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• Biome-BGC model was improved by modifying the ET and canopy multilayers modules. • The increasing trends of GPP, ET and WUE were more pronounced during 1999–2018. • Increasing NDVI mitigated the negative effect of precipitation. Extreme climate occurred frequently in subtropical region, which seriously affects carbon and water fluxes such as evapotranspiration (ET) and gross primary productivity (GPP) of terrestrial ecosystems. The process-based biome biogeochemical cycles (Biome-BGC) model is widely used for simulating carbon and water fluxes of forest ecosystems. However, the lack of the interaction information of climate, vegetation and soil, such as the hysteresis effect, canopy stratification on photosynthesis, impedes better simulations of the ecohydrological processes. Here, we tended to improve the simulation accuracy of Biome-BGC model at a subtropical forest on the Xin'an River in Southeastern China by reconstructing the precipitation series, modifying the ET and canopy multilayers modules, and optimizing the parameters. The spatiotemporal patterns of GPP, ET, water use efficiency (WUE) and their response to environmental factors across the Xin'an River Basin from 1982 to 2018 were further explored. The results showed that the improved model performed well, with the determination coefficient, root means square error and mean absolute error being 0.730, 1.522 gC/m2/d and 1.218 gC/m2/d for GPP, 0.857, 1.082 mm/d and 0.838 mm/d for ET, respectively. Basin-averaged GPP, ET and WUE increased during 1982–2018 and these increasing trends were more pronounced during 1999–2018. Significant positive trends of WUE occurred in the northeast corner. The increasing air temperature and precipitation respectively dominated the increase in GPP and ET, the increasing CO 2 concentration and NDVI mitigated the negative effect of extreme precipitation on WUE. Given that human activities such as afforestation have effectively reduced the extent of damage to forest ecosystems from extreme precipitation, we highlight an urgent need to formulate adaptation strategies aimed at reducing the risk of extreme climate in humid regions. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Water storage change occupies a large proportion in the precipitation partitioning at monthly scale. • Estimate monthly evapotranspiration by combining GRACE data into extended Budyko framework. • Budyko-E represents GRACE-E in arid regions comparatively better than it in humid regions. • Budyko monthly evapotranspiration outperforms four other global evapotranspiration products. Abstract The estimates of actual evapotranspiration (E) across basins and/or regions are still facing some difficulties because of the complex interactions amongst the components of the land-plant-atmosphere system, even though the basic physical mechanism of E is well understood. Presenting as a nonlinear relationship constrained by physical limits, the Budyko framework serves as a powerful tool used to estimate the averaged E at long-term scale. Given the ability of Gravity Recovery And Climate Experiment (GRACE) in effectively simulating the terrestrial water storage change (Δ TWS) at monthly scale, a model to estimate E was developed based on the Budyko framework with mean monthly parameter of multi-years, and was applied to the estimation of E for the 24 selected catchments in different climatic regions across China. Results indicate that for the majority of 24 catchments, the positive or negative trends in precipitation (P), potential evapotranspiration (E 0), runoff (R) and Δ TWS were not statistically significant during 2003–2013 at both annual and monthly scales, but the Δ TWS occupied a large proportion in the partitioning of P into R and E at monthly scale. The monthly Budyko parameter showed large variation within the year and also across these 24 catchments. The Budyko-modeled monthly E (Budyko-E) represented the GRACE-derived ones (GRACE-E) across these catchments well with both modeled E volumes and hydrograph shapes which were also consistent with GRACE-E series except some underestimation for peak E. Overall, the Budyko-E represented GRACE-E in the arid and semi-arid catchments comparatively better than it in the humid and semi-humid catchments due to the larger proportion of Δ TWS in the P partition. Among all the 24 selected catchments, the monthly Budyko-type E model represented the GRACE-E across Shixiali catchment (located in the Hai River basin) best, with the correlation coefficient (r), Nash-Sutcliffe coefficient (NSCE) and relative error (RE) between GRACE-E and simulated results being 0.996, 0.988, −0.033 in calibration and 0.994, 0.951, −0.111 in validation, respectively. Furthermore, using GRACE-E as the benchmark values, the Budyko-E outperformed four other global E products from the newly published Global Land Data Assimilation System with Noah Land Surface Model-2 (GLDAS_E), MODIS (MODIS_E), Japanese 25-year reanalysis product (JRA_E), and Zhang's method (Zhang_E). GLDAS_E was the best of four global E products at reproducing monthly variations, with relatively high r and NSCE values, small RE. While MODIS_E showed the poorest performance in reproducing the inter-annual E variations of these catchments, which was reflected by low r and NSCE values. This extended Budyko-E model can apply in other similar climatic regions and also may provide skill in evaluating the water resources since the preferable accuracy of E estimate. [ABSTRACT FROM AUTHOR]

It has become increasingly important to quantify carbon and water fluxes due to their roles in global warming and climate change, particularly for the agroecosystems. However, the dynamics of carbon and water fluxes have not been clearly recognized in rotation croplands with complex and changeable climate. Here, the variations of net ecosystem productivity (NEP), evapotranspiration (ET) and the water use efficiency (WUE, defined as NEP/ET), and their responses to the environmental factors were investigated in a wheat-maize rotation cropland across the Huaibei Plain of China over 2013–2015. The total average NEP, ET and average WUE respectively were 489.1 gC m-2, 315.8 kgH 2 O m-2, 4.4 gC kg-1H 2 O for wheat and 192.5 gC m-2, 249.0 kgH 2 O m-2, 3.6 gC kg-1H 2 O for maize, implying that wheat season sequestered more CO 2 than maize season. Spring drought and summer flood affected WUE of wheat and maize, respectively. Daily wheat WUE seemed to be more sensitive to changes in photosynthetically active radiation (PAR), vapor pressure deficit (VPD), soil water content (SWC) and canopy conductance (G c). PAR was the dominant factor controlling diurnal dynamics of NEP and ET, while the opposite effect of VPD on NEP was recognized. NEP increasing with PAR was limited by high VPD, which obviously when VPD exceeded 2 kPa during the maize season. Maize NEP was limited with VPD under high solar radiation (> 500 μmol m-2 s-1). WUEs of wheat and maize were negatively related to SWC and G c , and the sensitivity of WUE response to SWC and G c increased with the increase of PAR or VPD range. Sub-diurnal NEP against PAR, VPD or temperature showed clockwise hysteresis but ET against PAR or windspeed showed counter-clockwise hysteresis, and these hystereses were mainly caused by the interplay between evaporative demand and moisture supply, photosynthesis and carbon allocation of the agroecosystems. [Display omitted] • The wheat season sequestered more CO2 than the maize season. • Spring drought and summer flood affected WUE of wheat and maize, respectively. • Wheat WUE seemed to be more sensitive to changes in environmental factors. • PAR was the dominant factor controlling diurnal dynamics of NEP and ET. [ABSTRACT FROM AUTHOR]

