Magnitude and robustness associated with the climate change impacts on global hydrological variables for transient and stabilized climate states

Recent studies have assessed the impacts of climate change at specific global temperature targets using relatively short (30 year ) transient time-slice periods which are characterized by a steady increase in global mean temperature with time. The Inter-Sectoral Impacts Model Intercomparison Project Phase 2b (ISIMIP2b) provides trend-preserving bias-corrected climate model datasets over six centuries for four global climate models (GCMs) which therefore can be used to evaluate the potential effects of using time-slice periods from stabilized climate state rather than time-slice periods from transient climate state on climate change impacts. Using the H08 global hydrological model, the impacts of climate change, quantified as the deviation from the pre-industrial era, and the signal-to-noise (SN) ratios were computed for five hydrological variables, namely evapotranspiration (EVA), precipitation (PCP), snow water equivalent (SNW), surface temperature (TAR), and total discharge (TOQ) over 20 regions comprising the global land area. A significant difference in EVA for the transient and stabilized climate states was systematically detected for all four GCMs. In addition, three out of the four GCMs indicated that significant differences in PCP, TAR, and TOQ for the transient and stabilized climate states could also be detected over a small fraction of the globe. For most regions, the impacts of climate change toward EVA, PCP, and TOQ are indicated to be underestimated using the transient climate state simulations. The transient climate state was also identified to underestimate the SN ratios compared to the stabilized climate state. For both the global and regional scales, however, there was no indication that surface areas associated with the different classes of SN ratios changed depending on the two climate states (t-test, p > 0.01). Transient time slices may be considered a good approximation of the stabilized climate state, for large-scale hydrological studies and many regions and variables, as: (1) impacts of climate change were only significantly different from those of the stabilized climate state for a small fraction of the globe, and (2) these differences were not indicated to alter the robustness of the impacts of climate change

Historical and future changes in global flood magnitude - evidence from a model-observation investigation

To improve the understanding of trends in extreme flows related to flood events at the global scale, historical and future changes of annual maxima of 7 d streamflow are investigated, using a comprehensive streamflow archive and six global hydrological models. The models' capacity to characterise trends in annual maxima of 7 d streamflow at the continental and global scale is evaluated across 3666 river gauge locations over the period from 1971 to 2005, focusing on four aspects of trends: (i) mean, (ii) standard deviation, (iii) percentage of locations showing significant trends and (iv) spatial pattern. Compared to observed trends, simulated trends driven by observed climate forcing generally have a higher mean, lower spread and a similar percentage of locations showing significant trends. Models show a low to moderate capacity to simulate spatial patterns of historical trends, with approximately only from 12 % to 25 % of the spatial variance of observed trends across all gauge stations accounted for by the simulations. Interestingly, there are statistically significant differences between trends simulated by global hydrological models (GHMs) forced with observational climate and by those forced by bias-corrected climate model output during the historical period, suggesting the important role of the stochastic natural (decadal, inter-annual) climate variability. Significant differences were found in simulated flood trends when averaged only at gauged locations compared to those averaged across all simulated grid cells, highlighting the potential for bias toward well-observed regions in our understanding of changes in floods. Future climate projections (simulated under the RCP2.6 and RCP6.0 greenhouse gas concentration scenarios) suggest a potentially high level of change in individual regions, with up to 35 % of cells showing a statistically significant trend (increase or decrease; at 10 % significance level) and greater changes indicated for the higher concentration pathway. Importantly, the observed streamflow database under-samples the percentage of locations consistently projected with increased flood hazards under the RCP6.0 greenhouse gas concentration scenario by more than an order of magnitude (0.9 % compared to 11.7 %). This finding indicates a highly uncertain future for both flood-prone communities and decision makers in the context of climate change. © Author(s) 2020

Validity of estimating flood and drought characteristics under equilibrium climates from transient simulations

