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Global evapotranspiration from high-elevation mountains has decreased significantly at a rate of 3.923 %/a over the last 22 years
... The negative contribution of the southern and northern parts may be due to the reduction in vegetation damage and ET caused by excessive radiation. Excessive radiation may lead to photoinhibition in plants, reduce the efficiency of photosynthesis, and even cause leaf burns, damage plant health, and reduce ET [45]. The negative contribution of VPD to ET was significant in most regions, especially in the southern region. ...
The quantitative assessment of the impact of vegetation restoration on evapotranspiration and its components is of great significance in developing sustainable ecological restoration strategies for water resources in a given region. In this study, we used the Priestley-Taylor Jet Pro-pulsion Laboratory (PT-JPL) to simulate the ET components in the Helong section (HLS) of the Yellow River basin. The effects of vegetation restoration on ET and its components, vegetation transpiration (Et), soil evaporation (Es), and canopy interception evaporation (Ei) were separated by manipulating model variables. Our findings are as follows: (1) The simulation results are compared with the ET calculated by water balance and the annual average ET of MODIS products. The R2 of the validation results are 0.61 and 0.78, respectively. The results show that the PT-JPL model tracks the change in ET in the HLS well. During 2000–2018, the ET, Ei, and Es increased at a rate of 1.33, 0.87, and 2.99 mm/a, respectively, while the Et decreased at a rate of 2.52 mm/a. (2) Vegetation restoration increased the annual ET in the region from 331.26 mm (vegetation-unchanged scenario) to 338.85 mm (vegetation change scenario) during the study period, an increase of 2.3%. (3) TMP (temperature) and VPD (vapor pressure deficit) were the dominant factors affecting ET changes in most areas of the HLS. In more than 37.2% of the HLS, TMP dominated the change affecting ET, and vapor pressure difference (VPD) dominated the area affecting ET in 30.5% of the HLS. Overall, the precipitation (PRE) and VPD were the main factors affecting ET changes. Compared with previous studies that directly explore the relationship between many influencing factors and ET results through correlation research methods, our study uses control variables to obtain results under two different scenarios and then performs difference analysis. This method can reduce the excessive interference of influencing factors other than vegetation changes on the research results. Our findings can provide strategic support for future water resource management and sustainable vegetation restoration in the HLS region.
Aims: Caragana liouana has been widely planted in northwest China for ecological restoration which has induced the planted shrub landscape in the desert steppe zone and changed the structure and function of the original ecosystem. These plantations would affect the water vapor interactions between land surface and atmosphere eventually. Therefore, studying the evapotranspiration characteristics of the planted shrub communities in the desert steppe zone is significant for us to understand its ecohydrological mechanism and guide local ecological restoration.
Methods: The transpiration of shrubs and the evapotranspiration under shrub canopy in the desert steppe zone in Yanchi County, Ningxia, were measured by a coupled method of sap flow sensors and lysimeter from May to August in 2018. The effects of environmental factors on the evapotranspiration of the planted shrub communities were analyzed by path analysis.
Important findings: (1) The evapotranspiration obtained by the coupled method of sap flow sensors and lysimeter was consistent with the estimations based on the water balance method and the eddy covariance method. Thus, the coupled method of sap flow and lysimeter can be used to measure the evapotranspiration and its components of the planted shrub communities in the desert steppe zone. (2) The diurnal variations of transpiration rate of shrubs and the evapotranspiration rate under shrub canopy were consistent with each other, and both presented an obvious single peak at noon. The evapotranspiration of the planted shrub communities mainly occurred in day time. However, the time of the maximum transpiration rate of shrubs was 1 h later than that of the maximum evapotranspiration rate under shrub canopy. (3) The cumulative transpiration of shrubs was 83.6 mm, the daily average transpiration was 0.7 mm·d−1 , and the series curve of daily transpiration had a parabolic shape from May to August. The cumulative evapotranspiration under shrub canopy was 182.5 mm, and the daily average evapotranspiration was 1.5 mm·d−1 during the same period. The evapotranspiration under shrub canopy was higher than the transpiration of shrubs. It can be concluded that the water consumption in the planted shrub communities is mainly caused by evapotranspiration under shrub canopy. (4) The total evapotranspiration of the planted shrub communities from May to August was 266.1 mm, and the total precipitation in the same period was 222.6 mm. Therefore, the water balance in the planted shrub communities was in deficit. (5) Net radiation is the most important and direct driving factor that affected evapotranspiration of the planted shrub communities, and it coupled other environmental factors to drive the process of evapotranspiration. Therefore, the water deficit in the planted shrub communities encroaching desert steppe zone should be paid attention in ecological restoration and reconstruction.
水源涵养和生态系统过程与人类生产生活紧密相关，深入研究水源涵养对维持生态系统健康和人类社会可 持 续发展至关重要，对促进人与水和谐共生具有指导意义。随着对水源涵养研究与应用的深入，水源涵养内涵逐 渐 丰 富，评估方法愈加多样，但以往的研究中水源涵养定义模糊，缺乏对各种评估方法的综合性对比分析，因此急需 明 确 界定水源涵养内涵，分析各种估算方法的适用性。采用文献分析法系统梳理水源涵养研究历程，将其划分为认 识 与 萌芽期、理论发展期、定量计算和模型综 合 评 估 等 ４ 个 阶 段；从 定 义、水 量 与 功 能 等 ３ 个 方 面 明 确 界 定 水 源 涵 养 内 涵，从原理、时空尺度、适用范围及优缺点方面对已有水源涵养量估算方法进行对比和分析，并展望水源涵养研 究 的 未来发展方向。
Because of global warming, Earth’s ecosystems have been experiencing more frequent and severe heatwaves. Heatwaves are expected to tip terrestrial carbon sequestration by elevating ecosystem respiration and suppressing gross primary productivity (GPP). Here, using the convergent cross-mapping technique, this study detected positive bidirectional causal effects between GPP and respiration in two unprecedented European heatwaves. Heatwaves enhanced the causal effect strength of GPP on respiration rather than respiration on GPP across 40 site-years of observations. Further analyses and global simulations revealed spatial heterogeneity in the heatwave response of the causal link strength between GPP and respiration, which was jointly driven by the local climate and vegetation properties. However, the causal effect strength of GPP on respiration showed considerable uncertainties in CMIP6 models. This study reveals an enhanced causal link strength between GPP and respiration during heatwaves, shedding light on improving projections for terrestrial carbon sink dynamics under future climate extremes.
Evaporation (ET) is one of the crucial components of the water cycle, which serves as the nexus between global water, energy, and carbon cycles. Accurate quantification of ET is, therefore, pivotal in understanding various earth system processes and subsequent societal applications. The prevailing approaches for ET retrievals are either limited in spatiotemporal coverage or largely influenced by the choice of input data or simplified model physics, or a combination thereof. Here, using an independent mass conservation approach, we develop water-balance-based ET datasets (ET-WB) for the global land and the selected 168 major river basins. We generate 4669 probabilistic unique combinations of the ET-WB leveraging multi-source datasets (23 precipitation, 29 runoff, and 7 storage change datasets) from satellite products, in situ measurements, reanalysis, and hydrological simulations. We compare our results with the four auxiliary global ET datasets and previous regional studies, followed by a rigorous discussion of the uncertainties, their possible sources, and potential ways to constrain them. The seasonal cycle of global ET-WB possesses a unimodal distribution with the highest (median value: 65.61 mm per month) and lowest (median value: 36.11 mm per month) values in July and January, respectively, with the spread range of roughly ±10 mm per month from different subsets of the ensemble. Auxiliary ET products illustrate similar intra-annual characteristics with some over- or underestimation, which are completely within the range of the ET-WB ensemble. We found a gradual increase in global ET-WB from 2003 to 2010 and a subsequent decrease during 2010–2015, followed by a sharper reduction in the remaining years primarily attributed to the varying precipitation. Multiple statistical metrics show reasonably good accuracy of monthly ET-WB (e.g., a relative bias of ±20 %) in most river basins, which ameliorates at annual scales. The long-term mean annual ET-WB varies within 500–600 mm yr-1 and is consistent with the four auxiliary ET products (543–569 mm yr-1). Observed trend estimates, though regionally divergent, are evidence of the increasing ET in a warming climate. The current dataset will likely be useful for several scientific assessments centering around water resources management to benefit society at large. The dataset is publicly available in various formats (NetCDF, Mat, and Shapefile) at 10.5281/zenodo.8339655 (Xiong et al., 2023).