This study established a climate elasticity method based on Budyko hypothesis and enhanced it by selecting the most effective Budyko-type formula to strengthen the runoff change prediction reliability. The spatiotemporal variations in hydrologic variables (i.e., runoff, precipitation and potential evaporation) during historical period were revealed first and the climate elasticities of runoff were investigated. The proposed climate elasticity method was also applied to project the spatiotemporal variations in future runoff and its key influencing factors in 35 watersheds across China. Wherein, the future climate series were retrieved by consulting the historical series, informed by four global climate models (GCMs) under representative concentration pathways from phase five of the Coupled Model Intercomparison Project. Wang-Tang equation was selected as the optimal Budyko-type equation for its best ability in reproducing the runoff change (with a coefficient of determination and mean absolute error of 0.998 and 1.36 mm, respectively). Observed runoff presents significant decreasing trends in the northern and increasing trends in the southern regions of China, and generally its change is identified to be more sensitive to climatic variables in Hai River Basin and lower Yellow River Basin. Compared to the runoff during the reference period, positive change rates in the north and negative change rates in the south of China in the mid-21st century can be practically generalized from the majority of GCMs projections. This maybe resulted from the increasing precipitation, especially in parts of northern basins. Meanwhile, GCMs project a consistently upward trend in potential evaporation although significant decreasing trends occur in the majority of catchments for the historical period. The results indicate that climate change will possibly bring some changes to the water resources over China in the mid-21st century and some countermeasures of water resources planning and management should be taken. [ABSTRACT FROM AUTHOR]

Quantifying precipitation (P) partition into evapotranspiration (E) and runoff (Q) is of great importance for global and regional water availability assessment. Budyko framework serves as a powerful tool to make simple and transparent estimation for the partition, using a single parameter, to characterize the shape of the Budyko curve for a “specific basin”, where the single parameter reflects the overall effect by not only climatic seasonality, catchment characteristics (e.g., soil, topography and vegetation) but also agricultural activities (e.g., cultivation and irrigation). At the regional scale, these influencing factors are interconnected, and the interactions between them can also affect the single parameter of Budyko-type equations’ estimating. Here we employ the multivariate adaptive regression splines (MARS) model to estimate the Budyko curve shape parameter ( n in the Choudhury’s equation, one form of the Budyko framework) of the selected 96 catchments across China using a data set of long-term averages for climatic seasonality, catchment characteristics and agricultural activities. Results show average storm depth (ASD), vegetation coverage (M), and seasonality index of precipitation (SI) are three statistically significant factors affecting the Budyko parameter. More importantly, four pairs of interactions are recognized by the MARS model as: The interaction between CA (percentage of cultivated land area to total catchment area) and ASD shows that the cultivation can weaken the reducing effect of high ASD (>46.78 mm) on the Budyko parameter estimating. Drought (represented by the value of Palmer drought severity index < -0.74) and uneven distribution of annual rainfall (represented by the value of coefficient of variation of precipitation > 0.23) tend to enhance the Budyko parameter reduction by large SI (>0.797). Low vegetation coverage (34.56%) is likely to intensify the rising effect on evapotranspiration ratio by IA (percentage of irrigation area to total catchment area). The Budyko n values estimated by the MARS model reproduce the calculated ones by the observation well for the selected 96 catchments (with R = 0.817, MAE = 4.09). Compared to the multiple stepwise regression model estimating the parameter n taken the influencing factors as independent inputs, the MARS model enhances the capability of the Budyko framework for assessing water availability at regional scale using readily available data. [ABSTRACT FROM AUTHOR]

Future changes in reference evapotranspiration (ET0) are of increasing importance in assessing the potential impacts on hydrology and water resources systems of more pronounced climate change. This study assesses the applicability of the Statistical Downscaling Model (SDSM) in projecting ET0, and investigates the seasonal and spatial patterns of future ET0 based on general circulation models (GCMs) across the Haihe River Basin. The results indicate that SDSM can downscale ET0 well in term of different basin-averaged measures for the HadCM3 and CGCM3 GCMs. HadCM3 has a much superior capability in capturing inter-annual variability compared to CGCM3 and thus is chosen as the sole model to assess the changes in future ET0. There are three homogeneous sub-regions of the Haihe River Basin: Northwest, Northeast and Southeast. Change points are detected at around 2050 and 2080 under the A2 and B2 scenarios, respectively. The Northwest is revealed to have a slight to strong increase in ET0, while the Northeast and the Southeast tend to experience a pattern change from decrease to increase in ET0. EDITOR M.C. Acreman ASSOCIATE EDITOR J. Thompson [ABSTRACT FROM AUTHOR]

