Response Of Evapotranspiration Research Articles

Response of leaf nutrients, yield, growth parameters, and evapotranspiration of sweet corn (Zea mays L. saccharata Sturt) to shallow and saline groundwater depths

ABSTRACT This study was carried out to assess the effects of main and interactive groundwater salinities (GWS) (GWS1:0.38, GWS2: 5.0, and GWS3: 10 dS m−1) and groundwater depths (GWD) (GWD1: 0.30, GWD2: 0.55, and GWD3: 0.80 m) on yield and growth parameters of sweet corn. The canned seed yield in the GWS2 and GWS3 treatments reduced by 21.97% and 54.87%, respectively, as compared to the GWS1. Increasing GWS from 0.38 to 5.0 and 10.0 dS m−1 decreased aboveground fresh biomass by 16.67 and 37.95%, respectively. The groundwater contribution to ET varied from 26.80% to 87.10%, and capillary rise from groundwater to plant root zone decreased significantly with increasing GWD. The results showed that plant growth was not significantly affected under 0.38 dS m−1 GWS conditions at 0.55 m GWD, while the groundwater contribution to ET was about 52%. Stomata and SPAD values increased by 36.36% and 4.34%, respectively, in GWD3 treatment compared to GWD1 treatment. Besides, GWS decreased the leaf’s K+, K+/Na+, and Ca2+/Na+ ratios at different depths. In conclusion, it can be suggested that GWD should be at least 0.80 m to achieve high sweet corn production in regions with moderately or highly saline groundwater.

Using SWAT to model the response of evapotranspiration and runoff to varying land uses and climatic conditions in the Muringato basin, Kenya

Using SWAT to model the response of evapotranspiration and runoff to varying land uses and climatic conditions in the Muringato basin, Kenya

Estimation of evapotranspiration using all-weather land surface temperature and variational trends with warming temperatures for the River Source Region in Southwest China

• All-weather LST is used to overcome satellite TIR LST missing in cloudy conditions. • Estimation scheme for all-weather evapotranspiration (AWET) is proposed. • Response of AWET to the warming temperatures exhibits a spatiotemporal heterogeneity. Land surface temperature (LST) derived from satellite thermal infrared (TIR) remote sensing is widely used in estimating land surface evapotranspiration (ET) through energy balance theory. However, as satellite TIR remote sensing is frequently affected by clouds, the derived LST with spatial missing makes it impossible to estimate seamless ET for large areas. In this study, based on the all-weather LST (AWLST) generated through the synergistic use of TIR and passive microwave (PMW) remote sensing, we propose an estimation scheme of all-weather ET (AWET) for the River Source Region of Southwest China (RSR-SC) with complex environmental characteristics of frequent clouds and fog. Specifically, the parameters of the Surface Energy Balance System (SEBS), which have high impact on the model results, are first accurately determined by a global sensitivity analysis. Second, the parametric calculation schemes of turbulent exchange, such as surface roughness heights for momentum ( z 0m ) and heat ( z 0h ) transfer, and soil heat flux ( G 0 ) of the SEBS model are refined. Third, the daily ET temporal upscaling method is developed. Then a long-term AWET product for the RSR-SC is generated. Comparison against ground measurements from 12 eddy covariance (EC) sites indicates a good accuracy of the AWET product: the mean absolute percent error (MAPE) and root mean square error (RMSE) of the daily ET estimates are 36 % and 0.88 mm/d, respectively. From aspects of temporal variation of daily ET, monthly spatiotemporal distribution patterns, interannual variation of different land covers and spatial distribution comparison with other ET products, the generated AWET exhibits to be realistic, reflecting relatively subtle variations. Further investigation indicates that the response of surface ET to the warming temperatures exhibits the spatial heterogeneity of the change trend. This study highlights the importance of estimating daily AWET at a MODIS-like scale using AWLST data to significantly benefit water resource monitoring and conduct runoff prediction over large areas and watershed scales.