High Mountain Asia (HMA) has experienced a spatial imbalance in water resources in recent decades, partly because of a dipolar pattern of precipitation changes known as South Drying–North Wetting¹. These changes can be influenced by both human activities and internal climate variability2,3. Although climate projections indicate a future widespread wetting trend over HMA1,4, the timing and mechanism of the transition from a dipolar to a monopolar pattern remain unknown. Here we demonstrate that the observed dipolar precipitation change in HMA during summer is primarily driven by westerly- and monsoon-associated precipitation patterns. The weakening of the Asian westerly jet, caused by the uneven emission of anthropogenic aerosols, favoured a dipolar precipitation trend from 1951 to 2020. Moreover, the phase transition of the Interdecadal Pacific Oscillation induces an out-of-phase precipitation change between the core region of the South Asian monsoon and southeastern HMA. Under medium- or high-emission scenarios, corresponding to a global warming of 0.6–1.1 °C compared with the present, the dipolar pattern is projected to shift to a monopolar wetting trend in the 2040s. This shift in precipitation patterns is mainly attributed to the intensified jet stream resulting from reduced emissions of anthropogenic aerosols. These findings underscore the importance of considering the impact of aerosol emission reduction in future social planning by policymakers.
Compound drought‐heatwave (CDHW) events threaten ecosystem productivity and are often characterized by low soil moisture (SM) and high vapor pressure deficit (VPD). However, the relative roles of SM and VPD in constraining forest productivity during CDHWs remain controversial. In the summer of 2022, China experienced a record‐breaking CDHW event (DH2022). Here, we applied satellite remote‐sensing data and meteorological data, and machine‐learning techniques to quantify the individual contributions of SM and VPD to forest productivity variations and investigate their interactions during the development of DH2022. The results reveal that SM, rather than VPD, dominates the forest productivity decline during DH2022. We identified a possible critical tipping point of SM below which forest productivity would quickly decline with the decreasing SM. Furthermore, we illuminated the evolution of SM, VPD, evapotranspiration, forest productivity, and their interactions throughout DH2022. Our findings broaden the understanding of forest response to extreme CDHWs at the ecosystem scale.
The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUEeco), is widely expected to increase because of the rising atmospheric carbon dioxide concentration (Ca). However, little is known about the interactive effects of rising Ca and climate change on WUEeco. On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUEeco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.
The impact of atmospheric vapor pressure deficit (VPD) on plant photosynthesis has long been acknowledged, but large interactions with air temperature (T) and soil moisture (SM) still hinder a complete understanding of the influence of VPD on vegetation production across various climate zones. Here, we found a diverging response of productivity to VPD in the Northern Hemisphere by excluding interactive effects of VPD with T and SM. The interactions between VPD and T/SM not only offset the potential positive impact of warming on vegetation productivity but also amplifies the negative effect of soil drying. Notably, for high-latitude ecosystems, there occurs a pronounced shift in vegetation productivity's response to VPD during the growing season when VPD surpasses a threshold of 3.5 to 4.0 hectopascals. These results yield previously unknown insights into the role of VPD in terrestrial ecosystems and enhance our comprehension of the terrestrial carbon cycle's response to global warming.
The accurate inversion of actual evapotranspiration (ETa) at a regional scale is crucial for understanding water circulation, climate change, and drought monitoring. In this study, we produced a 1 km monthly ETa dataset for Turpan and Hami, two typical arid cities in northwest China, using multi-source remote sensing data, reanalysis information, and the ETMonitor model from 1980 to 2021. We analyzed the spatiotemporal variation of ETa using various statistical approaches and discussed the impact of climate and land use and cover changes (LUCC) on ETa. The results show the following: (1) the estimation results correlate well with ETa products on monthly scales (coefficient of determination (R2) > 0.85, root mean square error (RMSE) < 15 mm/month) with high reliability. (2) The ETa values were spatially distributed similarly to precipitation and LUCC, with the multi-year (1980–2021) average of 66.31 mm and a slightly fluctuating downward trend (−0.19 mm/a). (3) During the 42-year period, 63.16% of the study area exhibited an insignificant decrease in ETa, while 86.85% experienced pronounced fluctuations (coefficient of variation (CV) > 0.20), and 78.83% will show an upward trend in the future. (4) ETa was significantly positively correlated with precipitation (94.17%) and insignificantly positively correlated with temperature (55.81%). The impact of human activities showed an insignificant decreasing trend (85.41%). Additionally, the intensity of ETa varied considerably among land types, with the largest for cropland (424.12 mm/a). The results of the study have implications for promoting the rational allocation of regional water resources and improving water use efficiency in arid zones.
Increases in atmospheric CO2 concentration affect continental runoff through radiative and physiological forcing. However, how climate and land surface changes, and their interactions in particular, regulate changes in global runoff remains largely unresolved. Here we develop an attribution framework that integrates top-down empirical and bottom-up modelling approaches to show that land surface changes account for 73–81% of projected global runoff increases. This arises from synergistic effects of physiological responses of vegetation to rising CO2 concentration and responses of land surface—for example, vegetation cover and soil moisture—to radiatively driven climate change. Although climate change strongly affects regional runoff changes, it plays a minor role (19–27%) in the global runoff increase, due to cancellation of positive and negative contributions from different regions. Our findings highlight the importance of accurate model representation of land surface processes for reliable projections of global runoff to support sustainable management of water resources.
Surface-water availability, defined as precipitation minus evapotranspiration, can be affected by changes in vegetation. These impacts can be local, due to the modification of evapotranspiration and precipitation, or non-local, due to changes in atmospheric moisture transport. However, the teleconnections of vegetation changes on water availability in downwind regions remain poorly constrained by observations. By linking measurements of local precipitation to a new hydrologically weighted leaf area index that accounts for both local and upwind vegetation contributions, we demonstrate that vegetation changes have increased global water availability at a rate of 0.26 mm yr⁻² for the 2001–2018 period. Critically, this increase has attenuated about 15% of the recently observed decline in global water availability. The water availability increase is due to a greater rise in precipitation relative to evapotranspiration for over 53% of the global land surface. We also quantify the potential hydrological impacts of regional vegetation increases at any given location across global land areas. We find that enhanced vegetation is beneficial to both local and downwind water availability for ~45% of the land surface, whereas it is adverse elsewhere, primarily in water-limited or high-elevation regions. Our results highlight the potential strong effects of deliberate vegetation changes, such as afforestation programmes, on water resources beyond local and regional scales.
Terrestrial water storage (TWS) over the Tibetan Plateau, a major global water tower, is crucial in determining water transport
and availability to a large downstream Asian population. Climate change impacts on historical and future TWS changes, however,
are not well quantified. Here we used bottom-up and top-down approaches to quantify a significant TWS decrease (10.2 Gt yr–1)
over the Tibetan Plateau in recent decades (2002–2017), reflecting competing effects of glacier retreat, lake expansion and subsurface water loss. Despite the weakened trends in projected TWS, it shows large declines under a mid-range carbon emissions scenario by the mid-twenty-first century. Excess water-loss projections for the Amu Darya and Indus basins present a critical water resource threat, indicating declines of 119% and 79% in water-supply capacity, respectively. Our study highlights these two hotspots as being at risk from climate change, informing adaptation strategies for these highly vulnerable regions.