Summary As the indicator of atmospheric evaporative demand over a hypothetical reference surface, reference evapotranspiration (ET 0 ) is an important input to hydrological models. Future projections of ET 0 are of great importance in assessing the potential impacts of climate change on the hydrologic regime as well as water resources systems. Different estimating formulations and different input data reliabilities existing in practice determine there may be potential uncertainty in projection of future ET 0 change. Here we investigated the difference of future ET 0 response to climate change based on three approaches, i.e., more physically based Penman–Montieth equation with relatively uncertain downscaled data quality, more empirical temperature-based Hargreaves equation with more reliable downscaled input data, and statistical downscaling method with directly selecting ET 0 as predictands. The Hanjiang River Basin, a headwater source of famous South to North Water Diversion Project (SNWDP) in China was chosen as example to illustrate this issue. Although similar increase processes of ET 0 in the Hanjiang River Basin were suggested by three methods, the magnitude of ET 0 increase differs substantially, indicating that uncertainty still exist despite of approximate performance of these methods in simulating general trends. Whilst increasing aridity index and decreasing water surplus over the period of 2011–2099 would inevitably cause negative impacts on the implementation of the SNWDP and effective adapting measures are thus expected. [ABSTRACT FROM AUTHOR]

Abstract: As the most excellent indicator for hydrological cycle and a central link to water-balance calculations, the reference evapotranspiration (ET0) is of increasing importance in assessing the potential impacts of climate change on hydrology and water resources systems since the climate change has been becoming more pronounced. In this study, we conduct an investigation on the spatial and temporal changes in ET0 of the Haihe River Basin in present and future stages. The ET0 in the past five decades (1961–2010) are calculated by the Penman–Monteith method with historical climatic variables in 40 sites while the ET0 estimation for the future period of 2011–2099 is based on the related climatic variables projected by Coupled General Circulation Model (CGCM) multimodel ensemble projections in Phase 3 of the Coupled Model Intercomparison Project (CMIP3) using the Bayesian Model Average (BMA) approach. Results can be summarized for the present and future as follows. (1) No coherent spatial patterns in ET0 changes are seen in the whole basin. Half of the stations distributed mainly in the eastern and southeastern plain regions present significant negative trends, while only 3 stations in the western mountainous and plateau basin show significant positive trends. Radiation is mainly responsible for the ET0 change in the southern and eastern basin, whereas relative humidity and wind speed are the leading factors in the eastern coastal and north parts. (2) BMA ensemble method is competent to produce lower bias in comparison with other common methods in this basin. Future spatiotemporal ET0 pattern analysis by means of the BMA method based on the ensembles of four CGCMs suggested that although the spatial patterns under three scenarios are different in the forthcoming two decades, generally increasing trends can be found in the 21st century, which is mainly attributed to the significant increasing temperature. In addition, the implication of future ET0 change in agriculture and local water resources is discussed as an extension of this work. The results can provide beneficial reference and comprehensive information to understand the impact of climate change on the future water balance and improve the regional strategy for water resource and eco-environment management in the Haihe River Basin. [Copyright &y& Elsevier]

Although soil moisture (SM) is an important constraint factor of evapotranspiration (ET), the majority of the satellite-driven ET models do not include SM observations, especially the SM at different depths, since its spatial and temporal distribution is difficult to obtain. Based on monthly three-layer SM data at a 0.25° spatial resolution determined from multi-sources, we updated the original Priestley Taylor–Jet Propulsion Laboratory (PT-JPL) algorithm to the Priestley Taylor–Soil Moisture Evapotranspiration (PT-SM ET) algorithm by incorporating SM control into soil evaporation (Es) and canopy transpiration (T). Both algorithms were evaluated using 17 eddy covariance towers across different biomes of China. The PT-SM ET model shows increased R2, NSE and reduced RMSE, Bias, with more improvements occurring in water-limited regions. SM incorporation into T enhanced ET estimates by increasing R2 and NSE by 4% and 18%, respectively, and RMSE and Bias were respectively reduced by 34% and 7 mm. Moreover, we applied the two ET algorithms to the whole of China and found larger increases in T and Es in the central, northeastern, and southern regions of China when using the PT-SM algorithm compared with the original algorithm. Additionally, the estimated mean annual ET increased from the northwest to the southeast. The SM constraint resulted in higher transpiration estimate and lower evaporation estimate. Es was greatest in the northwest arid region, interception was a large fraction in some rainforests, and T was dominant in most other regions. Further improvements in the estimation of ET components at high spatial and temporal resolution are likely to lead to a better understanding of the water movement through the soil–plant–atmosphere continuum. [ABSTRACT FROM AUTHOR]

Evidence that the pan evaporation or reference evapotranspiration (ET0) as the indicator of atmospheric evaporation capability have decreased along with the continuous increase in temperature over the past decades (coined as 'evaporation paradox') has been reported worldwide. Here, we provide a nationwide investigation of spatiotemporal change of ET0 using meteorological data from 602 stations with the updated data (1961-2011). In addition, we explore the trigger mechanism by quantitative assessment on the contribution of climatic factors to ET0 change based on a differential equation method. In despite of different shift points regionally, our results suggest that the ET0 generally present decadal variations rather than monotonic response to climate change reported in previous studies. The significant decrease in net radiation dominate the decrease in ET0 before early 1990s in southern regions, while observed near-surface wind speed is the primary contributor to the variations of ET0 for the rest regions during the same periods. The enhancements of atmospheric evaporation capability after early 1990s are driven primarily by recent relative humidity limitation in China. From a continental scale view, as highly correlating with to Pacific Decadal Oscillation, the shift behaviors of ET0 is likely an episodic phenomenon of the ocean-atmosphere interaction in earth. [ABSTRACT FROM AUTHOR]

This study reveals a close connection between interannual variation of the Somalia Jet (SMJ) intensity in boreal spring and the northwest-southeast movement of the South Asian High (SAH) in the following summer based on multiple datasets and numerical experiments. It is found that the summer SAH tends to shift northwestward (southeastward) when the preceding spring SMJ is stronger (weaker). There are two ways by which the spring SMJ intensity affects the following summer SAH. One is via modulating sea surface temperature (SST) in the tropical Indian Ocean and the other is through modulating anomalous heating associated with the Indian summer monsoon rainfall. On one hand, increase in the spring SMJ intensity enhances southwesterly monsoon winds and upward latent heat flux and leads to SST decrease in the tropical western Indian Ocean. The tropical western Indian Ocean cooling further leads to a decrease (increase) in upper-level geopotential height to the southeastern (northwestern) side of the SAH, inducing a northwestward movement of the SAH. On the other hand, increase in the spring SMJ intensity strengthens the Indian summer monsoon and enhances rainfall and atmospheric heating over the Indian subcontinent. The enhanced Indian subcontinent heating induces strong upper-level positive geopotential height to the northwestern side of the SAH, contributing to a northwestward shift of the summer SAH. Results of this study indicate that variation of the spring SMJ intensity is an effective predictor in the prediction of the northwest-southeast movement of the SAH in the following summer. [ABSTRACT FROM AUTHOR]

