Climate Change Impacts on Flow and Suspended Sediment Yield in Headwaters of High-Latitude Regions—A Case Study in China’s Far Northeast

Effects of climate variability changes on runoff and erosion in the Western European Loess Belt region (NW, France)

Runoff and soil erosion are very pronounced in the Western European Loess Belt. In this study, the distributed physically-based model CLiDE is calibrated, validated, and applied to a catchment of this area (Dun, NW, France) to assess the hydro-sedimentary impacts of climate change scenarios. Despite considerable progress over the last decade in the study of runoff and soil erosion in the context of climate change, the effects of changes in the temporal variability of precipitation remain poorly understood, especially at the scale of a river basin. To examine these relationships more closely, we developed a stochastic weather generator to individually adjust the components that structure the temporal variability of rainfall. The climate scenarios considered represent projections to the year 2100 of the temporal variability of rainfall over NW Europe. The scenarios are based on historical daily rainfall records (1990-2012) and 4 exploratory assumptions: a 50 % decrease in the interannual rainfall regime (scenario 6yD), a 100 % increase in the interannual rainfall regime (scenario 6yI), a 50 % increase in the seasonal rainfall regime (scenario 1yI) and a 50 % increase in the synoptic rainfall regime (scenario 3dI). Simulated daily water and sediment discharges and erosion/deposition maps for each scenario are compared to those simulated for the situation without changes in rainfall. The time series were aggregated over different time intervals to allow for a multi-scale analysis of the differences. The results indicate that the model provides a satisfactory prediction of the catchment's water and sediment discharges, especially over the calibration period. Increased climate variability, whether on a synoptic (3dI), seasonal (1yI) or interannual (6yI) scale, leads to increased runoff and erosion. Increasing the synoptic rainfall variability (3dI) leads to the largest increase in mean annual runoff and erosion. Only the reduction of the interannual rainfall variability (6yD) provokes the decrease of these values.

Global assessment of future sectoral water scarcity under adaptive inner-basin water allocation measures

Water scarcity has become a major threat to sustainable development under climate change. To reduce the population exposure to water scarcity and improve universal access to safe drinking water are important targets of the Sustainable Development Goal (SDG) 6 in the near future. This study aims to examine the potential of applying adaptive inner-basin water allocation measures (AIWAM), which were not explicitly considered in previous studies, for mitigating water scarcity in the future period (2020-2050). By incorporating AIWAM in water scarcity assessment, nonagricultural water uses are assumed to have high priority over agricultural water use and thus would receive more water supply. Results show that global water deficit is projected to be ~3241.9 km³/yr in 2050, and severe water scarcity is mainly found in arid and semi-arid regions, e.g. Western US, Northern China, and the Middle East. Future warming climate and socioeconomic development tend to aggravate global water scarcity, particularly in Northern Africa, Central Asia, and the Middle East. The application of AIWAM could significantly mitigate water scarcity for nonagricultural sectors by leading to a decrease of global population subject to water scarcity by 12% in 2050 when compared to that without AIWAM. However, this is at the cost of reducing water availability for agricultural sector in the upstream areas, resulting in an increase of global irrigated cropland exposed to water scarcity by 6%. Nevertheless, AIWAM provides a useful scenario that helps design strategies for reducing future population exposure to water scarcity, particularly in densely populated basins and regions. Our findings highlight increasing water use competition across sectors between upstream and downstream areas, and the results provide useful information to develop adaptation strategies towards sustainable water management.

Climate Change Impacts on Soil Erosion and Sediment Yield in a Watershed

This study analyzed the influence of climate change on sediment yield variation, sediment transport and erosion deposition distribution at the watershed scale. The study was based on Gaoping River basin, which is among the largest basins in southern Taiwan. To carry out this analysis, the Physiographic Soil Erosion Deposition (PSED) model was utilized. Model results showed a general increase in soil erosion and deposition volume under the A1B-S climate change scenario. The situation is even worsened with increasing return periods. Total erosion volume and total sediment yield in the watershed were increased by 4–25% and 8–65%, respectively, and deposition volumes increased by 2–23%. The study showed how climate change variability would influence the watershed through increased sediment yields, which might even worsen the impacts of natural disasters. It has further illustrated the importance of incorporating climate change into river management projects.