Quantifying the contributions of climate and vegetation to the dynamics of evapotranspiration (ET) and water yield (i.e., precipitation minus ET) will help us better understand the changes in the water budget. In this study, we identified the contributions of climate variables (including precipitation, radiation, temperature, and relative humidity), human-disturbed vegetation, and natural vegetation to the trends in annual ET and water yield over vegetated mainland China during 2001-2020, using a process-based terrestrial ecosystem model and a joint-solution method with multiple sensitivity numerical experiments. Results showed that 46% of the study area experienced significant (p<0.05) increases in ET, with an overall increase of 2.32 mm y −1. Meanwhile, the overall trend in water yield was 2.56 mm y −1 but insignificant. Spatially, vegetation and precipitation are the dominant factors for ET trends over 55% and 32% of vegetated mainland China, respectively. Over the regions where vegetation dominates the ET trends, nearly half of these regions are covered by human-disturbed vegetation (e.g., cropland or regions with land cover changes), suggesting that anthropogenic activities play a crucial role in the hydrological cycle there. Concerning the trends in water yield, precipitation is the dominant factor over 64% areas. Human-disturbed vegetation and natural vegetation play similar roles and combined can explain the water yield trends over 30% areas. Our study highlights the spatial variations in the mechanisms behind changes in the water budget over mainland China, particularly in regions covered by human-disturbed vegetation. This finding should be considered in the existing and future national ecological recovery policies to maximize its eco-hydrological benefits.
The evaporative loss from global lakes (natural and artificial) is a critical component of the terrestrial water and energy balance. However, the evaporation volume of these water bodies—from the spatial distribution to the long-term trend—is as of yet unknown. Here, using satellite observations and modeling tools, we quantified the evaporation volume from 1.42 million global lakes from 1985 to 2018. We find that the long-term average lake evaporation is 1500 ± 150 km3 year−1 and it has increased at a rate of 3.12 km3 year−1. The trend attributions include an increasing evaporation rate (58%), decreasing lake ice coverage (23%), and increasing lake surface area (19%). While only accounting for 5% of the global lake storage capacity, artificial lakes (i.e., reservoirs) contribute 16% to the evaporation volume. Our results underline the importance of using evaporation volume, rather than evaporation rate, as the primary index for assessing climatic impacts on lake systems. While the evaporative water loss from global lakes is invisible, the volume is substantial. In recent decades, lake evaporation volume has been significantly increasing due to enhanced evaporation rate, melting lake ice, and expansion of water extent.
The changes in near-surface soil freeze-thaw cycles (FTCs) are crucial to understanding the related hydrological and biological processes in terrestrial ecosystems under a changing climate. However, long-term dynamics of soil FTCs at the hemisphere scale and the underlying mechanisms are not well understood. In this study, the spatiotemporal patterns and main driving factors of soil FTCs across the Northern Hemisphere (NH) during 1979–2017 were analyzed using multisource data fusion and attribution approaches. Our results showed that the duration and the annual mean area of frozen soil in the NH decreased significantly at rates of 0.13 ± 0.04 days/year and 4.9 × 10⁴ km²/year, respectively, over the past 40 years. These were mainly because the date of frozen soil onset was significantly delayed by 0.1 ± 0.02 days/year, while the end of freezing and onset of thawing were substantially advanced by 0.21 ± 0.02 and 0.15 ± 0.03 days/year, respectively. Moreover, the interannual FTC changes were more drastic in Eurasia than in North America, especially at mid-latitudes (30°–45° N) and in Arctic regions (>75° N). More importantly, our results highlighted that near-surface air temperature (Ta) and snowpack are the main driving factors of the spatiotemporal variations in soil FTCs. Furthermore, our results suggested that the long-term dynamics of soil FTCs at the hemisphere scale should be considered in terrestrial biosphere models to reduce uncertainties in future simulations.
Actual terrestrial evapotranspiration (ETa) is a key parameter controlling land–atmosphere interaction processes and water cycle. However, spatial distribution and temporal changes in ETa over the Tibetan Plateau (TP) remain very uncertain. Here we estimate the multiyear (2001–2018) monthly ETa and its spatial distribution on the TP by a combination of meteorological data and satellite products. Validation against data from six eddy-covariance monitoring sites yielded root-mean-square errors ranging from 9.3 to 14.5 mm per month and correlation coefficients exceeding 0.9. The domain mean of annual ETa on the TP decreased slightly (-1.45 mm yr-1, p<0.05) from 2001 to 2018. The annual ETa increased significantly at a rate of 2.62 mm yr-1 (p<0.05) in the eastern sector of the TP (long >90∘ E) but decreased significantly at a rate of -5.52 mm yr-1 (p<0.05) in the western sector of the TP (long <90∘ E). In addition, the decreases in annual ETa were pronounced in the spring and summer seasons, while almost no trends were detected in the autumn and winter seasons. The mean annual ETa during 2001–2018 and over the whole TP was 496±23 mm. Thus, the total evapotranspiration from the terrestrial surface of the TP was 1238.3±57.6 km3 yr-1. The estimated ETa product presented in this study is useful for an improved understanding of changes in energy and water cycle on the TP. The dataset is freely available at the Science Data Bank (10.11922/sciencedb.t00000.00010; Han et al., 2020b) and at the National Tibetan Plateau Data Center (10.11888/Hydro.tpdc.270995, Han et al., 2020a).
In this study, the temporal variation of actual evapotranspiration (AE) over China during 1980–2015 is analysed based on an ensemble of six reanalyses and a complementary‐relationship‐based AE dataset. The results reveal that annual mean AE in China increases significantly, and the major regime shift occurred around 1998. Accordingly, long‐term mean AE changes from 1999–2015 relative to 1980–1997 are computed to quantify the spatial pattern of AE trends. In general, annual AE increases significantly in southern and northwestern China but decreases significantly in northeastern China and eastern Tibetan Plateau. Then we examine the primary cause for changes in AE using the Budyko framework, in which the change of climate is represented by changes in precipitation and potential evapotranspiration and the shift in landscape characteristics is represented by the parameter n. Overall, increasing potential evapotranspiration is the main contributor to the increase of AE in southeastern China. Increasing potential evapotranspiration and the parameter n are the main contributors to the increase of AE in southwestern China. Decreasing AE in northeastern China and eastern Tibetan Plateau are respectively caused by changes in precipitation and the parameter n, whereas the combination of these two factors has led to increasing AE in northwestern China. It is shown here that the contribution of the parameter n is comparable to that of climate change in western China. A positive correlation is found between vegetation coverage and the parameter n; however, vegetation type, catchment slope, irrigation practices, and glacial meltwater also have an impact on regional n. A concurrent occurrence of the negative contribution of parameter n and positive vegetation coverage change is found in the forest land of northeastern and southeastern China. Decreasing irrigation water is likely the main reason for the negative contribution of n in cropland of the North China Plain.