Future flood and drought risks have been predicted to transition from moderate to high levels at global warmings of 1.5 °C and 2.0 °C above pre-industrial levels, respectively. However, these results were obtained by approximating the equilibrium climate using transient simulations with steadily warming. This approach was recently criticised due to the warmer global land temperature and higher mean precipitation intensities of the transient climate in comparison with the equilibrium climate. Therefore, it is unclear whether floods and droughts projected under a transient climate can be systematically substituted for those occurring in an equilibrated climate. Here, by employing a large ensemble of global hydrological models (HMs) forced by global climate models, we assess the validity of estimating flood and drought characteristics under equilibrium climates from transient simulations. Differences in flood characteristics under transient and equilibrium climates could be largely ascribed to natural variability, indicating that the floods derived from a transient climate reasonably approximate the floods expected in an equally warm, equilibrated climate. By contrast, significant differences in drought intensity between transient and equilibrium climates were detected over a larger global land area than expected from natural variability. Despite the large differences among HMs in representing the low streamflow regime, we found that the drought intensities occurring under a transient climate may not validly represent the intensities in an equally warm equilibrated climate for approximately 6.7% of the global land area

Understanding each other's models: a standard representation of global water models to support improvement, intercomparison, and communication

Global water models (GWMs) simulate the terrestrial water cycle, on the global scale, and are used to assess the impacts of climate change on freshwater systems. GWMs are developed within different modeling frameworks and consider different underlying hydrological processes, leading to varied model structures. Furthermore, the equations used to describe various processes take different forms and are generally accessible only from within the individual model codes. These factors have hindered a holistic and detailed understanding of how different models operate, yet such an understanding is crucial for explaining the results of model evaluation studies, understanding inter-model differences in their simulations, and identifying areas for future model development. This study provides a comprehensive overview of how state-of-the-art GWMs are designed. We analyze water storage compartments, water flows, and human water use sectors included in 16 GWMs that provide simulations for the Inter-Sectoral Impact Model Intercomparison Project phase 2b (ISIMIP2b). We develop a standard writing style for the model equations to further enhance model improvement, intercomparison, and communication. In this study, WaterGAP2 used the highest number of water storage compartments, 11, and CWatM used 10 compartments. Seven models used six compartments, while three models (JULES-W1, Mac-PDM.20, and VIC) used the lowest number, three compartments. WaterGAP2 simulates five human water use sectors, while four models (CLM4.5, CLM5.0, LPJmL, and MPI-HM) simulate only water used by humans for the irrigation sector. We conclude that even though hydrologic processes are often based on similar equations, in the end, these equations have been adjusted or have used different values for specific parameters or specific variables. Our results highlight that the predictive uncertainty of GWMs can be reduced through improvements of the existing hydrologic processes, implementation of new processes in the models, and high-quality input data

Analysis of parameter uncertainty and sensitivity in PCPF-1 modeling for predicting concentrations of rice herbicides

This paper demonstrates the procedures for probabilistic assessment of a pesticide fate and transport model, PCPF-1, to elucidate the modeling uncertainty using the Monte Carlo technique. Sensitivity analyses are performed to investigate the influence of herbicide characteristics and related soil properties on model outputs using four popular rice herbicides: mefenacet, pretilachlor, bensulfuron-methyl and imazosulfuron. Uncertainty quantification showed that the simulated concentrations in paddy water varied more than those of paddy soil. This tendency decreased as the simulation proceeded to a later period but remained important for herbicides having either high solubility or a high 1st-order dissolution rate. The sensitivity analysis indicated that PCPF-1 parameters requiring careful determination are primarily those involve with herbicide adsorption (the organic carbon content, the bulk density and the volumetric saturated water content), secondary parameters related with herbicide mass distribution between paddy water and soil (1st-order desorption and dissolution rates) and lastly, those involving herbicide degradations

Polar PAC (Polycyclic Aromatic Compound) mobilization and transfer in water in coal tar contaminated soils

