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Water use efficiency (WUE) is a central parameter for linking carbon and water exchange processes in terrestrial ecosystems. The Beijing-Tianjin Sand Source Region (BTSSR) in China has undergone tremendous vegetation restoration and climate change. Understanding the WUE responses to climate change and human activity and their relative contributions to the trends and inter-annual variations (IAVs) in WUE is necessary to improve water use efficiency and strengthen water resource management. The evapotranspiration (ET) dataset based on the model tree ensemble (MTE) algorithm which was a machine learning approach using flux-tower ET measurements and the GLASS GPP dataset, as well as the variance decomposition method, were used to analyze the spatiotemporal changes in water use efficiency and inherent water use efficiency (IWUE) and the impacts of climate change and human activities. The results showed that the annual WUE and IWUE exhibited significantly increase in most regions of the BTSSR. The trend of human activity played the most important role in the increases of WUE and IWUE, with relative contributions of 88.2% and 85.9%, respectively, followed by the IAV of human activity for WUE (6.1%) and the trend of climate change (8.7%) for IWUE. The contribution of IAV to climate change was relatively small. Moreover, WUE and IWUE were all positively correlated with precipitation and temperature in most regions. Our results indicated that ecological restoration projects had significantly improved water use efficiency in BTSSR and may decrease the water burden in the BTSSR., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Vegetation responses to rising atmospheric CO2 levels can significantly affect water availability (defined as precipitation minus evapotranspiration (ET)). While this effect has long been recognized and assessed for the mean state, its influence on interannual variability, which is more closely associated with extreme events, has yet to be comprehensively quantified. In this study, our primary focus is to evaluate the impacts of ET by plant physiology (denoted as ETPhy) on the mean and interannual variability of water availability under elevated CO2 using multiple CO2 sensitivity experiments from the coupled model intercomparison project phase 6. We show that the contribution of vegetation physiological effects to the mean water availability varies among regions, while it consistently contributes to variability by about 33%. Considering CO2 physiological effects alone, ETPhy exerts a more significant influence on the mean state than on variability, particularly in humid regions. Consequently, ETPhy contributes less than 5% to the variability of water availability in humid regions under rising CO2, whereas it accounts for about 20% of the mean state. This distinction could be attributed to the different mechanisms governing the mean and variability of ETPhy. Specifically, evaporation from CO2 physiological forcing is the most critical contributor to the variability of ETPhy in most regions while showing minimal impacts on the mean state. Our findings identify the divergent effects of ETPhy on the mean state and interannual variability of water availability under elevated CO2, as important in future climate projections. Plain Language Summary: Rising atmospheric CO2 concentrations impacts on vegetation physiological processes are expected to change the evapotranspiration (ET) by plant physiology (denoted as ETPhy), allowing for altering the mean and the interannual variability of water availability (defined as precipitation minus ET). Using an ensemble of Earth system model simulations, this study reveals that the impacts of CO2 physiological effects on the mean of water availability are region‐specific, but with a nearly consistent contribution to its interannual variability. Combined with the impact of ETPhy on the mean and variability of water availability resulting from CO2 physiological forcing, ETPhy typically contributes less than 5% to the variability of water availability in humid regions under rising CO2, whereas it accounts for about 20% of the mean state. This discrepancy could arise from the different mechanisms governing the mean and variability of ETPhy. These results underscore the importance of accounting for the effects of ETPhy in response to increasing CO2 in altering the mean and the interannual variability of regional water resources. Key Points: The effects of CO2 vegetation responses contribute variably in space to mean annual water availability but uniformly to variabilityThe evapotranspiration (ET) from CO2 physiological forcing (ETPhy) shows more impacts on the mean of water availability than its variabilityThe evaporation from CO2 physiological forcing plays an important role in the variability of ETPhy but has weak effects on the mean [ABSTRACT FROM AUTHOR]