Evapotranspiration (ET) is composed of soil evaporation (ETs), canopy transpiration (ETc), and canopy intercepted water evaporation (ETw), and it plays an indispensable role in the water and energy cycles of land surface processes. More accurate estimations of variations in ET are essential for natural hazard monitoring and water resource management. An improved algorithm for ETs with soil moisture and texture (SMT) and an optimized ETc based on the MOD16 model (MOD16‐STM) are proposed for the cold, arid, and semiarid regions of the Tibetan Plateau (TP). The nonlinear relationships between the soil surface resistance (rss) and soil surface hydration state in different soil textures were redefined by using five flux tower measurements over the TP. The value of the mean potential stomatal conductance per unit leaf area (CL) used for ETc calculation in grasslands was optimized at 0.0038 (m s⁻¹). The MOD16‐STM was compared with the MOD16 and a soil water index‐based Priestley‐Taylor algorithm (SWI‐PT) at five independent sites. The results showed that the half‐hourly, daily, and monthly estimations from the MOD16‐STM were closer to observations than those from the other two models. The results indicated that the MOD16‐STM increased R² by 0.2/0.05 and the index of agreement by 0.37/0.23 and deceased RMSE by 43.89/42.77 W m⁻² and the absolute mean bias (|MB|) by 46.23/45.89 W m⁻², when compared to the results of the MOD16 at daily/monthly scales. The results suggested that the MOD16‐STM will produce more accurate ET estimates over the flux sites and has great potential for ET estimation and land surface model improvement for the TP region.
Warming generally leads to increased evaporative demand, altering the amount of water needed for growing crops. For the Midwest, some studies have suggested that reaching yield targets by 2050 will not be possible without additional precipitation or large expansion of irrigation. Here, we show that this claim is not supported by the historical summer climate trends, which indicate that the warming of daily average temperatures is largely driven by increases in minimum temperatures, while maximum temperatures have decreased. This has translated into a net decrease in vapor pressure deficit (VPD) and potential evapotranspiration (PET). With the increasing rainfall, this suggests that crop water deficits have likely become less frequent in the region despite the warming climate. By projecting these trends into 2050 and ancillary use of a crop model, we estimate minor changes in PET that would have minimal effects on corn yields (<6%) under persistence of these trends.
Human-induced climate change impacts the hydrological cycle and thus the availability of water resources. However, previous assessments of observed warming-induced changes in dryness have not excluded natural climate variability and show conflicting results due to uncertainties in our understanding of the response of evapotranspiration. Here we employ data-driven and land-surface models to produce observation-based global reconstructions of water availability from 1902 to 2014, a period during which our planet experienced a global warming of approximately 1 °C. Our analysis reveals a spatial pattern of changes in average water availability during the driest month of the year over the past three decades compared with the first half of the twentieth century, with some regions experiencing increased and some decreased water availability. The global pattern is consistent with climate model estimates that account for anthropogenic effects, and it is not expected from natural climate variability, supporting human-induced climate change as the cause. There is regional evidence of drier dry seasons predominantly in extratropical latitudes and including Europe, western North America, northern Asia, southern South America, Australia and eastern Africa. We also find that the intensification of the dry season is generally a consequence of increasing evapotranspiration rather than decreasing precipitation.
This study quantifies the influences of climate variation and vegetation greening on the changes in evapotranspiration (ET) over 110 humid catchments of China from 1982 to 2016. A process‐based model considering climate and vegetation changes is employed to estimate ET. The simulated ET compared well with the observed values based on the flux measurements and water balance equation, with the coefficients of determination above 0.62. During the study period, environmental change of the study area was characterized by the increases in air temperature and leaf area index, that is, climate warming and vegetation greening. Annual ET increased at 104 catchments with the trends ranging from 0.01 to 6.2 mm yr⁻². The numerical experiment approach and sensitivity method are used to attribute ET changes to climate variation and vegetation greening. The results show that the contributions of climate variation to ET changes estimated by the two methods are 90.6% and 91.3%, respectively, whereas the contributions of vegetation greening to ET changes are only 9.4% and 8.7%, indicating that climate variation controls ET changes in the study area. However, vegetation greening alters the proportions of ET components, which contributes 59.0%–62.0% to the increase in the ratio of transpiration to ET. This study quantifies the roles of climate and vegetation changes in the dynamics of ET, suggesting that more concerns should be paid on the interactions between ecological processes and hydrological cycle in humid regions.
Vegetation cover exerts a strong control on land‐atmosphere interactions. To quantify the relative effects of external forcing (climate change) versus internal forcing (anthropogenic activity) on recent vegetation change over the Tibetan Plateau (TP), we apply an ecohydrological diagnostic framework, developed from earlier work. We compare vegetation change during 1986–2015 based on NDVI (Normalized Difference Vegetation Index) data with changes in environmental conditions (European Centre for Medium‐Range Weather Forecasts Reanalysis 5th generation, ERA5). Results show that external forcing is the dominant factor behind significant vegetation change over the southeastern TP during 1986–2015. In the area with significant vegetation changing, 60.5%/41.5% of pixels have experienced a respective wetting/drying of climate, which in turn has supported greening/browning during 1986–2005/1996–2015. However, during the greening/browning transition in the latter period, the proportion of internal forcing on browning increased from 5.62% to 19.4%, indicating that anthropogenic factors are playing an increasingly role on impacting vegetation change in recent decades.
Evapotranspiration (ET) is a pivotal process for ecosystem water budgets and accounts for a substantial portion of the global energy balance. In this paper, the exited actual ET main datasets in global scale, and the global ET modeling and estimates were focused on discussion. The Source energy balance (SEB) models, empirical models and other process-based models are summarized. Accuracy for ET estimates by SEB models highly depends on accurate surface temperature retrieval, and SEB models are hard to apply in large heterogeneous surface. The Penman–Monteith (PM) equations are thought to be with considerable sound mechanism. However, it involves large number of parameters, which are not all global available. A simplified PM equation by Priestley and Taylor (PT) is found to perform well on well-watered surface. For both PM and PT equations in estimating ET, the key is to consider the constraint from surface resistance primarily water stress. Empirical models are simple but the accuracy of which highly depends on training samples. Coupling satellite data into ET models can improve ET estimates with higher resolution spatiotemporal information inputs; However, finding the most proper way to estimate global ET remains problematic. Several reasons for this issue are also analyzed in this review.
Evapotranspiration (ET) is usually difficult to estimate at the regional scale due to scarce direct measurements. This study uses the water balance equation to calculate the regional ET with observations of precipitation, runoff, and terrestrial water storage changes (TWSC) in nine exorheic catchments of China. We compared the regional ET estimates from a water balance perspective with and without considering TWSC (ETWB: ET estimates with considering TWSC, and ETPQ: ET estimates from precipitation minus runoff without considering TWSC). Results show that the regional annual ET ranges from 417.7 mm/yr to 831.5 mm/yr in the nine exorheic catchments based on the water balance equation. The impact of ignoring TWSC on calculating ET is notable, as the root mean square errors (RMSEs) of annual ET between ETWB and ETPQ range from 12.0–105.8 mm/yr (2.6%–12.7% in corresponding annual ET) among the exorheic catchments. We also compared the estimated regional ET with other ET products. Different precipitation products are assessed to explain the inconsistency between different ET products and regional ET from a water balance perspective. The RMSEs between ET estimates from Gravity Recovery and Climate Experiment (GRACE) and ET from land surface models can be reduced if the deviation of precipitation forcing data is considered. ET estimates from Global Land Evaporation Amsterdam Model (GLEAM) can be improved by reducing the uncertainty of precipitation forcing data in three semiarid catchments. This study emphasizes the importance of considering TWSC when calculating the regional ET using a water balance equation and provides more accurate ET estimates to help improve modeled ET results.
