of streamflow changes, the recently developed DBEST algorithm was used to provide a better view of the underlying reasons. The Budyko method and the HBV model were used to investigate the decreasing streamflow, and DBEST detected a non-abrupt change in the streamflow trend, indicating the impacts of human activity in the region. Remote sensing analysis confirmed this finding by distinguishing land-use change in the region. The algorithm found an abrupt change in precipitation, reflecting the impacts of climate variation on streamflow. The final assessment showed that the observed streamflow reduction is associated with both climate variation and human influence. The combination of increased irrigated area (from 9 to 19% of the total basin area), reduction of forests (from 11 to 3%), and decreasing annual precipitation has substantially reduced the streamflow rate in the basin. The developed framework can be implemented in other regions to thoroughly investigate human vs. climate impacts on the hydrological cycle, particularly where data availability is a challenge.

Impacts of run‐of‐river hydropower on coho salmon (Oncorhynchus kisutch): the role of density‐dependent survival

AbstractPredicting whether anthropogenic sources of mortality have negative consequences at the level of population dynamics is challenged by mechanisms like density‐dependent survival that can amplify or offset the loss of individuals from anthropogenic disturbances. Run‐of‐river (RoR) hydropower is a growing industry that can cause frequent mortality of salmonid fry through rapid reductions in streamflow, leading to stranding on dewatered shores. However, whether individual‐level impacts reduce population growth rates or increase local extinction risk is difficult to predict. We used a stochastic stage‐structured matrix model to evaluate how the timing and magnitude of anthropogenic flow fluctuations impacted population abundance and extinction risk of coho salmon (Oncorhynchus kisutch), which spend up to 1.5 yr in many streams regulated by RoR hydropower. We additionally assessed how the timing (spring, winter) and strength (weak, moderate, high) of natural density‐dependent bottlenecks experienced by salmon in freshwaters tempers or amplifies the potential for RoR‐induced mortality to scale to emergent population dynamics. We compared population sizes and the 45‐yr probability of quasi‐extinction under 12 scenarios that varied the frequency (0–20 events per year) and magnitude (1–10% mortality per event) of RoR‐induced flow fluctuations, as well as the timing and strength of density‐dependent bottlenecks occurring during the first year in freshwater. We found that even mild flow fluctuations by RoR hydropower can impact coho salmon population dynamics, especially if density dependence is weak or occurs early in freshwater residency (spring). When density dependence was strong and during winter, the potential for population‐level impact was lessened, but populations still declined by 13–42% when RoR‐induced mortality was severe (5–10%) or frequent (10–20 events/yr). We conclude that strong density‐dependent survival bottlenecks could partially mitigate the loss of fry from anthropogenic flow fluctuations, especially if bottlenecks occur late in freshwater residency, but not for all intensities of flow fluctuations. Even with strong density dependence in winter, our models predict declining populations by up to 70% under strong and very frequent flow fluctuations, which should serve to caution those tasked with regulating flows in streams affected by RoR hydropower.

Contrasting Impacts of a Hotter and Drier Future on Streamflow and Catchment Scale Sediment Flux in the High Andes

AbstractFluvial sediment transport in high mountain regions is uniquely sensitive to changes in hydroclimate that alter the timing and magnitude of streamflow contributions from snowmelt, glaciers, and other cryoforms. In the high Andes of Argentina and Chile, mountain river systems serve millions of people in foothill communities and large metropolitan areas alike, yet these same rivers can also carry high sediment loads that compromise water quality and infrastructure. Regional climate predictions are also increasingly forecasting a warmer and drier future that is likely to alter hydrologic regimes and sediment transport dynamics. In this study, we use hydrologic modeling techniques and sediment rating curves compiled from historical data to propagate a suite of endmember climate projections to estimate changes in streamflow and consequent impacts on catchment‐scale sediment transport on both the Argentine and Chilean sides of the high Andes. Our results show reductions in streamflow of up to 57% under scenarios with reduced precipitation and even larger decreases in sediment flux, while outcomes under a warming scenario with no changes to precipitation were highly variable. Our results further show that high winter streamflow events may increase in magnitude in Chile while mountain communities in Argentina may increasingly face water shortages. These results indicate that different adaptation and mitigation strategies may be needed on either side of the Andes in response to a changing hydroclimate.