The most basic features of climatological normals and variability are useful for describing observed or likely future climate fluctuations. Pan evaporation (Epan) is an important indicator of climate change; however, current research on Epan has focused on its change in mean rather than its variability. The variability of monthly Epan from 1961 to 2020 at 969 stations in China was analyzed using a theoretical framework that can distinguish changes in Epan variance between space and time. The Epan variance was decomposed into spatial and temporal components, and the temporal component was further decomposed into interannual and intra-annual components. The results show that the variance in Epan was mainly controlled by the temporal component. The time variance was mainly controlled by intra-annual variance, decreasing continuously in the first 30 years, and slightly increasing after the 1990s. This is mainly due to the fact that the decrease of wind speed and the increase of water vapor pressure deficit with the temperature increase offset each other and inhibit the variability of Epan. The variance decreased more in the northern region, whereas it exhibited a small decrease or slight increase in the southern region. The reduction in seasonality was dominated by spring, followed by summer. The differences in Epan variability in space and season were mainly caused by the differing rates of change in evaporation driving forces, such as a greater reduction in wind speed in the northern region and spring. Significance Statement: The purpose of this study is to better understand how the variability of evaporation changes rather than in mean under climate change. This is important because the variability is useful to describe the observed or likely future fluctuations, and a small fluctuation may have large impacts on water practices, such as agricultural production. Our findings showed that the temporal and spatial variability of evaporation decreased due to its drivers offsetting each other. However, because the drivers are numerous and continuously changing under climate change, it is necessary to pay attention to its mean and variability for serving water resources practice. [ABSTRACT FROM AUTHOR]

Based on the large ensembles of the half a degree additional warming, prognosis, and projected impacts historical, +1.5 and +2 °C experiments, we quantify changes in the magnitude of water availability (i.e., precipitation minus actual evapotranspiration; a function of monthly precipitation flux, latent heat flux, and surface air temperature) below normal conditions (less than median, e.g., 20th percentile water availability). We found that, relative to the historical experiment, water availability below normal conditions of the +1.5 and +2 °C experiments would decrease in the midlatitudes and the tropics, indicating that hydrological drought is likely to increase in warmer worlds. These cause more (less) people in East Asia, Central Europe, South Asia, and Southeast Asia (West Africa and Alaska/Northwest Canada) to be exposed to water shortage. Stabilizing warming at 1.5 °C instead of 2 °C would limit population impact in most of the regions, less effective in Alaska/Northwest Canada, Southeast Asia, and Amazon. Globally, this reduced population impact is ~117 million people. Plain Language Summary: This study emerges from the lack of scientific investigations to inform climate policy about differences between two global warming targets (i.e., 1.5 and 2 °C) for the "Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5°C." We seek to understand the following: How would water availability below normal conditions (the drier end of hydrological extremes) change at these targets? How would they affect the water shortage of human society? Could we limit the impact by stabilizing the global warming at 1.5 °C instead of 2 °C? To address these questions, we employ the HAPPI (half a degree additional warming, prognosis, and projected impacts) experiments, explicitly designed to differentiate impacts between these targets. Relative to the historical period, future water availability below normal conditions (less than median, e.g., 20th percentile or lower) would decrease in the midlatitudes and the tropics; the globe and most of the regions would endure water shortages. Relative to the 2 °C warming target, stabilizing temperature increase at 1.5 °C would constrain adverse impact on people suffering water shortages in most of the regions (particularly Central Europe, East Africa, East Asia, South Asia, and West Africa) but ineffective in Alaska/Northwest Canada, Southeast Asia, and Amazon. A global sum of this reduced risk is ~117 million people. Key Points: The first evaluation of global freshwater availability below normal conditions in +1.5 versus +2 °C worlds is conductedRelative to the historical period, future water availability below normal conditions would decrease in the midlatitudes and the tropicsRisk of freshwater shortages in a +1.5 °C world is less than a +2 °C world [ABSTRACT FROM AUTHOR]