Satellite remote sensing combined with water balance calculations provides a promising approach to estimating evapotranspiration (ET), a critical variable in water-energy exchange. Here we compare ET estimates from terrestrial and atmospheric water balances, multisource remote sensing (AVHRR, GLEAM, and MOD16), and a land surface model (GLDAS NOAH) for headwaters on the Tibetan Plateau (TP), including headwaters of the Brahmaputra River (HBR), Salween (HSR), Mekong (HMR), Yangtze (HYR), and Huang (Yellow) (HHR) rivers for the 2003‒2012 period. Results show that: (1) ET estimated from terrestrial and atmospheric water balances agrees closely in three basins (HMR, HYR, and HHR) but has large discrepancies in the other two basins (HBR and HSR), mainly caused by uncertainties in the terrestrial water balance; (2) agreement between various ET products and water balance-derived ET baselines is highest for GLEAM in two basins (HMR and HYR) and GLDAS NOAH in another two basins (HSR and HHR); and (3) large discrepancies between water balance-derived ET and all ET products are found in the most glacierized HBR, which may reflect the importance of sublimation in the ET process. The decadal mean ET based on water balance-derived ET baselines is highest in the HHR (447 mm/yr) and HSR (430 mm/yr) and lowest in the HBR (238 mm/yr), ranging from 51 to 78% of mean precipitation in the five TP headwaters. These findings have important implications for ET estimation on the TP headwaters, which greatly influences downstream water availability.
Atmospheric vapor pressure deficit (VPD) is a critical variable in determining plant photosynthesis. Synthesis of four global climate datasets reveals a sharp increase of VPD after the late 1990s. In response, the vegetation greening trend indicated by a satellite-derived vegetation index (GIMMS3g), which was evident before the late 1990s, was subsequently stalled or reversed. Terrestrial gross primary production derived from two satellite-based models (revised EC-LUE and MODIS) exhibits persistent and widespread decreases after the late 1990s due to increased VPD, which offset the positive CO 2 fertilization effect. Six Earth system models have consistently projected continuous increases of VPD throughout the current century. Our results highlight that the impacts of VPD on vegetation growth should be adequately considered to assess ecosystem responses to future climate conditions.
Evapotranspiration is the key driving factor of the earth's water cycle, and an important component of surface water and energy balances. Therefore, it also reflects the geothermal regulation function of ecohydrological process. The Qinghai-Tibet Plateau is the birthplace of important rivers such as the Yangtze River and the Yellow River. The regional water balance is of great significance to regional ecological security. In this study, ARTS, a dual-source remote sensing evapotranspiration model developed on a global scale, is used to evaluate the actual evapotranspiration (ET) in the Qinghai-Tibet Plateau from 1982 to 2014, using meteorological data interpolated from observations, as well as FPAR and LAI data obtained by satellite remote sensing. The characteristics of seasonal. interannual and dynamic changes of evapotranspiration were analyzed. The rates at which meteorological factors contribute to evapotranspiration are calculated by sensitivity analysis and multiple linear regression analysis, and the dominant factors affecting the change of evapotranspiration in the Qinghai-Tibet Plateau are discussed. The results show that: (1) The estimated values can explain more than 80% of the seasonal variation of the observed values (R 2 = 0.80, P < 0.001), which indicates that the model has a high accuracy. (2) The evapotranspira-tion in the whole year, spring, summer and autumn show significant increasing trends in the past 30 years, but have significant regional differences. Whether in the whole year or in summer, the southern Tibetan Valley shows a significant decreasing trend (more than 20 mm per 10 years), while the Ali, Lhasa Valley and Haibei areas show increasing trends (more than 10 mm per 10 years). (3) Sensitivity analysis and multiple linear regression analysis show that the main factor driving the interannual change trend is climate warming, followed by the non-significant increase of precipitation. However, vegetation change also has a considerable impact, and together with climate factors, it can explain 56% of the interannual variation of evapotranspiration (multiple linear regression equation R 2 = 0.56, P < 0.001). The mean annual evapotranspiration of low-cover grassland was 26.9% of high-cover grassland and 21.1% of medium-cover grassland, respectively. Considering significant warming and insignificant wetting in the Qinghai-Tibet Plateau, the increase of surface evapotranspiration will threaten the regional ecological security at the cost of glacial melting water. Effectively protecting the ecological security and maintaining the sustainable development of regional society are difficult and huge challenges.
Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.
Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural
and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should
be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available
renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion
people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is
expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living
under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect.
Reducing current vulnerability will be the first step to prepare for such anticipated changes.
Drylands are particularly sensitive to climate change, and soil water availability is critical for dryland ecosystems. Climate change and large-scale ecological restoration have led to significant vegetation greening and soil moisture (SM) variations in the drylands. However, the influencing mechanisms of climate change and vegetation greening on SM variability were not well understood. In this study, the spatiotemporal variations of climate variables, vegetation cover, surface SM (SM_surf) and root-zone SM (SM_root) along aridity gradient in the drylands of northern China during 1981-2018 were investigated. The random forest model and partial least-squares structural equation modelling (PLS-SEM) analysis were employed to quantify the contributions of climate change and vegetation greening to SM variability and identify the influencing pathways. The climate change showed temporal trends of wetting to drying before 2000 and drying to more wetting in 2000-2018, and vegetation greening was significant in 2000-2018. The variations of SM displayed obvious east-negative and west-positive trends. The temporal trend of SM deficit decreased during 1981-2018 in most regions, and SM showed a significant increasing trend in all regions of different aridity gradient after 2000. The correlations of climate factors and vegetation cover with SM became stronger in the area with lower aridity. The vegetation greening-induced SM change was limited compared to climate change. The climate factors can directly impact SM availability, and also indirectly influence SM availability through vegetation (e.g., precip-itation→LAI→SM_surf). The SM_surf had positive responses to vegetation greening, especially in semiarid sub-area. In contrast, vegetation greening exacerbated SM_root consumption in semiarid and dry subhumid subareas, which became intensive after 2000. This study fills the knowledge gap in the underlying reasons for the variations of both surface and root zone SM in water-limited areas and reveals the potential risks of SM depletion, particularly in dry subhumid areas.
A R T I C L E I N F O Edited by Jing M. Chen Keywords: Evaporation Transpiration Soil moisture active passive mission SMAP L4_SM product A B S T R A C T The interplay between soil moisture and evapotranspiration modulates the water available to sustain soil evaporation and influences canopy stomatal conductance controls on vegetation transpiration. Modeling this behavior remains challenging. Indeed, satellite remote sensing based Penman-Monteith (PM) ET models tend not to directly consider soil moisture constraints on evaporation and transpiration due to a lack of consistent soil moisture data. To address this issue, we modified a PM model to include satellite enhanced surface and root zone soil moisture from the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) mission. The resulting model was used to produce global 9-km daily ET estimates, including contributing fluxes from soil evaporation, transpiration and evaporation of canopy-intercepted precipitation. The global PM ET estimates were assessed using in situ sap flow and AmeriFlux measurements, upscaled FLUXCOM latent heat flux, as well as against other independent global ET data products (GLEAM, GLDAS, SSEBop, and LandFlux-EVAL). The modelled transpiration showed similar seasonal variation and positive correlation to in situ sap flow measurements available from several forest sites (R 2 = 0.85; p < 0.01). When compared against AmeriFlux data, the PM ET estimates showed favorable agreement with annual ET measurements extracted from 34 diverse sites (R 2 = 0.58; p < 0.01; RMSE = 227 mm yr − 1). The PM 8-day ET results also reflected a similar hemispheric seasonality as the global FLUXCOM record (R 2 = 0.94-0.98; p < 0.01), while comparisons against other global ET products showed moderate mean differences of between 49 and 107 mm yr − 1 (11-25%) over the global domain. Our PM ET estimates varied up to 52% in response to SMAP surface soil moisture dynamics, displaying stronger surface (0-5 cm depth) than root zone (0-100 cm) soil moisture sensitivity. While PM ET sensitivity to soil moisture was greater in arid climate regions, it was also significant in humid climate zones, with analysis indicating that the inclusion of soil moisture predominantly acts as a sustaining influence on ET, especially in moisture limited drylands. PM ET sensitivity to temperature was stronger in humid forest regions relative to other climate and land cover regimes. Overall, the model results clarify the influence of soil moisture hetero-geneity on the global ET pattern as informed by satellite-based estimates of surface and root zone soil moisture. Potential enhancements to the spatial and vertical resolution of soil moisture inputs are expected to enable further ET improvements through more realistic model representation of soil and plant available water.
