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Forest disturbance threshold is defined as a critical disturbance level (e.g., % of forest cover change) in forested landscapes above which significant hydrological impacts are detected. Determining disturbance thresholds is critically important for supporting forest management to ensure the sustaining of ecological and hydrological functions. However, there are very few quantitative evaluations of forest disturbance thresholds globally. In this study, we applied a well‐tested methodology (the modified double mass curve) to derive the long‐term, continuous hydrological response curves and then to quantify forest disturbance thresholds on annual streamflow in 42 forested watersheds in British Columbia, Canada. The results show that forest disturbance thresholds for significant and cumulative hydrological impacts vary from 7% to 52% of cumulative equivalent clear‐cut area with an average of 17% or from 8% to 52% of disturbed area with an average of 19%. Climate (inter‐annual and intra‐annual) and watershed properties exert critical controls on forest disturbance thresholds. Watersheds with greater snowfall proportions (more annual precipitation falling as snow), more desynchronizations (temporal mismatching) of energy demand and water supply at the intra‐annual scale, less diverse ecosystems, and larger watershed sizes have lower forest disturbance thresholds. Given the present forest disturbance levels in the central interior of British Columbia, about half (53%) of the forested watersheds have already crossed the average disturbance threshold. These results highlight that watershed planning and management using forest disturbance thresholds must carefully consider local climate and watershed properties. The methodology can be effectively and robustly extended elsewhere around the globe. Plain Language Summary: This first‐ever study uses the hydrological response curves to quantify forest disturbance thresholds on annual streamflow in 42 forested watersheds in the interior of British Columbia, Canada. We find that forest disturbance thresholds range from 7% to 52% of cumulative equivalent clear‐cut area with the average of 17%. Climate and watershed properties play critical roles in controlling disturbance thresholds. Watersheds characterized by more annual precipitation falling as snow, more mismatching of energy and water at the intra‐annual scale, larger watershed size, and less diverse ecosystems exhibit lower disturbance thresholds. Management implications and future research needs for preserving hydrological functions are discussed. Key Points: A new approach with the hydrological response curves is used to quantify forest disturbance thresholdsForest disturbance thresholds for cumulative impacts on annual streamflow vary from 7% to 52% of cumulative equivalent clear‐cut area with the average of 17%Climate and watershed properties exert critical controls on the thresholds [ABSTRACT FROM AUTHOR]

The Xingu River Basin (XRB) in the Brazilian Amazon region has a great relevance to the development of northern Brazil because of the Belo Monte hydropower plant and its crescent agribusiness expansion. This study aimed to evaluate the potential of the Lavras Simulation of the Hydrology (LASH) model to represent the main hydrological processes in the XRB and simulate the hydrological impacts in the face of land-use change scenarios. Following the trend of the most relevant agribusiness evolution in the XRB, four agribusiness scenarios (S) were structured considering the increase in grasslands (S1: 50% over the native forest; S2: 100% over the native forest) and soybean plantations (S3: 50% over the native forest; S4: 100% over native forest). Average hydrographs were simulated, and the frequency duration curves (FDC) and average annual values of the main hydrological components for each scenario were compared. The results showed that, in general, changes in land use based on deforestation in the XRB would lead to an increase in flood streamflow and a reduction in baseflow. The increases in direct surface runoff varied from 4.4% for S1 to 29.8% for S4 scenarios. The reduction in baseflow varied from −1.6% for S1 to −4.9% for S2. These changes were reduced when the entire XRB was analyzed, but notable for the sub-basins in its headwater region, where the scenarios were more effective. [ABSTRACT FROM AUTHOR]

It is imperative to understand the interconnectedness of water use and hydrological impacts for water policy design underlying varying hydrological conditions across space and over time. However, such analysis remains difficult, constrained by the lack of appropriate modeling tools that fully integrate water policies, water use, and hydrological processes with high spatiotemporal resolutions. To address this challenge, this study proposes a distributed policy design scheme featuring spatially variable and temporally dynamic policies for conjunctive surface water‐groundwater management in large river basins. A fully integrated modeling framework is developed to tightly couple (a) an agent‐based model for farmers' water use under distributed water policies and (b) a physically based hydrological model for surface water‐groundwater processes. The modeling framework is applied to the Heihe River Basin to assess water use and hydrological impacts under distributed water policies. By using the distributed policy scheme to adjust a water policy (e.g., groundwater tax) across space and over time, we found that hydrological outcomes can be improved without adversely reducing agricultural water supply. For example, by shifting the implementation of a high groundwater tax from dry to wet years, a rise of the water table by 0.28 m (0.03–0.95 m across different irrigation districts) can be achieved while the total water supply is maintained at a similar level. Furthermore, hydrological externality effects among nearby districts can be explicitly identified and quantified based on assessments of spatially varying water policies. This study highlights the need for water policy design to consider spatiotemporal variations in the physical hydrological system. Key Points: This study proposes and evaluates a distributed policy design scheme featuring spatially variable and temporally dynamic water policiesIt is possible to mitigate aquifer depletion without reducing the total water supply by adjusting water policies between dry and wet yearsHydrological externality effects among nearby districts are explicitly quantified by assessing spatially varying water management policies [ABSTRACT FROM AUTHOR]