Assessment and Mitigation of Streamflow and Sediment Yield under Climate Change Conditions in Diyala River Basin, Iraq

The impact of climate change on the streamflow and sediment yield in the Derbendkhan and Hemrin Watersheds is an important challenge facing the water resources of the Diyala River in Iraq. The Soil and Water Assessment Tool (SWAT) was used to project this impact on streamflow and sediment yield until year 2050 by applying five climate models for scenario A1B involving medium emissions. The models were calibrated and validated based on daily observed streamflow and sediment recorded for the periods from 1984 to 2013 and 1984 to 1985, respectively. The Nash–Sutcliffe efficiency and coefficient of determination values for the calibration (validation) were 0.61 (0.53) and 0.6 (0.62) for Derbendkhan and Hemrin, respectively. In addition, the average of the future predictions for the five climate models indicated that the streamflow (sediment yield) for the Derbendkhan and Hemrin Watersheds would decrease to 49% (43.7%) and 20% (30%), respectively, until 2050, compared with the observed flow of the base period from 1984 to 2013. The spatial analysis showed that 10.4% and 68% of the streamflow comes from Iraqi parts of the Derbendkhan and Hemrin Watersheds, respectively, while 10% and 60% of the sediment comes from the Iraqi parts of the Derbendkhan and Hemrin Watersheds, respectively. Deforestation of the northern part of the Hemrin Watershed is the best method to decrease the amount of sediment entering the Hemrin Reservoir.

Assessment of sediment and organic carbon exports into the Arctic ocean: The case of the Yenisei River basin

The export of organic carbon export by the rivers to the oceans either as particulate organic carbon (POC) or dissolved organic carbon (DOC) is very sensitive to climate change especially in permafrost affected catchments where soils are very rich in organic carbon. With global warming, organic carbon export in both forms is expected to increase in Arctic regions. It should affect contemporary biogeochemical cycles in rivers and oceans and therefore modify the whole food web. This study tries to understand complex processes involved in sediment, POC and DOC riverine transport in the Yenisei River basin and to quantify their respective fluxes at the river outlet. The SWAT (Soil and Water Assessment Tool) hydrological model is used in this study to simulate water and suspended sediment transfers in the largest Arctic river. POC and DOC export have been quantified with empirical models, adapted from literature for the study case. First, the hydrological model has been calibrated and validated at a daily time step for the 2003-2008 and the 2009-2016 periods respectively, and its output has been compared with field data for water and sediment fluxes. Based on conceptualization of transfer processes, calibration on climate and soil properties has been performed in order to correctly represent hydrology and sediment transfer in permafrost basins. Second, calibration of empirical models for DOC/POC transport have been performed by comparing their output with field data, available from 2003 to 2016. Our study reveals that SWAT is capable of correctly representing hydrology, sediment transfer, POC and DOC fluxes and their spatial distribution at a daily timescale, and outlines the links between these fluxes and permafrost features. Our simulation effort results in specific sediment, POC and DOC fluxes of 2.97 t km^-2 yr^-1, 0.13 t km^-2 yr^-1 and 1.14 t km^-2 yr^-1 for the period 2003-2016 which are in the range of previous estimates. About 60% of the total fluxes of sediment, DOC and POC to the Arctic Ocean are exported during the two months of the freshet. Spatial analysis show that permafrost-free areas have returned higher daily organic carbon export than permafrost affected zones, highlighting the thawing permafrost effect on carbon cycle in climate change feedback.

Multidecadal Sediment Balance Modelling of a Cascade of Alpine Reservoirs and Perspectives Based on Climate Warming

This paper presents a comprehensive methodology to model and determine the annual sediment balance of a complex system of interconnected reservoirs, based on the detailed interpretation of a multi-decadal data series of reservoir management and modelling of sediment fluxes. This methodology is applied to the reservoirs of Oberaar, Grimsel, Räterichsboden, and Trift, which are located in the Swiss Alps. Additionally, the effects of climate warming on the annual sediment yield are investigated. Modelling results show that at present, the hydropower cascade formed by Oberaar, Grimsel, and Räterichsboden retains about 92% of the annual sediment yield, of which only the finest fraction leaves the system and enters the river network. Very fine sediments (d < 10 μm) account for 28% of the total sedimentation rate and in the case of Oberaar, it can reach up to 46% of the total sedimentation rate. Under a climate warming scenario, both sediment yield and runoff are expected to increase in terms of the annual average throughout the XXIst century, which will likely lead to greater annual inputs of sediments to the reservoirs. This, in turn, will lead to a higher sedimentation rate and suspended sediment concentration in the reservoirs, unless active management of the sediment fluxes is implemented.