Characterization of dominant factors on evapotranspiration with seasonal soil water changes in two adjacent forests in the semiarid Loess Plateau

Evapotranspiration is an essential process of ecosystem water consumption and is controlled by meteorological factors and soil water conditions. In this study, evapotranspiration based on soil water budget was investigated with respect to major influencing factors during the growing seasons of 2012–2015 for two adjacent forest communities in the semiarid Loess Plateau region: a natural secondary forest dominated by oak (Quercus liaotungensis) and a plantation of exotic black locust (Robinia pseudoacacia). Based on the combination of precipitation, soil water storage, and evapotranspiration dynamics, four distinct periods were differentiated to explore the evapotranspiration response to the main meteorological factors. The correlation matrixes showed that, in most investigated days, evapotranspiration was limited by soil water storage, while an increase in soil water storage shifted dominance from water-related to energy-related factors. The stepwise regression of ≥5-day dry events in each period had a better fitness when paired with the relationship between the summations of evapotranspiration and dominant factors (R2 > 0.7, p < 0.01 for all) than the stepwise results for daily averages. A receiver operating characteristic (ROC) curve was applied to validate the division and determine thresholds. All divisions were thoroughly validated, and the soil water storage thresholds of the black locust (120.2 mm and 150.7 mm) were higher than those of the oak (105.8 mm and 137.9 mm). Our results suggest that the oak had an adaptive water consumption strategy, while the black locust had a higher sensitivity to soil water insufficiency. The method for determining the threshold of soil water storage to separate the dominant factors of evapotranspiration was validated. The approach may be applied to other semiarid forest communities with distinct evapotranspiration response patterns.

Diversified evapotranspiration responses to climatic change and vegetation greening in eight global great river basins

Accurately quantifying the impacts of climate change on global hydrological cycles is essential for water resource management. Integrated with the Global Land Surface Satellite (GLASS) leaf area index (LAI), the actual evapotranspiration (ETa) from 1981 to 2012 in eight major river basins (Amazon, Mississippi, Yangtze, Mekong, Lena, Murray-Darling, Nile, and Rhine) was simulated with the VIP (Vegetation Interface Processes) ecohydrological dynamic model. The sensitivities and contributions of ETa to climatic change and vegetation greening were explored with the elasticity coefficient method. The results showed that the simulated monthly ETa was consistent with the eddy covariance measurements (determination coefficient (R2) ranged from 0.5 to 0.85, p < 0.01). The simulated annual ETa was also in good agreement with those from the annual water balance (R2 = 0.89, p < 0.01) across the eight basins. The relative annual ETa trends in the Amazon and Mississippi basins were negative, approximately −0.09 % year−1, whereas the annual increasing trends in other basins ranged from 0.16 % year−1 to 0.39 % year−1. In the Yangtze and Mekong basins, ETa was more sensitive to vegetation greening, whereas climatic changes were the primary drivers of ETa tendencies in the other six basins in which the dominant driving factors were different, shifted from precipitation in the Mississippi, Murray-Darling and Nile basins to solar radiation in the Amazon basin. Specifically, precipitation and air temperature equivalently contributed to the ETa trend in the Lena Basin. The variations in ETa were strongly linked with El Niño events in the Amazon basin (energy-limited region) and Murray-Darling basin (water-limited region), reflecting the impacts of global climate extremes.

Integration of carbon dioxide concentration in a simplified process-based model for evapotranspiration estimation in an old-growth forest

Evapotranspiration (ET) is one of the most difficult hydrologic variables to estimate. In the context of climate change, the rise of CO2 emissions may improve photosynthesis but depress transpiration thus increasing plant water use efficiency (WUE). Therefore, it is of great importance and necessity to take elevated atmospheric CO2 concentration (Ca) as a limiting factor for improved assessment of ET and WUE toward a better ecosystem management. In this study, we developed and incorporated a Ca stress function into a modified process-based transpiration model and extended its application from transpiration to total evapotranspiration simulation. The original simplified process-based model (denoted as BTA by combining developers’ last name initials), previously modified version with added soil moisture (BTA-θ) and the extended version in this study with CO2 concentration (BTA-Ca) were tested for daily ET simulation at the Dinghushan forest ecosystem in south China. The results show that eddy covariance based forest ET was well simulated by the models, while the two modified models outperformed the original one. Despite the limited improvement of accuracy, the BTA-Ca model is more superior and meaningful regarding the physical mechanisms. Moreover, we evaluated the biophysical responses of ET and GPP to the major environmental factors, which shows that ET and GPP responded similarly to environmental changes with close linkages between the two. This study proposed a Ca stress function and provides an effective way of coupling water and carbon exchange processes for ET simulations, which can be applied for forest water budget assessment under climate change.