Satellite remote sensing technology provides a promising means for near real-time monitoring of crop water status and requirements in agricultural and hydrological applications. Estimation of actual evapotranspiration (ETa) often requires thermal information; however, not every satellite is equipped with a thermal sensor, which limits the estimation of ETa. To address this limitation, here we propose a satellite-based ETa estimation model, OPTRAM-ET, based on the optical trapezoid model (OPTRAM) estimates of soil moisture and a vegetation index (VI). We applied the OPTRAM-ET model to Sentinel-2 and Landsat-8 satellite data and evaluated the model for ETa estimates using 16 eddy covariance flux towers in the United States and Germany with different landcover types, including agriculture, orchard, permanent wetland, and foothill forests. Next, OPTRAM-ET was compared with the conventional land surface temperature (LST)-VI model. The proposed OPTRAM-ET model showed promising performance over all the studied landcover types. In addition, OPTRAM-ET showed comparable performance to the conventional LST-VI model. However, since the OPTRAM-ET model does not need thermal data, it benefits from higher spatial and temporal resolution data provided by ever-increasing drone- and satellite-based optical sensors to predict crop water status and demand. Unlike the LST-VI model, which needs to be calibrated for each satellite image, a temporally-invariant region-specific calibration is possible in the OPTRAM-ET model. Therefore, OPTRAM-ET is substantially less computationally demanding than the LST-VI model.
Study region
The ecological barrier region is located in the northeastern of the Qinghai-Tibet Plateau, western China, serves as an important ecological security barrier and water conservation zone. Owing to its unique geographical location, this region has a sensitive and fragile ecosystem.
Study focus
This study comprehensively quantified the water conservation volume, analyzed the spatiotemporal variations and its influencing factors, predicted the future changes of water conservation under different climate scenarios.
New hydrological insights for the region
The results showed that the average water conservation volume of the ecological barrier region was approximately 254 × 10⁸ m³ from 1980 to 2019 with a increasing trend. The spatial distribution of water conservation showed a decreasing trend from southeast to northwest. There is a significant positive correlation between precipitation and water conservation (P < 0.01), the water conservation function in the area with elevation between 3 500–4 500 m and slope less than 10°is obviously stronger than other areas. From 2021–2100, the overall trend of water conservation in the study region is increasing, only part of the Yellow River source’s water conservation has a decreasing trend. This research provides scientific support for optimal allocation of water resources and sustainable development in alpine regions.
Water conservation (WC) is a key ecosystem service offered by dryland mountains given its effect in flood prevention and drought relief. However, the consequence of climate change on WC services is largely unknown, particularly in mountain areas with limited data availability. By using a parametric model driven by ground and satellite remote sensing data, the spatial distribution of WC and its inter-annual trends and variability were assessed in the Qilian Mountains to differentiate the main factors for WC dynamics. Our results showed that the spatiotemporal variation of modeled WC and remotely sensed soil moisture was consistent. Mean annual WC showed a contrary trend with increasing annual mean air temperature and precipitation and was higher in forests than grasslands and deserts. In the context of warm and humid climates, percentage of the area with a significant increase and decrease in annual WC from 2000 to 2019 was 32.2% and 4.2% (p < 0.05), respectively. WC declines appeared mainly in a forest steppe zone at an altitude of 2600–3300 m, as increased vegetation growth boosted water consumption and eventually led to soil drying. Increases in WC were largely attributed to increasing precipitation and decreasing solar radiation, making a contribution of 39.4 and 38.4%, respectively. WC was positively correlated with precipitation but negatively related to land surface temperature (p < 0.05). A negative relationship between WC and solar radiation was prevalent in deserts. Precipitation and vegetation growth explained 50.1% and 22.8%, respectively, of the inter-annual variability in WC. The findings of this study highlight the importance of evapotranspiration in controlling WC dynamics in the forest steppe zone of a dryland mountain, where a risk from reduced water conservation may increase under future climate warming. To conserve grassland and desert ecosystems is more beneficial to improve the WC function of the Qilian Mountains as compared to afforestation.
While terrestrial evapotranspiration (ET) from the Tibetan Plateau (TP) plays a key role in modulating water storage change in the Asian Water Tower, the magnitude, trend, and drivers of ET remain poorly understood in this region due partially to sparse ground measurements. This study used a water-carbon coupled biophysical model, Penman-Monteith-Leuning Version 2 (PML_V2), to characterize the variations in ET across TP during 1982–2016 and its drivers. Model parameters of PML_V2 were calibrated against ground-observed data from 14 eddy-covariance flux towers. Plot- and basin-scale validations demonstrate that the PML_V2 is robust enough in simulating both magnitude and trend in ET. The 35-year mean annual ET rates decrease from the southeastern to the northwestern TP, leading to a TP-averaged value of 353 ± 24 mm yr⁻¹. Soil evaporation is the main component (64%) of ET, followed by plant transpiration (31%) and canopy evaporation (5%). From 1982 to 2016, TP-averaged ET increased significantly with a rate of 1.87 ± 0.25 mm yr⁻² (p < 0.001) due primarily to precipitation enhancement. Spatially, precipitation is the dominant driver that controls ET trend over most parts of TP except certain regions in the southeastern and eastern TP, where net radiation and temperature do so instead, respectively. This is because 68% of the TP area is dryland with the aridity index < 0.65. While LAI appears less important than climate factors over much of TP, its relative contribution to ET trend exceeds 20% in many parts of eastern TP, indicating that vegetation change played a nonnegligible role in regulating annual ET variations over certain regions where LAI varied substantially. Our results are of vital importance for facilitating the understanding of hydrological processes over the Asian Water Tower.
Terrestrial evapotranspiration (ET) refers to a key process in the hydrological cycle by which water is transferred from the Earth's surface to lower atmosphere. With spatiotemporal variations, ET plays a crucial role in the global ecosystem and affects vegetation distribution and productivity, climate, and water resources. China features a complex, diverse natural environment, leading to high spatiotemporal heterogeneity in ET and climatic variables. However, past and future ET trends in China remain largely unexplored. Thus, by using MOD16 products and meteorological datasets, this study examined the spatiotemporal variations of ET in China from 2000 to 2019 and analyzed what is behind changes, and explored future ET trends. Climate variation in China from 2000 to 2019 was statistically significant and had a remarkable impact on ET. Average annual ET increased at a rate of 5.3746 mm yr⁻¹ (P < 0.01) during the study period. The main drivers of the trend are increasing precipitation and wind speed. The increase in ET can also be explained to some extent by increasing temperature, decreasing sunshine duration and relative humidity. The zonation results show that the increase in temperature, wind speed, and precipitation and the decrease in relative humidity had large and positive effects on ET growth, and the decrease in sunshine duration had either promoting or inhibiting effects in different agricultural regions. Pixel-based variations in ET exhibited an overall increasing trend and obvious spatial volatility. The Hurst exponent indicates that the future trend of ET in China is characterized by significant anti-persistence, with widely distributed areas expected to experience a decline in ET. These findings improve the understanding of the role of climate variability in hydrological processes, and the ET variability in question will ultimately affect the climate system.