A Multi-GCM Assessment of the Climate Change Impact on the Hydrology and Hydropower Potential of a Semi-Arid Basin (A Case Study of the Dez Dam Basin, Iran)

In this paper, the impact of climate change on the climate and discharge of the Dez Dam Basin and the hydropower potential of two hydropower plants (Bakhtiari and Dez) is investigated based on the downscaled outputs of six GCMs (General Circulation Models) and three SRES (Special Report on Emission Scenarios) scenarios for the early, mid and late 21st century. Projections of all the scenarios and GCMs revealed a significant rise in temperature (up to 4.9 °C) and slight to moderate variation in precipitation (up to 18%). Outputs of the HBV hydrologic model, enforced by projected datasets, show a reduction of the annual flow by 33% under the climate change condition. Further, analyzing the induced changes in the inflow and hydropower generation potential of the Bakhtiari and Dez dams showed that both inflow and hydropower generation is significantly affected by climate change. For the Bakhtiari dam, this indicates a consistent reduction of inflow (up to 27%) and electricity generation (up to 32%). While, in the Dez dam case, the inflow is projected to decrease (up to 22%) and the corresponding hydropower is expected to slightly increase (up to 3%). This contrasting result for the Dez dam is assessed based on its reservoir and hydropower plant capacity, as well as other factors such as the timely releases to meet different demands and flow regime changes under climate change. The results show that the Bakhtiari reservoir and power plant will not meet the design-capacity outputs under the climate change condition as its large capacity cannot be fully utilized; while there is room for the further development of the Dez power plant. Comparing the results of the applied GCMs showed high discrepancies among the outputs of different models.

Effects of Vegetation Density and Arrangement on Sediment Budget in a Sediment-Laden Flow

Understanding the effects of riparian vegetation under sediment-laden flow is becoming crucial due to the increase in frequency of extreme weather events. This study designed three densities and nine random distributions of bent flexible vegetation in flume experiments under sediment-laden flow. Sediments were continually added to the flume at a rate of 21 kg/h to simulate a natural river environment in a sediment-laden flow. The results showed that the evolutionary process of bed form under sediment-laden flow could be divided into four stages: scouring, development, recovery, and deposition stages, forming a dynamic cycle. Dunes were formed and backwater caused them to develop upstream, while structural resistance developed the dunes downstream. Contrary to clear water regime, sediments were deposited upstream of the vegetation area and scour occurred behind the vegetation. In addition, the vertical velocity profile showed to be dependent on the vegetation structure and four clear zones were identified: fixed, bent, canopy, and developed zones. The findings from this study provide crucial information towards river management through understanding the diverse vegetation effects under sediment-laden flows.

Long-Term Suspended Sediment Concentrations and Loads from a Relatively Undisturbed Agroforested Catchment in the Northwest of the Iberian Peninsula

The suspended sediment dynamics in small catchments are difficult to estimate accurately because they result from the coupling of complex processes occurring at different scales. In this study, the dynamics of suspended sediment concentrations (SSC) and loads were assessed in an agroforested humid catchment in NW Spain, based on a long-term rainfall, discharge and suspended sediment dataset (12 hydrological years) from high-frequency monitoring. The results highlight the episodic nature of sediment transport in the study area, given that about 78% of SS was exported over 10% of the study period. The SS transport was related to runoff generation and flooding, although sediment availability also played an important role in SS transport. The SS load was mainly driven by high-magnitude rainfall events, while intense rainfall episodes generated high SSC peaks. The mean annual suspended sediment yield was relatively low from a quantitative stand point (10 Mg km−2 y−1); however, during 11% of the monitoring, SS concentrations exceeding the threshold threatened surface water quality (Freshwater Fish Directive 78/659/EEC and Directive 75/440/EEC), mainly during runoff events, indicating the need to adopt management practices in order to reduce or mitigate sediment loss during such episodes.