Little Ruaha catchment has been recognized for its potential to support multi-projects including irrigated schemes, urban water supply and providing significant inflow to Mtera reservoir for hydropower generation and ultimately Julius Nyerere Hydropower Plant (JNHPP). Despite the potential, the catchment has experienced declining flows in the recent years. This study assessed the likely changes in streamflow due to future climate change in the Little Ruaha catchment for the period 2025-2060. General Circulation Model (GCM) datasets from ACCESS1.0, CNRM-CM5 and BCC-CSM1 models and RCP4.5 and RCP8.5 greenhouse gas concentration scenarios were selected as the representative scenarios. Impact of climate change on stream flows was assessed using the calibrated NAM hydrological model. The impact assessment results show that under the climate change scenario (2025–2060), the monthly maximum and minimum temperatures will increase in the range of 0.8 °C to 2 °C for both RCP4.5 and RCP8.5 scenarios. For the case of rainfall, average annual rainfall is expected to increase by about 10% compared to the baseline. However, the inter-annual variability of rainfall for the period between 2025 and 2060 shows the decreasing trend for RCP 8.5. The simulation results show that streamflow will decrease by about 30% and 6% for RCP4.5 and RCP8.5, respectively. [ABSTRACT FROM AUTHOR]

A key motivation for managing invasive alien plant (IAP) species is their impacts on streamflows, which, for the wetter half of South Africa, are about 970 m³∙ha−1∙a−1 or 1 444 mill. m³∙a−1 (2.9% of naturalised mean annual runoff), comparable to forest plantations. However, the implications of these reductions for the reliability of yields from large water supply systems are less well known. The impacts on yields from the WCWSS were modelled under three invasion scenarios: ‘Baseline’ invasions; increased invasions by 2045 under ‘No management’; and under ‘Effective control’ (i.e. minimal invasions). Monthly streamflow reductions (SFRs) by invasions were simulated using the Pitman rainfall−runoff catchment model, with taxon-specific mean annual and low-flow SFR factors for dryland (upland) invasions and crop factors for riparian invasions. These streamflow reduction sequences were input into the WCWSS yield model and the model was run in stochastic mode for the three scenarios. The 98% assured total system yields were predicted to be ±580 million m³∙a−1 under ‘Effective control’, compared with ±542 million m³∙a−1 under ‘Baseline’ invasions and ±450 mill. m³∙a−1 in 45 years’ time with ‘No management’. The ‘Baseline’ invasions already reduce the yield by 38 mill. m³∙a−1 (two thirds of the capacity of the Wemmershoek Dam) and, in 45 years’ time with no clearing, the reductions would increase to 130 mill. m³∙a−1 (capacity of the Berg River Dam). Therefore IAP-related SFRs can have significant impacts on the yields of large, complex water supply systems. A key reason for this substantial impact on yields is that all the catchments in the WCWSS are invaded, and the invasions are increasing. Invasions also will cost more to clear in the future. So, the best option for all the water-users in the WCWSS is a combined effort to clear the catchments and protect their least expensive source of water. [ABSTRACT FROM AUTHOR]

This study aimed to assess the hydrological and erosive effects of different levels of the fire severity in the drainage basin of the Trionto River (Calabria, southern Italy), which was partially burned by intense fires during the summer 2017. The analysis focused on the identification of wildfire areas using a supervised classification of remote sensing images with the minimum distance algorithm. The level of severity of each fire was then discriminated based on a procedure proposed by the U.S. Department of Agriculture and adapted to the study area. To evaluate how wildfire occurrence affects the hydrological behaviour at the basin scale, the SCS-Curve Number model was used to document pre- and post-fire conditions in relation with the level of fire severity. Finally, the influence on erosion was analysed for analogous conditions at the basin scale using the RUSLE equation. The effects on hydrological balance and soil loss were evaluated by comparing the pre-fire value with three different post-fire scenarios: (a) different levels of severity on the surface covered by the fire (real case); (b) maximum level of severity on the surface covered by the fire; (c) total loss of the canopy and formation of a hydrophobic layer on the surface soil. The results confirmed that the level of severity of the forest fires, combined with climatic factors, morphological conditions, and the pedological characteristics of the basin, significantly influence changes to the hydrology and rates of erosion. Moreover, these impacts proved to be mainly dependent on the consequent, often notable, heterogeneity in the spatial distribution of burned areas with different severity. [ABSTRACT FROM AUTHOR]