High-throughput mapping of latent heat flux (λET) is critical to efforts to optimize water resources management and to accelerate forest tree breeding for improved drought tolerance. Ideally, investigation of the energy response at the tree level may promote tailored irrigation strategies and, thus, maximize crop biomass productivity. However, data availability is limited and planning experimental campaigns in the field can be highly operationally complex. To this end, a multi-platform multi-sensor observational approach is herein developed to dissect the λET signature of a black poplar (Populus nigra) breeding population (“POP6”) at the canopy level. POP6 comprised more than 4600 trees representing 503 replicated genotypes, whose parents were derived from contrasting environmental conditions. Trees were trialed in two adjacent plots where different irrigation treatments (moderate drought [mDr] and well-watered [WW]) were applied. Data collected from satellite and unmanned aerial vehicles (UAVs) remote sensing as well as from ground-based proximal sensors were integrated at consistent spatial aggregation and combined to compute the surface energy balance of the trees through a modified Priestley-Taylor method. Here, we demonstrated that λET response was significantly different between WW and mDr trees, whereby genotypes in mDr conditions exhibited larger standard deviations. Importantly, genotypes classified as drought tolerant based on the stress susceptibility index (SSI) presented λET values significantly higher than the rest of the population. This study confirmed that water limitation in mDr settings led to reduced soil moisture in the tree root zone and, thus, to lower λET. These results pave the way to breeding poplar and other bioenergy crops with this underexploited trait for higher λET. Most notably, the illustrated work demonstrates a multi-platform multi-sensor data fusion approach to tackle the global challenge of monitoring landscape-scale ecosystem processes at fine resolution.
The Tibetan Plateau (TP), known as the “Third Pole”, is a climate-sensitive and ecology-fragile region. In this study, the spatio-temporal trends of soil moisture (SM) and vegetation were analyzed using satellite-based ESA CCI SM and MODIS LAI data respectively in the growing season during the last 20 years (2000-2019) over the TP covering diverse climate zones. The climatic drivers (precipitation and air temperature) of SM and LAI variations were fully investigated by using both ERA5 reanalysis and observation-based gridded data. The results reveal the TP is generally wetting and significantly greening in the last 20 years. The SM with significant increasing trend accounts for 21.80% (fraction of grid cells) of the TP, and is about twice of the SM with significant decreasing trend (10.19%), while more than half of the TP (58.21%) exhibits significant increasing trend of LAI. Though the responses of SM and LAI to climatic factors are spatially heterogeneous, precipitation is the dominant driver of SM variation with 48.36% (ERA5) and 32.51% (observation-based) precipitation data showing the strongest significant positive partial correlation with SM. Temperature rise largely explains the vegetation greening though precipitation also plays an important role in vegetation growth in arid and semi-arid zones. The combined trend of SM and LAI indicates the TP is mainly composed of wetting and greening areas, followed by drying and greening regions. The change rate of SM is negative at low altitudes and becomes positive as altitude increases, while the LAI value and its change rate decrease as altitude increases.
China has increased its vegetation coverage and enhanced its terrestrial carbon sink through ecological restoration since the end of the 20th century. However, the temporal variation in vegetation carbon sequestration remains unclear, and the relative effects of climate change and ecological restoration efforts are under debate. By integrating remote sensing and machine learning with a modelling approach, we explored the biological and physical pathways by which both climate change and human activities (e.g., ecological restoration, cropland expansion, and urbanization) have altered Chinese terrestrial ecosystem structures and functions, including vegetation cover, surface heat fluxes, water flux and vegetation carbon sequestration (defined by gross and net primary production, GPP and NPP). Our study indicated that during 2001~2018, GPP in China increased significantly at a rate of 49.1~53.1 TgC/yr2, and the climatic and anthropogenic contributions to GPP gains were comparable (48%~56% and 44%~52%, respectively). Spatially, afforestation was the dominant mechanism behind forest cover expansions in the farming‐pastoral ecotone in northern China, on the Loess Plateau and in the southwest karst region, while climate change promoted vegetation cover in most parts of southeastern China. At the same time, the increasing trend in NPP (22.4~24.9 TgC/yr2) during 2001~2018 was highly attributed to human activities (71%~81%), particularly in southern, eastern and northeastern China. Both GPP and NPP showed accelerated increases after 2010 because the anthropogenic NPP gains during 2001~2010 were generally offset by the climate‐induced NPP losses in southern China. However, after 2010, the climatic influence reversed, thus highlighting the vegetation carbon sequestration that occurs with ecological restoration.
The Ningxia Hui Autonomous Region (NX) in Northwest China has been challenged by water scarcity and drought for decades. In this study, to understand the spatio-temporal variation, cause analysis and relationship with atmospheric circulation of ET0 in Ningxia, ET0 and other climate factors at 20 national climate stations from 1957 to 2018 were analyzed. Results showed that ET0 in NX (Ningxia), NYR (Northern Yellow River Irrigation Area) and SMA (Southern Mountain Area) had increased significantly at annual scale, whilst the CAZ (Central Arid Zone) was the opposite trend, and ET0 had a trend of first rise and then decline from north to south in spatial distribution. ET0 was most sensitive to RH and Tmax at annual scale in Ningxia, while the greatest contribution rates were Tmax and SD. Ningxia was becoming drier in the past decades. The abrupt change in ET0 at approximately 1990, and it’s long and short period were 25a(15a) and 10a(5a) at annual scale, respectively. The four teleconnection indices could be used to predict changes in ET0 at annual and autumn scale, while the ENSO and PDO could predict changes in ET0 of summer and IOD and AO could predict changes in ET0 of spring and winter.
The Lake Naivasha Basin in Kenya has experienced significant land use cover changes (LUCC) that has been hypothesized to have altered the hydrological regime in recent decades. While it is generally recognized that LUCC will impact evapotranspiration (ET), the precise nature of such impact is not very well understood. This paper describes how land use conversions among grassland and croplands have influenced ET in the Lake Naivasha Basin for the period 2003 to 2012. MODIS data products were used in combination with the European Centre for Medium-Range Weather Forecasts (ECMWF) data sets to model ET using the Surface Energy Balance System (SEBS). The results indicate that conversions from grassland to cropland accounted for increases in ET of up to 12% while conversion from cropland back to grasslands (abandonment) reduced ET by ~4%. This suggests that recently cultivated agricultural lands could increase local water demands, while abandonment of the farms could decrease the water loss and eventually increase the water availability. Also, recovery of ET following re-conversion from cropland to grassland might be impeded due to delayed recovery of soil properties since parts of the catchment are continuously being transformed with no ample time given for soil recovery. The annual ET over the 10 years shows an estimated decline from 724 mm to 650 mm (~10%). This decline is largely explained by a reduction in net radiation, an increase in actual vapour pressure whose net effect also led to decrease in the air-surface temperature difference. These findings suggest that in order to better understand LUCC effects on water resources of Lake Naivasha, it is important to take into account the effect of LUCC and climate because large scale changes of vegetation type from grassland to cropland substantially will increase evapotranspiration with implications on the water balance.
The partitioning of evapotranspiration (ET) is a critical factor in the terrestrial water balance and global water cycle, and understanding the partitioning across terrestrial biomes and the relationships between ET partitions and potential influencing factors is critical for predicting future ecosystem feedbacks. Based on an optimized Priestly-Taylor Jet Propulsion Laboratory model, we partitioned ET into three components transpiration (T), canopy interception evaporation (EI), and soil evaporation (ES). We found the components of EI to be significant with the ratio of EI to precipitation ranging from 0.02 to 0.29 across different biomes. The T/ET ratio ranged from 0.29 to 0.72 with obvious differences across biomes and with ratios generally lower than in previous studies with isotope-based methods. The (T + EI)/ET ratio was limited to a relatively narrow band from 0.57 to 0.86. The T/ET values show an obvious decreasing trend with increasing annual precipitation, but there was no significant correlation between T/ET and annual leaf area index.