International audienceDue to the coal transformation activities occurring between the 18th and 20th centuries, about 200 000 sites in Europe have been diagnosed nowadays as polluted by polycyclic aromatic hydrocarbons (PAHs). Because of their toxicity, mutagenic and carcinogenic properties, 16 PAHs, listed as priority pollutants by the US EPA, are regulated and measured for site diagnosis or monitoring during remediation treatment. However, associated to PAHs, polar PACs (OPACs and N-PACs) occur and could induce a risk for water resources. O-PACs and N-PACs are present in the initial pollution (coal tars) as PAHs but can also be produced during remediation treatments or during natural attenuation processes. Moreover, recent studies revealed that O-PACs and N-PACs (i) are leached in higher proportion than PAHs and (ii) can be equally or more toxic than corresponding parent PAHs. All these reasons imply increasing of our knowledge on the mechanisms involved in polar PAC mobilization and transfer to water, especially by comparison to PAHs. In fine, should polar PACs be included in coal tar contaminated site diagnosis in order to improve risk evaluation? In this project, the mobilization and transfer of polar PACs and PAHs were assessed via leaching experiments of aged contaminated soils from two former coking plants (Lorraine, France), which have undergone aging processes. A multiscale approach was used: batch (50 g), laboratory column (2 kg) and pilot scale column (2 m3 lysimeter). In a first step, PAC mobilization was investigated at laboratory scale under various conditions of temperature, ionic strength, PAC availability and water flow rate. PAC transfer was also assessed in situ with a lysimeter column. Dissolved Organic Carbon (DOC) and dissolved PAC concentrations (including 16 PAHs, 11 O-PACs and 5 N-PACs) were measured. Batch experiments, carried out on soils with a high available contamination, revealed that PAC (LMW PAH and polar PAC) mobilization was mainly controlled by dissolution processes according to the Raoult law. For aged soils (low pollutant availability), the mobilization of PACs was lower, especially for LMW-PAHs and polar PACs. In both case, HMWPAHs were mainly released in association with colloids. The temperature increased the mobilization of all PACs while an increase in ionic strength caused a decrease in all PAC mobilization, with a lower impact on polar PACs compared to PAHs. Whatever the column scale (laboratory and field lysimeter), results tend to show a higher release of polar PACs. Polar PACs exhibited intermediate behavior between PAHs (hydrophobic compounds) and DOC. These first results showed that (i) the pollution availability is a fundamental parameter for PAC transfer, and (ii) that polar PACs seem to behave similarly to PAHs but with a higher mobilization and transfer rate. All these findings underline the need to include polar PACs in diagnosis and monitoring of potentially contaminated sites

Magnitude and robustness associated with the climate change impacts on global hydrological variables for transient and stabilized climate states

Recent studies have assessed the impacts of climate change at specific global temperature targets using relatively short (30 year ) transient time-slice periods which are characterized by a steady increase in global mean temperature with time. The Inter-Sectoral Impacts Model Intercomparison Project Phase 2b (ISIMIP2b) provides trend-preserving bias-corrected climate model datasets over six centuries for four global climate models (GCMs) which therefore can be used to evaluate the potential effects of using time-slice periods from stabilized climate state rather than time-slice periods from transient climate state on climate change impacts. Using the H08 global hydrological model, the impacts of climate change, quantified as the deviation from the pre-industrial era, and the signal-to-noise (SN) ratios were computed for five hydrological variables, namely evapotranspiration (EVA), precipitation (PCP), snow water equivalent (SNW), surface temperature (TAR), and total discharge (TOQ) over 20 regions comprising the global land area. A significant difference in EVA for the transient and stabilized climate states was systematically detected for all four GCMs. In addition, three out of the four GCMs indicated that significant differences in PCP, TAR, and TOQ for the transient and stabilized climate states could also be detected over a small fraction of the globe. For most regions, the impacts of climate change toward EVA, PCP, and TOQ are indicated to be underestimated using the transient climate state simulations. The transient climate state was also identified to underestimate the SN ratios compared to the stabilized climate state. For both the global and regional scales, however, there was no indication that surface areas associated with the different classes of SN ratios changed depending on the two climate states (t-test, p > 0.01). Transient time slices may be considered a good approximation of the stabilized climate state, for large-scale hydrological studies and many regions and variables, as: (1) impacts of climate change were only significantly different from those of the stabilized climate state for a small fraction of the globe, and (2) these differences were not indicated to alter the robustness of the impacts of climate change