The Beijing–Tianjin–Hebei (BTH) region has encountered increasingly severe and frequent haze pollution during recent decades. This study reveals that El Niño–Southern Oscillation (ENSO) has distinctive impacts on interannual variations of haze pollution over BTH in early and late winters. The impact of ENSO on the haze pollution over the BTH is strong in early winter, but weak in late winter. In early winter, ENSO-related sea surface temperature anomalies generate double-cell Walker circulation anomalies, with upward motion anomalies over the tropical central-eastern Pacific and tropical Indian Ocean, and downward motion anomalies over the tropical western Pacific. The ascending motion and enhanced atmospheric heating anomalies over the tropical Indian Ocean trigger atmospheric teleconnection propagating from the north Indian Ocean to East Asia, and result in the generation of an anticyclonic anomaly over Northeast Asia. The associated southerly anomalies to the west side lead to more serious haze pollution via reducing surface wind speed and increasing low-level humidity and the thermal inversion. The strong contribution of the Indian Ocean heating anomalies to the formation of the anticyclonic anomaly over Northeast Asia in early winter can be confirmed by atmospheric model numerical experiments. In late winter, vertical motion and precipitation anomalies are weak over the tropical Indian Ocean related to ENSO. As such, ENSO cannot induce a clear anticyclonic anomaly over Northeast Asia via atmospheric teleconnection, and thus has a weak impact on the haze pollution over BTH. Further analysis shows that stronger ENSO-induced atmospheric heating anomalies over the tropical Indian Ocean in early winter are partially due to higher mean SST and precipitation there. Significance Statement: There exist large discrepancies regarding the contribution of El Niño–Southern Oscillation (ENSO) events to the wintertime haze pollution over North China. Several studies have indicated that ENSO has a weak impact on the haze pollution over North China. However, some studies have argued that ENSO events can exert impacts on the occurrence of haze pollution over North China. In this study, we present evidence to demonstrate that ENSO has distinctive impacts on interannual variations of the haze pollution over the Beijing–Tianjin–Hebei (BTH) region in North China in early and late winters. Specifically, ENSO has a strong impact on the haze pollution over BTH in early winter, whereas the impact of ENSO on the haze pollution over BTH is fairly weak in late winter. Results of this study could reconcile the discrepancy of previous studies about the impact of ENSO on the haze pollution over North China. [ABSTRACT FROM AUTHOR]

• Evaluate ET estimation uncertainty derived from remote sensing ET models. • Discuss ET estimation uncertainty derived from reanalysis products. • Investigate the effect of the uncertainty in eddy covariance tower observations. Since evapotranspiration (ET) is the intrinsic link between global energy and water cycle, remote sensing-based models have been developed for regional and global scale ET on heterogeneous land surface over the past four decades. In view of the significantly different physical mechanisms and mathematical expressions among remote sensing ET models as well as data availability and quality control process among the model input products, it is necessary to investigate the uncertainties of the multiple sources in actual ET estimation. Here, three remote sensing ET models, including the PT-DTsR model, the PM-mod model and the PML model, were simultaneously driven by three meteorological reanalysis products, resulting in nine calculation schemes to analyze the combined effect of the models and the input datasets. The Sobol' sensitivity method was also adopted for identifying the influential model parameters and in turn understanding the model process. The results indicated that estimates from nine calculation schemes showed great differences in the magnitude and temporal variation, explaining 20–50% of ET variability over all sites. Additionally, schemes compared with both uncorrected and corrected energy balance observations, as well as schemes using meteorological variables from three reanalysis products and Eddy Covariance tower observations, verified that the uncertainties in latent heat flux data observations caused by the energy budget mis-closure problem and spatial scale mismatch have propagated into the ET estimation. Our study is a beneficial reference for the uncertainties in remote sensing-based methods, and thus can provide guidance for the future development of ET models. [ABSTRACT FROM AUTHOR]

This study investigated the change in precipitation concentration due to global warming based on the output of one experiment (a scenario of 1% CO2 increase per year) from 12 general circulation models provided by the Coupled Model Intercomparison Project Phase 5 (CMIP5). Two different indices were used to describe precipitation concentration: (a) concentration index (CI), which measures the evenness of total precipitation on wet days, and (b) precipitation concentration degree (PCD), which measures the evenness of the annual precipitation distributions over time. We found that widespread increases in CI were distributed over all land areas except for some arid areas, indicating a less uniform distribution of precipitation on wet days in a warming climate caused by CO2 increasing. We also found that the spatial patterns of changes in PCD are complex, with large regional differences. All of the results suggest a global‐scale readjustment of precipitation distribution in magnitude and timing. This kind of readjustment may have significant impacts on climatic and hydrological events and thus cause severe ecological and environmental damage. [ABSTRACT FROM AUTHOR]