Highlights • The ED approach can make robust improvement for the PD method. • The ED approach can better identify and quantify the contribution of meteorological observations. • The wind function: f q (u 2) = 3.977 × 10 - 8 (1 + 0.505 u 2) is recommended in the PenPan model in D20 E pan estimation. • The ED approach can be used in attribution analysis in complicated hydrological models. Abstract In predicting how droughts and hydrological cycles might change in a changing climate, change of pan evaporation (E pan) is one crucial element to be understood. The derived partial differential (PD) form of the PenPan equation is a prevailing attribution approach worldwide. However, small biases exist and the application of PD method is limited within the derivation of partial differential form of the equation, which impede the attribution analysis in hydrology. Here we designed a series of numerical experiments by detrending each climatic variable, i.e., an experimental detrending (ED) approach, to attribute changes of E pan over China. We compared the attribution results using these two methods and further analyzed the plausible advantages of ED approach. The comparison shows that both ED approach and PD method perform well in attributing changes of E pan and to the input meteorological variables in China over 1960–2017. The first advantage of ED approach is that it can help make robust adjustment for the PD method in attribution analysis. Another advantage lies in its ability to attribute to the observed meteorological variables in China, when the PD method fails to quantify the contribution of relative humidity and air temperature in net radiation in E pan attribution analysis. We highlight that the ED approach is recommended in attribution analysis for hydrologic research. Together with the adjusted PD method, both methods can assist a better understanding and prediction of water-energy cycles change in a changing climate. [ABSTRACT FROM AUTHOR]

Climate change and its potential threats on water security call for reliable predictions of evapotranspiration (ET) and runoff Q at different time scales, but current knowledge of the differences in their predictability between humid and nonhumid regions is limited.Based on spatially distributed catchments inChina, the authors characterized their predictability and provided plausible explanations. Using the Budyko framework, it was confirmed that annualET is predictable in nonhumid regions but less predictable in humid regions, and annualQ is predictable in humid regions but less reliable in nonhumid regions. Themain cause of the varied predictability lies in the variation of water storage change DS in the water balance equation. It affects both the estimation and the variability ofQin nonhumid catchmentsmore than that in humid catchments, which increases the challenge of predicting annual Q in nonhumid regions, while the opposite effect occurs in annual ET prediction between humid and nonhumid catchments.Moreover, the differences between the controlling factors of ET variability in different regions add more differences in their predictability. The dominant control of precipitation makes it easy to predict annual ET in nonhumid regions. By contrast, precipitation, potential evaporation, and their covariance take considerable effort to determine annualETvariations,which leads to less reliableETestimation and predictability in humid catchments. Therefore, one can accurately predict annual ET in nonhumid catchments andQ in humid catchments based on commonly used hydrological models.With proper consideration of ΔS, the predictability of annual ET and Q in both humid and nonhumid catchments can be improved. [ABSTRACT FROM AUTHOR]

Abstract: From the water balance analysis using Eagleson''s ecohydrological model in the non-humid regions of China, the allocation of evpotranspiration flux among canopy transpiration, surface interception, and bare soil evaporation is analyzed and the quantitative relationship between the vegetation and soil moisture is derived. It implies that change of vegetation coverage due to climate variation and human activity can be potentially predicted through the soil moisture change. Combination of the Budyko''s water-energy balance model and the Eagleson''s ecohydrological model is used for predicting the impact of climate and vegetation changes on the catchment water balance, which shows that the combined model has the capacity of capturing the regional soil-vegetation-atmosphere interaction, and is useful for assessing the long-term effect of vegetation changes on catchment hydrology, and hence it has implications to the water resources management in the non-humid regions of China. [Copyright &y& Elsevier]

• The annual precipitation remained stationary during 1958–2018 over the basin. • The annual water level and discharger remained stationary for period 1890–1970. • We analyzed the hydrological regime of the Changjiang River during the past 100. • The water level and discharge were in perfect synchronization prior to 1980. • About 50% discharge change in flood and non-flood seasons was attributed to TGD. The assumption that natural system changes in a constant range runs through the practice of water resources application. However, under the changing environment, whether hydrological characteristics have changed is a crucial and important foundation for flood control and drought resistance. Based on daily water level, discharge, and precipitation for the last 100 or 60 years, this study analyzed the hydrological characteristics and their changes as results of the operation of the Three Gorges Dam (TGD) in the Changjiang (Yangtze) River. Results showed that the monthly precipitation did not significantly change for the longer period and the annual precipitation remained stationary during 1959–2018. However, the water level and discharge significantly increased during most months from January to March and significantly decreased from August to November. Yichang station, the nearest station to the TGD, changed the most, which annual runoff and water level series (1959–2018) showed obvious non-stationarity behavior. The same results were showed in period 1890–2018, while the process remained stationary for the period 1890–1970. 50.15% of the discharge at Yichang station decrease from June to November was attributed to water storage of the TGD and 57.57% of its increase during other months was attributed to recharge of the TGD. The TGD had a greater impact on the discharge, while the water level was affected by both the TGD and Gezhouba Dam. The trends of the water level and discharge were in perfect synchronization prior to 1980, but went out of sync in terms of a different direction or rate after 1980 given the operation of Gezhouba Dam. The non-stationarity behavior of discharge and water level and the change of their laws make it difficult to directly apply the relationship among precipitation-discharge-water level in the past, which brings great challenges to the planning and management of water resources. [ABSTRACT FROM AUTHOR]