The Biophysical Impacts of Deforestation on Precipitation: Results from the CMIP6 Model Intercomparison

AbstractDeforestation can impact precipitation through biophysical processes and such effects are commonly examined by models. However, previous studies mostly conduct deforestation experiments with a single model and the simulated precipitation responses to deforestation diverge across studies. In this study, 11 Earth system models are used to robustly examine the biophysical impacts of deforestation on precipitation, precipitation extremes, and the seasonal pattern of the rainy season through a comparison of a control simulation and an idealized global deforestation simulation with clearings of 20 million km2of forests. The multimodel mean suggests decreased precipitation, reduced frequency and intensity of heavy precipitation, and shortened duration of rainy seasons over deforested areas. The deforestation effects can even propagate to some regions that are remote from deforested areas (e.g., the tropical and subtropical oceans and the Arctic Ocean). Nevertheless, the 11 models do not fully agree on the precipitation changes almost everywhere. In general, the models exhibit higher consistency over the deforested areas and a few regions outside the deforested areas (e.g., the subtropical oceans) but lower consistency over other regions. Such intermodel spread mostly results from divergent responses of evapotranspiration and atmospheric moisture convergence to deforestation across the models. One of the models that has multiple simulation members also reveals considerable spread of the precipitation responses to deforestation across the members due to internal model variability. This study highlights the necessity of robustly examining precipitation responses to deforestation based on multiple models and each model with multiple simulation members.

Spatiotemporal Variations of Evapotranspiration in Amazonia Using the Wavelet Phase Difference Analysis

AbstractThe relationships and seasonal‐to‐annual variations among evapotranspiration (ET), precipitation (P), terrestrial water storage anomalies (TWSA), radiation (downward shortwave radiation, DSR), and phenology (leaf area index, LAI) are complex across the Amazon basin. To analyze how ET is controlled by these influencing factors, we used wavelet phase difference (WPD) to investigate the effects of P, TWSA, DSR, and LAI on ET at different spatiotemporal scales. The Amazon‐scale averaged ET has strong correlations with these factors at the annual and multi‐year periodicities. The patterns of WPDs have south‐north and west‐east divides due to the significant variation in climatic conditions. The results demonstrate that ET is mainly affected by water and energy availability while vegetation regulates both processes. The deep soil moisture/groundwater can provide strong subsidies to ET during the meteorological dry season in the water‐limited area of Amazon. The WPD can well reflect the responses of ET to the variations of P, TWSA, DSR, and LAI, and the process of vegetation sustaining ET in the dry years in the water‐limited area of the Amazon.

Precipitation-Use Efficiency and Its Conversion with Climate Types in Mainland China

The impacts of climate change on ecosystem productivity and water resources over a long term in China are not well quantified. Precipitation-use efficiency (PUE) is a key parameter that describes carbon and water exchange in terrestrial ecosystems. Research on the response of regional PUE to climate change and its driving forces is of great significance to climate-change mitigation and the sustainable development of regional ecology. Based on an improved actual evapotranspiration (ETa) model, the responses of ETa, net primary productivity (NPP), and PUE to climate change in different climatic regions of China were analyzed; the contributions of various environmental factors to PUE changes were quantified; and the conversion characteristics and regulatory mechanisms of the PUE regime in different climatic regions were identified. The results indicate that the improved ETa model, after considering the limiting effect of energy on ETa in humid regions, can simulate the ETa distribution in China well. Over the past 58 years (1960–2017), ETa and NPP have increased in the western regions and decreased in the eastern regions, with the boundary at 103° E. PUE presents a “low-high-low” spatial distribution from northwest to southeast in China. It is noteworthy that there was a zonal distribution for a high value area of PUE, which coincided with the summer monsoon transition zone. The soil moisture (SM) increase in arid regions is the main driving force of the PUE increase, whereas the annual net radiation (Rn) change in humid regions is the main driving force of the PUE change. The transition zone is the conversion zone, where the prevailing factor limiting vegetation growth transitions from water to energy.