As one of the most influential oceanic and atmospheric oscillations in the Earth system, El Niño‐Southern Oscillation (ENSO) has modulated numerous geophysical processes. This is particularly true for the Yangtze River Basin (YRB), which is vulnerable to Asian Monsoon and faces serious hydrological hazards. In this study, the co‐variability between lag–lead precipitation and sea surface temperature anomalies was evaluated utilizing singular value decomposition (SVD) method. Moreover, certain teleconnections between ENSO and streamflow were identified by wavelet methods. In addition, the contribution of related atmospheric variables was revealed by composite analysis. Results indicate that there are strong associations in lag–lead seasons between the wet condition (dry condition) and September–November (December–February) mature ENSO phase. Significant common power and coherence signals between the ENSO indices and the streamflow occur in the 4–8, 8–16 and 16–32 seasonal scales. Meanwhile, the activity cycle of the ENSO indices ahead of streamflow increases from the mid‐lower reaches to the source region. In addition, the Western Pacific Subtropical High is strengthened during the mature ENSO phase. Anomalous sinking motions and divergent water vapour flux occupy the YRB, reducing the precipitation and leading to the dry condition in the source region until the following March–May. On the other hand, ascending movements and abundant water vapour flux coming from northern Pacific, equatorial western Pacific and the Bay of Bengal result in the wet condition in the mid‐lower reaches. [ABSTRACT FROM AUTHOR]

Reference evapotranspiration (ET0), which is an indicator of atmospheric evaporation demand over a hypothetical reference surface, is expected to alter along with global climate change. In this study, the changes and driving forces of historical ET0 and its future projections in Xinjiang, China, were comprehensively conducted. The trend analysis revealed that regional ET0 decreased significantly at a rate of −4.1 mm/year2 during 1961–1993 and increased at a rate of 3.4 mm/year2 during 1994–2010. To explore the possible causes, the contributions of major climatic variables to the ET0 trends were derived based on the differential equation method. During 1961–1993, the decreasing trend of ET0 was primarily ascribed to the wind speed. The integrated negative effect from the decrease in wind speed (U) and increase in relative humidity (RH) was more significant than the positive effect from the increase in air temperature (T), resulting in the decrease in ET0. During 1994–2010, combined with increasing T and decreasing RH, the increase in U offset the effect of the net radiation (Rn) decrease and led to an increase in ET0. Future ET0 projections over three periods (2010–2039, 2040–2069, and 2070–2099, which are named as 2020s, 2050s, and 2080s, respectively) were conducted based on the general circulation models under two representative concentration pathways scenarios (RCP4.5 and RCP8.5). A continuous increasing trend in ET0 was demonstrated in the 21st century. The increase in ET0 will increase the irrigated water resource consumption and bring new challenge to water resource management in this area. [ABSTRACT FROM AUTHOR]

Abstract Phosphorus (P) is an indispensable resource for plant growth, and under the background with P consumption increase in crop system, the agriculture and crop production may be affected through the changes of phosphorus use efficiency (PUE) due to the human interference and climate change. Therefore, it is of great importance to investigate the responses of PUE to human interference and climate change from the perspectives of historical simulation and future projection. In this study, the changes of PUE from 2001 to 2015 in the middle and lower reaches of the Yangtze River (MLRYR) of China were evaluated by means of empirical PUE-model based on the substance flow analysis (SFA) method. Their responses to future human interference and climate scenarios during 2016–2030 were investigated by driving PUE-model with three neural networks (BP, GA-BP and PSO-BP) as well as the multiple downscaling climatic data (HadGEM2-ES, BCC-CSM1.1 (m) and GFDL-ESM2M) with the help of a statistical downscaling method (SDSM). The results showed that the PUE values of the three crops (rice, wheat and maize) were 53.61%, 36.22%, and 32.56%, respectively, during the period from 2001 to 2015 and 54.11%, 38.92% and 32.78%, respectively, during the period from 2016 to 2030. Consequently, the PUE showed an increasing trend from history to future. However, the overall PUE level was not high, with a value of not more than 60%, and notably for wheat and maize, the PUE was less than 40%. Meanwhile, the precipitation affected the leaching of P because increasing in precipitation may result in a relatively high increase in the probability of low-PUE. Moreover, great potential exists for enhancing the PUE in this region, and measures to improve PUE management practices have been proposed, such as, raising the crop yield, keeping the arable area and balancing the fertilization, etc. The present study provides a beneficial reference to comprehensively understand the impacts of human interference and climate change on the PUE and further improves regional sustainable strategies for future P resource management. Highlights • Evaluating the responses of PUE to human interference and climate change from historical simulation and future projections. • Predicting the future PUE by driving PUE-model with the multiple neural networks and GCMs. • Besides the rice, the PUE values of wheat and maize were less than 40% during 2001–2030. [ABSTRACT FROM AUTHOR]

As the only connecting term between water balance and energy budget in the earth-atmospheric system, evapotranspiration ( ET ) is considered the most excellent indicator for the activity for the water and energy cycle. Under the background of global change, regional ET estimates, components partitioning as well as their spatial and temporal patterns recognition are of great importance in understanding the hydrological processes and improving water management practices. This is particularly true for the Tibetan Plateau (TP), one of most sensitive and vulnerable region in response to the environment change in the earth. In this study, with flux site observation data and monthly ET data from the monthly water balance method incorporating the terrestrial water storage changes from the Gravity Recovery and Climate Experiment satellite (GRACE) production as the multiple validations, the long-term daily ET in the TP was retrieved by a modified Penman-Monteith-Leuning (PML) model with considering evapotranspiration over snow covered area during 1982–2012. The spatial and temporal changes of partitioned three components of ET , i.e., soil evaporation ( E s ), transpiration through the stomata of plant ( E c ) and canopy interception ( E i ), were investigated in the TP. Meanwhile, how the ET components contribute to ET changes and respond to the change in environmental factors in the TP was revealed and discussed. The results indicate that E s dominates ET in most areas of the TP with the mean annual ratio of 65.7%, except southeastern regions where the vegetation coverage is high. Although regional average ET and three main components all present obvious increase trends during the past decades, high spatial heterogeneity for their trends are identified in the TP. Moreover, a mixed changing pattern can be apparently found for E s in southeastern area, E c and E i in northwestern and southeastern area. Spatially, the ET variation are mainly attributed to change in E s , followed by E c and E i . In addition, compared with evaporation-related meteorological variables, precipitation and leaf area index (LAI) provide the more dominated contributions to the changes of ET and E i in the TP, while LAI is overwhelming contribution factor for E c changes. As for E s changes, although precipitation play the important role, the effects of other factors are also non-negligible. [ABSTRACT FROM AUTHOR]