Abstract Ensuring water is available for human use usually requires dams and reservoirs that severely affect river ecosystems. Hydrological alteration by river damming represents one of the most prominent human impacts on freshwater ecosystems. This paper presents a methodological approach to assessing the natural patterns of flow variability and hydrological alterations in Mediterranean rivers. The methodology first defines the admissible range of flow variability under a natural flow regime. Then, the methodology measures the environmental impact of flow regulation according to the inferred hydrological alteration and classifies rivers into homogeneous groups according to the magnitude, frequency, duration and seasonality of the impacts. This paper applies the proposed methodology to thirty-three river segments below large reservoirs located in the three largest Spanish watersheds (Duero, Ebro and Tajo). For each segment, this study evaluates the general impoundment characteristics and geographical contexts and calculates the flow impacts and the divergence between the circulating flows and the reference area of admissible flow variability on a daily basis for the period 2000–2010. This assessment of dam regulation impacts provides a reference to plan measures for mitigating these impacts. The advantages of this flow regulation impact analysis over other analyses are discussed in terms of the implementation of environmental water costs as suggested by the European Water Framework Directive. This approach expands the current understanding of water environmental costs and represents a practical management tool for decreasing the environmental impacts of flow regulation and for increasing water-use efficiency. Highlights • Natural range of flow variability discriminates distinct hydrologic regime patterns. • Flow regulation impacts are not significantly influenced by climate or geomorphology. • Flow regulation impacts strongly depend on dam operation and water use. • Flow reductions in winter and increases in summer are common in Mediterranean rivers. • Deviations from natural flow variability should inform damming environmental costs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: This study used climate change projections from different regional approaches to assess hydrological effects on the Thukela River Basin in KwaZulu-Natal, South Africa. Projecting impacts of future climate change onto hydrological systems can be undertaken in different ways and a variety of effects can be expected. Although simulation results from global climate models (GCMs) are typically used to project future climate, different outcomes from these projections may be obtained depending on the GCMs themselves and how they are applied, including different ways of downscaling from global to regional scales. Projections of climate change from different downscaling methods, different global climate models and different future emissions scenarios were used as input to simulations in a hydrological model to assess climate change impacts on hydrology. A total of 10 hydrological change simulations were made, resulting in a matrix of hydrological response results. This matrix included results from dynamically downscaled climate change projections from the same regional climate model (RCM) using an ensemble of three GCMs and three global emissions scenarios, and from statistically downscaled projections using results from five GCMs with the same emissions scenario. Although the matrix of results does not provide complete and consistent coverage of potential uncertainties from the different methods, some robust results were identified. In some regards, the results were in agreement and consistent for the different simulations. For others, particularly rainfall, the simulations showed divergence. For example, all of the statistically downscaled simulations showed an annual increase in precipitation and corresponding increase in river runoff, while the RCM downscaled simulations showed both increases and decreases in runoff. According to the two projections that best represent runoff for the observed climate, increased runoff would generally be expected for this basin in the future. Dealing with such variability in results is not atypical for assessing climate change impacts in Africa and practitioners are faced with how to interpret them. This work highlights the need for additional, well-coordinated regional climate downscaling for the region to further define the range of uncertainties involved. [Copyright &y& Elsevier]

Summary: Although no single weather-related event can be directly attributed to climate change, new techniques make it possible to estimate how much the chance of an event has been altered by anthropogenic emissions. This paper looks at the floods that occurred in England in Autumn/Winter 2000, by using large ensembles of 1-year climate model simulations representing April 2000–March 2001. These represent an industrial climate and four estimates of an hypothetical non-industrial climate (without historical greenhouse gas emissions), and are used to drive hydrological models for eight catchments in England. The simulated flows are used to assess the impact of historical emissions on the chance of occurrence of extreme floods in each catchment, through calculation of the fraction of attributable risk (FAR). Combining results for the four non-industrial climates, positive median values of FAR indicate that, for all but one catchment, emissions are likely to have led to an increased chance of flooding in the October–December period. Definitive conclusions are difficult however, as there are wide bands of uncertainty in FAR, with distributions generally spanning no attributable difference in risk (FAR =0). One catchment shows a decreased flood chance (negative median FAR), due to its high permeability, but an analysis of the effect of antecedent conditions shows that a longer period of climate data than 1year is probably required to obtain more representative values of FAR for such catchments. The inclusion of snowfall/snowmelt is also shown to be important for floods over the October–March period, as the reduced likelihood of snowmelt-induced floods in the warmer temperatures of the industrial climate moderates the increased flood chance due to other sources of flooding. [ABSTRACT FROM AUTHOR]