Enzymatic hydrolysis is a critical process for the cellulase-mediated lignocellulosic biorefinery to produce sugar syrups that can be converted into a whole range of biofuels and biochemicals. Such a process operating at high-solid loadings (i.e., scarcely any free water or roughly ≥ 15% solids, w/w) is considered more economically feasible, as it can generate a high sugar concentration at low operation and capital costs. However, this approach remains restricted and incurs "high-solid effects", ultimately causing the lower hydrolysis yields with increasing solid loadings. The lack of available water leads to a highly viscous system with impaired mixing that exhibits strong transfer resistance and reaction limitation imposed on enzyme action. Evidently, high-solid enzymatic hydrolysis involves multi-scale mass transfer and multi-phase enzyme reaction, and thus requires a synergistic perspective of transfer and biotransformation to assess the interactions among water, biomass components, and cellulase enzymes. Porous particle characteristics of biomass and its interface properties determine the water form and distribution state surrounding the particles, which are summarized in this review aiming to identify the water-driven multi-scale/multi-phase bioprocesses. Further aided by the cognition of rheological behavior of biomass slurry, solute transfer theories, and enzyme kinetics, the coupling effects of flow-transfer-reaction are revealed under high-solid conditions. Based on the above basic features, this review lucidly explains the causes of high-solid hydrolysis hindrances, highlights the mismatched issues between transfer and reaction, and more importantly, presents the advanced strategies for transfer and reaction enhancements from the viewpoint of process optimization, reactor design, as well as enzyme/auxiliary additive customization. • High-solid enzyme hydrolysis includes multi-scale transfer and multi-phase reaction. • Porous particle properties determine water distribution and enzyme/solute transfer. • Particle rheological behavior link enzyme reaction with transfer. • Deficient water availability is a key limitation to enzyme action in viscous system. • Cellulase customization with auxiliary enzyme/additive benefits the mass transfer. [ABSTRACT FROM AUTHOR]

• The paired-catchment approach was improved and successfully utilized to detect reservoir impacts on flow regime alteration. • Reservoirs primarily homogenize temporal runoff variations, reshape flow duration curve, and reduce runoff sensitivity to precipitation. • The extent of flow regime alteration caused by reservoirs varies depending on their functionalities and regulatory capacities. The 20th century witnessed a substantial surge in global reservoir construction, representing a significant milestone and resulting in a profound anthropogenic alteration of flow regimes. Despite this surge, a comprehensive large-sample quantification of reservoir impacts on flow regime alteration has been impeded by limitations in statistical and modeling methodologies. Here we present an improved paired-catchment method tailored to discern the impact of reservoirs on flow regime within the context of climate change. Employing this method, we identified and analyzed 177 paired catchments from a selection of over 12,000 global catchments. Our findings highlight the effectiveness of the improved method, demonstrating that reservoirs predominantly alleviate high flows while enhancing low flows. This serves to moderate the annual periodicity of streamflow signals while amplifying their low-frequency components. Furthermore, predetermined water releases play a significant role in diminishing the seasonality of low flows and reducing the sensitivity of runoff to precipitation. Additionally, the extent of flow regime alteration caused by reservoirs is closely linked to their functionalities and regulatory capacity. For example, reservoirs designed for irrigation and water supply show minimal enhancement of low flow values, while the magnitude of reservoir-induced changes rises with higher regulatory capacity. The improved method can be further employed in the future to investigate the impact of reservoirs on extreme hydrological events such as droughts and floods. It emerges as a crucial scientific tool for effective water resource management and the mitigation of water-related disasters in the Anthropocene era. [ABSTRACT FROM AUTHOR]