Representing atmospheric evaporating capability for a hypothetical reference surface, potential evapotranspiration (PET) determines the upper limit of actual evapotranspiration and is an important input to hydrological models. Due that present climate models do not give direct estimates of PET when simulating the hydrological response to future climate change, the PET must be estimated first and is subject to the uncertainty on account of many existing formulae and different input data reliabilities. Using four different PET estimation approaches, i.e., the more physically Penman (PN) equation with less reliable input variables, more empirical radiation-based Priestley-Taylor (PT) equation with relatively dependable downscaled data, the most simply temperature-based Hamon (HM) equation with the most reliable downscaled variable, and downscaling PET directly by the statistical downscaling model, this paper investigated the differences of runoff projection caused by the alternative PET methods by a well calibrated abcd monthly hydrological model. Three catchments, i.e., the Luanhe River Basin, the Source Region of the Yellow River and the Ganjiang River Basin, representing a large climatic diversity were chosen as examples to illustrate this issue. The results indicated that although similar monthly patterns of PET over the period 2021–2050 for each catchment were provided by the four methods, the magnitudes of PET were still slightly different, especially for spring and summer months in the Luanhe River Basin and the Source Region of the Yellow River with relatively dry climate feature. The apparent discrepancy in magnitude of change in future runoff and even the diverse change direction for summer months in the Luanhe River Basin and spring months in the Source Region of the Yellow River indicated that the PET method related uncertainty occurred, especially in the Luanhe River Basin and the Source Region of the Yellow River with smaller aridity index. Moreover, the possible reason of discrepancies in uncertainty between three catchments was quantitatively discussed by the contribution analysis based on climatic elasticity method. This study can provide beneficial reference to comprehensively understand the impacts of climate change on hydrological regime and thus improve the regional strategy for future water resource management. [ABSTRACT FROM AUTHOR]

Accounting for over 70% of global water withdrawals, irrigation plays a crucial role in the development of agriculture. Irrigation water requirement ( WIRR ) will be influenced by climate change due to the alteration in soil water balances, evapotranspiration, physiology and phenology under global warming. This is particularly true for rice, a high water-consuming crop. Therefore, exploring the impact of climate change on rice WIRR is of great significance for the sustainable utilization of water resources and food security. This paper aims to investigate spatially and temporally the responses of rice WIRR to climate change in the middle and lower reaches of the Yangtze River (MLRYR), which is one of the most important rice farming districts in China. With the help of the specially developed rice growing period calculation method and water balance model coupled with rice irrigation scheduling, the impacts of climate change on WIRR for early rice, late rice and single cropping rice during the historical (1961–2012) and future (2011–2100) periods were evaluated. Meanwhile, to consider the uncertainty from climate models in future projection, four GCMs under RCP2.6, RCP4.5 and RCP8.5 emission scenarios from the 5th Coupled Model Intercomparison Project (CMIP5) were employed as the input of the water balance model. The results indicate the following: (1) The days of growing period ( DGP ) for all three types of rice display shortened trends in historical and most future periods. However, in the middle region of the MLRYR, the DGP for early rice and late rice would increase by up to10 days in 2080s under RCP8.5 scenario. (2) Over the past 50 years, the WIRR significantly decreased by 1.58 and 2.10 mm yr −1 for late rice and single cropping rice, respectively. While for early rice, the WIRR only slightly decreased by 0.13 mm yr −1 . (3) Projected future WIRR would increase for all three types of rice in the whole region under RCP4.5 and RCP8.5 scenarios (up to 100 mm), but decrease for single cropping and late rice in the southeast region (up to 40 mm). The results can provide beneficial reference and comprehensive information to understand the impact of climate change on the agricultural water balance and improve the regional strategy for water resource utilization and agricultural management in the middle and lower reaches of the Yangtze River. [ABSTRACT FROM AUTHOR]

• We derived an analytical solution to the impact of seasonal vegetation changes on ET. • Global ET elastic response to seasonal LAI changes has a north-south shift trajectory. • The weak negative ET elastic response to vegetation changes was found under low LAI. Seasonal variation of vegetation profoundly affects the water cycle. However, the seasonal divergence of evapotranspiration (ET) sensitivity in response to vegetation variations has not been fully understood. Here we derived an analytical solution to examine the impact of seasonal vegetation changes on ET with an extended Budyko framework based on an improved ET algorithm with improved water balance constraints. Results reveal a clear seasonal divergence of ET sensitivity to vegetation coverage changes across climate regimes and biomes. Generally, the high ET sensitivity to vegetation coverage has a clear north-south shift trajectory from spring to winter. For moderate-humid regions (0.7 < aridity index < 1.0), vegetation exhibits higher importance in altering ET in March-September. While for moderate-dry regions (1.0 < aridity index < 1.4), the sensitivity of ET to vegetation changes is the highest in September-November. Moreover, the spatial-temporal pattern of ET sensitivity to seasonal vegetation changes is different between short vegetation cover and forest. Additionally, negative ET sensitivity to vegetation coverage changes was discovered in regions with seasonal precipitation of less than 500 mm and sparse vegetation coverage (predominant land cover types of grassland, scrubland, and savannas). In summary, our study provides an analytical solution to estimate ET sensitivity to seasonal vegetation changes within the extended Budyko framework. The results highlight the difference in hydrological response to vegetation dynamics across seasons and vegetation types. [ABSTRACT FROM AUTHOR]