Delivering the latest global water resource simulation results to the public

Increasing demands from governments and businesses for accurate and actionable information regarding water security have led to the development of various interactive websites that provide water risk indicators. However, these websites often lack adequate descriptions of the sources and limitations of their datasets and failure to take any uncertainty associated with water risk indicators into account can have serious consequences. Furthermore, websites that only provide data for specific time points, such as mid-century and end-of-century, may be less useful. To address these limitations, we present the H08 Water Risk Tool, a scientifically rigorous platform designed to assess historical and future water resources.The H08 Water Risk Tool incorporates advanced hydrological simulations that utilize the latest modeling techniques, input data, and parameter calibration methods. Our validation indicates that the temporal variation and spatial distribution characteristics of the historical water risk indicators produced by the H08 Water Risk Tool agree with previous observations. This platform provides the public with high-quality water risk indicators, as well as the tools necessary to interpret these indicators. To ensure complete transparency, the H08 Water Risk Tool uses open-source tools and provides detailed information on its methodology and assumptions.Despite the ongoing Water Action Decade initiative launched by the United Nations, avoiding a global water crisis will require considerable work. The H08 Water Risk Tool will be a defining asset providing comprehensive and realistic assessments of water scarcity, both current and future, as well as helping to prioritize adjustments to climate change

Parameters controlling oxygenated polycyclic aromatic compounds mobilization in pah contaminated soils

International audienceIn PAH-contaminated sites, the 16 US EPA PAHs are the only compounds considered in the frame of site characterization and monitoring. However, O-PACs (oxygenated polycyclic aromatic compounds) are associated with PAHs and occur in high concentration on these sites (in the range of 10 to 50% of the total PAH level). They are emitted from the same sources as PAHs but can also be formed by PAH degradations during natural attenuation and remediation. These O-PACs are (i) characterized by a higher polarity and consequently, a potential higher mobility in the environment than PAHs, and (ii) can exhibit a higher toxicity. That is why, this project aims to study parameters controlling O-PAC mobilization and transfer in soils to better evaluate their potential risk for groundwater resources and define if O-PACs should be included in site characterization or monitoring. In this work, O-PAC mobilization from two PAH contaminated soils sampled at former coking plant site is assessed by means of a Design Of batch lixiviation Experiments (DOE) under various conditions (temperature, ionic strength, pollutant availability, pH and natural organic matter). Dissolved organic carbon (DOC) and PAC concentrations (including 16 PAHs and 11 O-PACs) are measured after SPE extraction of the water samples. Column-leaching experiments will then be carried out under saturated and unsaturated conditions to assess O-PAC transfer to uncontaminated soils. In addition to these laboratory-controlled experiments, in situ experiments are carried out to assess PAC release and transfer in a lysimeter column and groundwater is regularly sampled on a pilot site

Aging as the main factor controlling PAH and polar-PAC (polycyclic aromatic compound) release mechanisms in historically coal-tar-contaminated soils

International audienceIn industrial sites, historically contaminated by coal tar (abandoned coking and manufactured gas plants), other families of organic pollutants than the 16 PAHs (polycyclic aromatic hydrocarbons) classified by the US-EPA can occur and induce potential risk for groundwater resources. Polar PACs (polycyclic aromatic compounds), especially oxygenated and nitrogenated PACs (O-PACs and N-PACs), are present in the initial pollution and can also be generated over time (i.e., O-PACs). Their aqueous solubilities are much greater than those of the PAHs. For these reasons, we need to increase our knowledge on polar PACs in order to better predict their behavior and the potential on-site risk. Batch leaching tests were carried out under various conditions of temperature, ionic strength, and availability of pollutants to determine the mechanisms and key parameters controlling their release. The results show a release of low-molecularweight PAHs and polar PACs mainly by dissolution, while higher molecular weight PAHs are mainly released in association with colloids. Aging mainly controls the former mechanism, and ionic strength mainly controls the latter. Temperature increased both dissolution and colloidal mobilization. The Raoult law predicts the PAC equilibrium concentration for soils presenting high pollutant availability, but this law overestimates PAC concentration in aged soils (low pollutant availability). This is mainly due to limitation of PAC diffusion within coal-tar particles with aging. The most soluble PACs (especially polar PACs) are the most sensitive to aging. For better prediction of the PAC behavior in soils and water resources management, aging needs to be taken into account