The Food and Agriculture Organization of the United Nations (FAO) has recommended the Penman-Monteith (FAO56 P-M) method as a standard method for estimating reference evapotranspiration (ET0) and for evaluating other methods. But the FAO56 P-M method requires many parameters that are not available in many developing regions of high elevation in Tibet. Meanwhile, the low pressure, strong solar radiation, intensive evaporation, and frequent heat transfer are special meteorological phenomena in high-elevation areas. Accordingly, the basic objective of this study is to develop a new equation requiring fewer parameters for simulating the ET0 at high-elevation stations. When solar radiation, relative humidity and/or wind speed data are missing, a possible exception is the Hargreaves-Samani (HS) method which has shown reasonable ET0 results with a global validity according to the FAO's Irrigation and Drainage Paper No. 56. Therefore, the new equation (HS-E) based on HS equation and elevation was developed using the meteorological data of nine stations in the Tibet Plateau from 1981 to 1990. Then the HS-E and HS equation, which requires less meteorological data to calculate ET0, were evaluated as compared to the FAO56 P-M method. Results indicate that the computations of the improved HS-E model are obviously better than the HS model for the areas of higher than 2000m under the conditions of different time scale. The new mode, which enhanced the practical applications and computational accuracy of ET0, can make up the shortage of HS model that the ET0 would have negative values in arctic-alpine region of Tibet. Compared with the ET0 calculated by FAO56 P-M equation, the daily ET0 and monthly ET0 calculated by HS-E model were analyzed. The Nash-Sutcliffe efficiency coefficient (NSE), root-mean-square error (RMSE) and Mean Relative Error(MRE)for the daily ET0 calculated by HS-E model are 0.8, 0.53 mm/d and 13.80% and for the monthly ET0 are 0.84, 11.90 mm/month and 12.50%, respectively, which indicated that the HS-E model is high-quality and can calculate the ET0 more exactly. Considering the error result of different time scales, the larger the time scale, the better the results can be obtained by the HS-E model. In conclusion, the HS-E model is suitable and accurate in the high-altitude district, which can be recommended as a simple method for calculating ET0 in the area higher than 2000m.
In order to making true the exact calculation of reference crop evapotranspiration (ET0) and increase the accuracy with the lack of meteorological data in the large area, the Yangtze River Basin is divided into upstream, midstream and downstream according to the altitude of the stations. A new method of space distribution based on Inverse Distance Weighted Interpolation method is raised which can present every substream, including upstream, midstream and downstream. This method can include the connection of different stations. There are 102 meteorological stations which can provide 50 years of daily meteorological data from 1963 to 2013. In this research, we used the methods of Penman-Monteith (P-M), Hargreaves-Samani (H-S), Irmark-ALLEN (I-A), Pristley-Taylor (P-T), Makkink (M-K), Penman-Van Bavel (PVB), 1948 Penman (48-PM) to calculate daily ET0 of every stations of the Yangtze River Basin. The method of Penman-Monteith can be used to be the standard method to calculate standard daily ET0 to evaluate other methods because of its accuracy. The coefficient of Nash-Sutcliffe, the daily relative root mean square error, the consistency coefficient of Kendall can be used to evaluate the precision index of the method. The result showed that the best method of daily ET0 imitative effect compared with P-M is PVB in the three substreams, because the slope of the imitative equation of PVB in upstream is 0.946, the slope in midstream is 1.065, and the slope in downstream is 1.005. The method of Pristley-Taylor has a better effect in the midstream and downstream, and the slopes of the imitative equation are 1.030 and 1.201.The method of Makkink also has a good effect in the midstream and downstream, and the slopes are 0.857 and 0.936. The determination coefficient of daily ET0 imitative equation of these six methods all achieved very significant levels (α =0.01) in three substreams. The methods of Pristley-Taylor and Penman-Van Bavel have high calculation accuracy in all area of the Yangtze River Basin, and the highest absolute error of monthly ET0 is 0.55 mm/d using the method of Pristley-Taylor, at the same time the highest absolute error of monthly ET0 is 0.48 mm/d using the method of Penman-Van Bavel. The effect of the methods of Hargreaves-Samani and Irmark-ALLEN are worse than other methods to calculate monthly ET0 in the whole Yangtze River Basin. The method of Pristley-Taylor is the best method to calculate ET0 in the upstream of the Yangtze River Basin, because the daily relative root mean square error is 0.341 mm/d, the coefficient of Nash-Sutcliffe is 0.886, and the consistency coefficient of Kendall is 0.829. The method of Penman-Van Bavel is the best method to calculate ET0 in the midstream and downstream of the Yangtze River Basin. In the midstream, the daily relative root mean square error is 0.201 mm/d, the coefficient of Nash-Sutcliffe is 0.973, and the coefficient of Nash-Sutcliffe is 0.926. In the downstream, the daily relative root mean square error is 0.306 mm/d, the coefficient of Nash-Sutcliffe is 0.954, and the consistency coefficient of Kendall is 0.869. In the Yangtze River Basin, the relative error of Pristley-Taylor and Penman-Van Bavel are the lowest among these methods which are less than 35%, the relative error of is the highest among these methods which is more than 40%. In conclusion, the method of Pristley-Taylor and Penman-Van Bavel are the best methods to calculate ET0 in the Yangtze River Basin, the calculation process are simple at the same time. The method of Pristley-Taylor and Penman-Van Bavel can be a simplified recommendation of calculating ET0 in the Yangtze River Basin. © 2016, Chinese Society of Agricultural Engineering. All right reserved.
Trend analyses of evaporation data were conducted for 48 sites on the Canadian Prairies for three analysis periods. Significant trends were identified using the Mann–Kendall statistical test for trend and a bootstrap resampling technique. Trends in calculated evaporation were compared with trends in input variables used to calculate evaporation for all sites and with trends in pan evaporation for four sites. June, July, August, October and warm season evaporation revealed significant trends that were mainly decreasing. The longest analysis period identified an increasing trend in April. Increasing trends were typically in the more northern regions and decreasing trends in the more southern regions. Examining causal mechanisms for evaporation revealed that wind speed had more of an influence on decreasing trends and vapour pressure deficit had more of an influence on increasing trends.
Numerous models of evapotranspiration have been published that range in data-driven complexity, but global estimates require a model that does not depend on intensive field measurements. The Priestley–Taylor model is relatively simple, and has proven to be remarkably accurate and theoretically robust for estimates of potential evapotranspiration. Building on recent advances in ecophysiological theory that allow detection of multiple stresses on plant function using biophysical remote sensing metrics, we developed a bio-meteorological approach for translating Priestley–Taylor estimates of potential evapotranspiration into rates of actual evapotranspiration. Five model inputs are required: net radiation (Rn), normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), maximum air temperature (Tmax), and water vapor pressure (ea). Our model requires no calibration, tuning or spin-ups. The model is tested and validated against eddy covariance measurements (FLUXNET) from a wide range of climates and plant functional types—grassland, crop, and deciduous broadleaf, evergreen broadleaf, and evergreen needleleaf forests. The model-to-measurement r2 was 0.90 (RMS = 16 mm/month or 28%) for all 16 FLUXNET sites across 2 years (most recent data release). Global estimates of evapotranspiration at a temporal resolution of monthly and a spatial resolution of 1° during the years 1986–1993 were determined using globally consistent datasets from the International Satellite Land-Surface Climatology Project, Initiative II (ISLSCP-II) and the Advanced Very High Resolution Spectroradiometer (AVHRR). Our model resulted in improved prediction of evapotranspiration across water-limited sites, and showed spatial and temporal differences in evapotranspiration globally, regionally and latitudinally.
Forestry management policies in China have changed direction to encourage sustainability while balancing land-use, economic growth, and demand for forest products.