The concept of elasticity has been widely employed to quantify the hydrological response to changes in climate and catchments properties. To separate the effect of different climatic variables on runoff, the potential evaporation (E 0 ) elasticity of runoff needs to be presented in term of observed climate variables. To fully reflect the effects of maximum and minimum temperatures and reduce the influence of the correlations of radiation with sunshine duration and relative humidity on the assessment results, we decompose the E 0 elasticity into five evaporation-related elasticities (i.e., sunshine duration, maximum and minimum temperature, wind speed and relative humidity) via the first-order differentiation of the FAO 56 Penman equation. As the catchment runoff is frequently affected by the land use/cover change, we also consider changes in catchment characteristics and derive a catchment alteration elasticity based on the Budyko framework. An application was carried out in 30 catchments with widespread climatic types in China. For the two periods (i.e., the baseline period and the changed period) divided by the Pettitt test, the contributions of different climatic variables and land use/cover conditions to runoff change were quantified. In general, the alteration of catchment characteristics and climatic change should be mainly responsible for changes in runoff in water-limited and humid basins, respectively. Although the elasticity of maximum temperature are usually higher than that of minimum temperature, the contributions to runoff change present the opposite direction. Furthermore, additional analysis indicated some overestimation in relative humidity elasticities in the previous studies, further emphasizing the necessity of our extension to alleviate the influence of correlation between climatic variables to the assessment results. Moreover, the results of model performance versus model complexity showed that the choice of model complexity still depends on the goal in the climatic and anthropogenic effects on runoff. If the climatic effect as a whole was of interest, the method based on two parameters elasticities would be more applicable compared to that based on multi-parameter elasticities because the simple model provides higher confidence in modelling the overall change in runoff. [ABSTRACT FROM AUTHOR]

Accurate estimation of global evapotranspiration (ET) is critical to understand the water and energy cycles in the Earth system. Satellite-driven ET algorithms serve as an effective way to estimate the global ET. However, many algorithms have been designed independently of water balance constraints, which potentially limit their ability to estimate ET in water-limited and high interception regions. As ET remains one of the most uncertain terms in the global water budgets, incorporating water balance constraints into algorithms should improve the performance of ET estimates. In this study, we developed a general solution (denoted PEW) based on the proportionality hypothesis to incorporate available water control into the widely used Priestley Taylor-Jet Propulsion Laboratory (PT-JPL) ET algorithm. Simulated performances of the PEW model and PT-JPL algorithm were evaluated against 106 FLUXNET eddy covariance (EC) towers data at the site scale. Meanwhile, model results were compared at the global scale with the means of the widely used ET products. We found that the PEW model has smaller errors than the original PT-JPL algorithm, with the greatest improvements in water-limited regions and areas characterized by the high interception. Moreover, by incorporating the water balance constraints into the ET algorithm, the PEW model has the ability to distinguish variations in ET affected by El Nino-Southern Oscillation. In summary, our study offers a convincing evidence regarding the incorporation of water balance constraints into remote sensing algorithms for more accurately mapping global terrestrial ET with an enhanced understanding of ET variation under climate change. This model is the first of its kind among remote-sensing models to provide global land ET estimation with the proportionality hypothesis-based water balance constraints. • We develop a new ET algorithm based on proportionality hypothesis and PT-JPL model. • A water balance constraint is introduced to this ET algorithm and applied globally. • The new ET algorithm is validated globally and shows better performance than PT-JPL model. • ET improved most in water-limited regions and areas having a high interception. [ABSTRACT FROM AUTHOR]

• Magnitude of the sub-diurnal AET - VPD hysteresis varies with biomes and climates. • Variation of the hysteresis is mostly attributable to evaporative demand limitation. • Soil moisture supply limitation amplifies the hysteresis during the dry season. • We provide some novel features to diagnose limiting mechanisms of the hysteresis. Hysteresis between sub-diurnal actual evaporation (AET) (or one of its components, transpiration) and vapor pressure deficit (VPD) at the species or individual ecosystem level has been extensively studied, but the global variation and seasonal variability of this hysteresis across biomes and climates is yet to be fully explored and the limiting mechanisms remain unclear. We hypothesize that the sub-diurnal AET - VPD hysteresis results from the interplay between evaporative demand and soil moisture supply limitations. To test our hypothesis, we quantify the sub-diurnal AET - VPD hysteresis across a broad range of biomes and climates based on the observations from the 89 FLUXNET sites (703 site-years) across the globe. We find that the magnitude of hysteresis varies with biomes and climates and is mostly attributable to evaporative demand limitation in all ten sampled biomes. In seasonally dry locations, however, low soil moisture availability amplifies the hysteresis during the dry season. Sensitivity analysis using a hydraulic model suggests that most ecosystems exhibiting seasonal drought display a more isohydric behavior during the dry season, while shift toward a more anisohydric response during the wet season. Our findings have important implications for understanding sub-diurnal dynamics between vegetation and its surrounding environment, reducing uncertainties in AET simulation at fine spatial and temporal scales, and improving understanding of the ecosystem response to hydrologic stress. [ABSTRACT FROM AUTHOR]

Climate change in North China would result in significant changes in temperature, precipitation and their spatial/temporal distributions. Consequently, these induced changes will have profound effects on the hydrological cycle and water resources in both agricultural and natural ecosystems. Panjiakou reservoir in the middle Luanhe River basin-a tributary of the Haihe River basin-is one of the important sources of water for industrial and agricultural development in Beijing, Tianjin and Hebei province, China. Any significant change in the magnitude and/or timing of runoff from the reservoir induced by changes in climatic variables would have significant implication for the economic prosperity in North China. This paper investigates the impacts of climate change on hydrological processes in the Luanhe River basin as follows. Firstly, spatial and temporal patterns of precipitation, temperature and runoff at both annual and seasonal scales from 1957 to 2000 in the Luanhe River basin are analyzed using Mann-Kendall trend analysis, linear regression methods and inverse distance weighted interpolation. For the impact study, four Global Climate Models (GCMs) (named CSIRO, HadCM3, CNRM and GFDL) were used to produce precipitation and temperature data under A2 scenario by mean of a widely used quantile-quantile transformation. Projected meteorological variables were used to force a two-parameter hydrologic model to simulate the hydrological response to climate change in the future (2021-2050). Moreover, a sensitivity analysis is conducted to assess how precipitation and temperature affect the runoff. Results suggested that most part of the Luanhe River basin was dominated by significant increasing trends of temperature and no significant trends of precipitation in annual and seasonal scale during the past decades. Annual, spring and autumn runoffs present significant decreasing trends in the Panjiakou reservoir basin. Meanwhile, runoff is more strongly related to precipitation than to temperature. All GCMs projected precipitation and temperature series after bias correction indicated increasing temperature and increasing precipitation trends for the period 2021-2050 except that CNRM showed a slight decreasing trend in precipitation. Great enhancements can be found in projected runoff except CNRM by driving the two-parameter water balance model. The study provides valuable information on the assessment of the impact of the climate change on water resources in the Luanhe River basin as well for allocating and designing water resources projects. [ABSTRACT FROM AUTHOR]