As climate change could have considerable influence on hydrology and corresponding water management, appropriate climate change inputs should be used for assessing future impacts. Although the performance of regional climate models (RCMs) has improved over time, systematic model biases still constrain the direct use of RCM output for hydrological impact studies. To address this, a distribution-based scaling (DBS) approach was developed that adjusts precipitation and temperature from RCMs to better reflect observations. Statistical properties, such as daily mean, standard deviation, distribution and frequency of precipitation days, were much improved for control periods compared to direct RCM output. DBS-adjusted precipitation and temperature from two IPCC Special Report on Emissions Scenarios (SRESA1B) transient climate projections were used as inputs to the HBV hydrological model for several river basins in Sweden for the period 1961 -2100. Hydrological results using DBS were compared to results with the widely-used delta change (DC) approach for impact studies. The general signal of a warmer and wetter climate was obtained using both approaches, but use of DBS identified differences between the two projections that were not seen with DC. The DBS approach is thought to better preserve the future variability produced by the RCM, improving usability for climate change impact studies. [ABSTRACT FROM AUTHOR]

After the SWAT (Soil and Water Assessment Tool) model was calibrated and validated to historic flow records for the current land use conditions, two additional land cover scenarios (a prehistoric land cover and a potential maximum plantation pine cover) were used to evaluate the impacts of land cover change on total water yields, groundwater flow, and quick flow in the Motueka River catchment, New Zealand. Low-flow characteristics and their potential impacts on availability for water abstraction and for support of in-stream habitat values were focused on. The results showed that the annual total water yields, quick flow and baseflow decreased moderately in the two scenarios when compared with the current actual land use. The annual water balance for the pine potential land cover scenario did not differ substantially from the prehistoric scenario for the catchment as whole. However, there were more notable differences among individual tributary catchments, which could be attributed to the relative area of land cover altered and location of those catchments. Simulated low flows for the prehistoric and potential pine land cover scenarios were both significantly lower than the low flows for the current land use. In summary, under the current land use conditions, both annual water yield and low flow are higher than was the case before human intervention in the area or in a maximum commercial reforestation scenario. [ABSTRACT FROM AUTHOR]

Climate change affects natural systems, leading to increased acceleration of global water cycle and substantial impacts on the productivity of tropical rivers and the several ecosystem functions they provide. However, the anticipated impacts of climate change in terms of frequency and intensity of extreme events (e.g., droughts and floods) on hydrological systems across regions could be substantially different. This study therefore aims to assess the impacts of climate change on the streamflow of a large river basin located in central Australia (Cooper Creek-Bulloo River Basin). Modified version of the hydrological model Hydrologiska Byråns Vattenbalansavdelning (HBV) was used in this study to generate daily streamflow. This model was first calibrated (2001–2010) and then validated for two independent periods (1993–1997 and 2011–2015). The model depicted a good performance in simulating observed streamflow. Climate projection data from multiple general circulation models (ACCESS1.0, CanESM2, CESM1-CAM5, CNRM-CM5, GFDL-ESM2M, HadGEM2-CC, MIROC5, NorESM1-M, ACCESS1-0, ACCESS1-3, CCSM4, CNRM-CM5, CSIRO-Mk3.6, GFDL-CM3, GFDL-ESM2M, HadGEM2, MIROC5, MPI-ESM-LR, and NorESM1-M) in various forms (raw, statistically downscaled, dynamically downscaled, and bias adjusted) were considered in this study. Results showed that three high resolution dynamically downscaled and bias adjusted models (ACCESS1-3, CNRM-CM5, and MPI-ESM-LR) from Terrestrial Ecosystem Research Network (TERN) dataset (v1.0.2) have better performance than other models considered, that is, in terms of capturing observed precipitation over the basin. Future climate projections of ensemble of these three models forced with RCP 4.5 and RCP 8.5 emission scenarios were then used to generate streamflow for 2050s (2040–2069) and 2080s (2070–2099). Results of the study indicated that mean annual precipitation was projected to decrease by up to −8% in 2050s and temperature was projected to increase by up to 4.66 °C in 2080s under the average and extreme emission scenarios, respectively. Mean annual, mean seasonal (December–February, March–May, June–August, September–November), and mean monthly streamflow were projected to decrease under different emission scenarios in 2050s and 2080s. These results indicate decreased water availability in the future as well as water cycle intensification. These changes in streamflow might have impacts on agriculture, natural ecosystem, and could lead to water restrictions. The outcome of this study can directly feed into frameworks for sustainable management of water resources and support adaptation strategies that rely on science and policy to improve water resources allocation in the region. [ABSTRACT FROM AUTHOR]