Exploring the characteristic of the extreme climatic events, especially future projection is considerably important in assessing the impacts of climatic change on hydrology and water resources system. We investigate the future patterns of climate extremes (2001-2099) in the Haihe River Basin (HRB) derived from Coupled General Circulation Model (CGCM) multimodel ensemble projections using the Bayesian Model Average (BMA) approach, under a range of emission scenarios. The extremes are depicted by three extreme temperature indices (i.e., frost days (FD), growing season length (GSL), and T >90th percentile (TN90)) and five extreme precipitation indices (i.e., consecutive dry days (CDD), precipitation ≥10 mm (R10), maximum 5-day precipitation total (R5D), precipitation >95th percentile (R95T), and simple daily intensity index (SDII)). The results indicate frost days display negative trend over the HRB in the 21st century, particularly in the southern basin. Moreover, a greater season length and more frequent warm nights are also projected in the basin. The decreasing CDD, together with the increasing R10, R5D, R95T, and SDII in the 21st century indicate that the extreme precipitation events will increase in their intensity and frequency in the basin. Meanwhile, the changes of all eight extremes climate indices under A2 and A1B scenarios are more pronounced than in B1. The results will be of practical significance in mitigation of the detrimental effects of variations of climatic extremes and improve the regional strategy for water resource and eco-environment management, particularly for the HRB characterized by the severe water shortages and fragile ecological environment. [ABSTRACT FROM AUTHOR]

Spatiotemporal changes in climatic extremes in the Yellow River Basin from 1959 to 2008 were investigated on the basis of a suite of 27 climatic indices derived from daily temperature and precipitation data from 75 meteorological stations with the help of the Mann-Kendall test, linear regression method and GIS technique. Furthermore, the changes in the probability distribution of the extreme indices were examined. The results indicate: (1) The whole basin is dominated by significant increase in the frequency of warm days and warm nights, and dominated by significant decrease in the frequency of cold days and cold nights. Although trends in absolute temperature indices show less spatial coherence compared with that in the percentile-based temperature indices, overall increasing trends can be found in Max Tmax (TXx), Min Tmax (TXn), Max Tmin (TNx) and Min Tmin (TNn). (2) Although the spatial patterns and the number of stations with significant changes for threshold and duration temperature indices are also not identical, general positive trends in warm indices (i.e., summer days (SU25), tropical nights (TR20), warm spell duration indicator and growing season length) and negative trends in cold indices (i.e., frost days, ice days and cold spell duration indicator) can be found in the basin. Annual nighttime temperature has increased at a faster rate than that in daytime temperature, leading to obvious decrease in diurnal temperature range. (3) The changes in precipitation indices are much weaker and less spatially coherent compared with these of temperature indices. For all precipitation indices, only few stations are characterized by significantly change in extreme precipitation, and their spatial patterns are always characterized by irregular and insignificant positive and negative changes. However, generally, changes in precipitation extremes present drying trends, although most of the changes are insignificant. (4) Results at seasonal scale show that warming trends occur for all seasons, particularly in winter. Different from that in other three seasons, general positive trends in max 1-day precipitation (Rx1DAY) and max 5-day precipitation (Rx5DAY) are found in winter. Analysis of changes in probability distributions of indices for 1959-1983 and 1984-2008 indicate a remarkable shift toward warmer condition and a less pronounced tendency toward drier condition during the past decades. The results can provide beneficial reference to water resource and eco-environment management strategies in the Yellow River Basin for associated policymakers and stakeholders. [ABSTRACT FROM AUTHOR]

As one of the most important crops cultivated in China, rice contributes to approximately 28% of total yield. In despite of the substantial production, rice productivity is gravely affected by ongoing climate change and reduction of available water resources. Thus, assessing the responses of rice water consumption and productivity to more pronounced climate change is of great significance to water resources management in terms of relieving the resources shortage and meeting the food demand. In this study, the yield and water resources utilization during 1961–2010 in two typical rice plantation regions of China were evaluated using validated rice model ORYZA2000. Subsequently, their responses to future climate scenarios of 21 century were investigated through driving ORYZA2000 with downscaling climatic projections from GCMs under four RCPs emission scenarios. To quantify the water resources utilization in rice production from multiple perspectives, the water footprint (WF) and three water productivity indices (WP I , WP U and WP ET) were integrated for assessing the regional agricultural water stress in this paper. The results revealed that the annual average linear inclining rates of WF in two stations (Kaifeng and Kunshan) were 3.86 m3/ t and 2.62 m3/ t, respectively. Moreover, compared with the green water footprint (WF g), the blue water footprint (WF b) is projected to significantly increase in future. The water productivity (WP) would decrease in two stations under four RCPs scenarios except that the WP u and WP ET of Kunshan under RCP2.6 and RCP4.5 scenario in 2020s, 2050s and 2080s. Hence, this study provides insights into comprehensively understand the influences of climate change on food security and sheds lights on the regional strategy for future water resource management. Unlabelled Image • The study pioneers the use of ORYZA model along with GCM outputs in rice WF assessment under climate change. • The rice WFs of two stations are projected to increase in future. • Decrease in CWU green of Kunshan does not affect incremental WF g due to the decline in future rice yield. • Compared with WF b , the WF g of study area play a dominant role in the growth of future rice WF. [ABSTRACT FROM AUTHOR]