Pinus tabulaeformis Forests Have Higher Carbon Sequestration Potential Than Larix principis-rupprechtii Forests in a Dryland Mountain Ecosystem, Northwest China

Carbon sinks in terrestrial ecosystems can be significantly increased by afforestation, which will slow global warming. However, it is still unclear how different plantations influence the carbon sink and how they respond to environmental factors, especially in drylands. In this study, eddy correlation method (EC) was used to measure carbon and water fluxes and environmental factors of two artificial forests (Larix principis-rupprechtii and Pinus tabulaeformis) in the dryland of Northwest China, and the responses of evapotranspiration (ET), net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) to environmental factors were also assessed. Results showed that the L. principis-rupprechtii forest ecosystem had higher water use efficiency (WUE), light use efficiency (LUE), GPP, and RECO than the P. tabulaeformis forest ecosystem. However, the proportion of net ecosystem production (NEP) to GPP in the P. tabulaeformis forest ecosystem (62.89%) was higher than that in the L. principis-rupprechtii forest ecosystem (47.49%), indicating that the P. tabulaeformis forest ecosystem had the higher carbon sequestration efficiency. In addition, the CO2 and H2O fluxes in the L. principis-rupprechtii forest ecosystem were more sensitive to environmental factors, compared with the P. tabulaeformis forest ecosystem. Further, the RECO of the L. principis-rupprechtii forest ecosystem was more sensitive to temperature changes, which implies that the L. principis-rupprechtii forest ecosystem will release more CO2 than the P. tabulaeformis forest ecosystem with a warming climate. Therefore, the P. tabulaeformis forest ecosystem may have better carbon sequestration potential. These results are important for understanding the effects of climate change on the CO2 and H2O cycles in coniferous plantation ecosystems in drylands.

Environmental controls on water use efficiency in a hilly tea plantation in southeast China

Knowledge of water use efficiency (WUE) at the ecosystem level is imperative as it is a critical ecophysiological index reflecting the coupling processes between carbon sequestration and water consumption. However, the temporal patterns of WUE and their underlying regulating factors during different seasons in tea plantations, which are under intense pruning practice, remain poorly understood. Based on carbon and water vapor flux data obtained by the eddy covariance technique in a subtropical hilly tea plantation in southeast China during 2014–2018, this study analyzed the seasonal variations in gross primary productivity (GPP), evapotranspiration (ET) and ecosystem WUE, and discussed the response of GPP, ET and WUE to environmental variables during different seasons. The results showed that the mean annual GPP, ET and WUE ranged from 1426.61 ~ 1896.05 g C m−2, 607.05 ~ 805.83 mm and 2.22 ~ 2.68 g C kg−1 H2O, respectively. At the daily time scale, GPP and ET responded significantly and positively to variations in air temperature (Ta), water vapor pressure deficit (VPD), and net radiation (Rn) in both the pruning season and late growing season, but daily WUE had a significantly negative response to variations in Ta, VPD and Rn. Rn was identified as the dominant factor in regulating GPP and ET during both the pruning season and late growing season, while the dominant factor controlling daily variations in WUE was VPD during the pruning season and Ta during the late growing season. Such results were mainly due to the divergent responses of carbon assimilation and water loss to different environmental variations and management strategies. These findings highlighted the importance of understanding the coupled processes between carbon assimilation and water loss in tea plantations.

Modeling revealed the effect of root dynamics on the water adaptability of phreatophytes

Given their high root plasticity, phreatophytes have flexible water use strategies and can dynamically adjust their rooting depth for the effective uptake of water from soils and shallow aquifers. By this strategy, phreatophytes are strongly ecologically resilient to water stress and thus are commonly grown in drylands. In this study, we used a modified soil–hydrological model, Hydrus-1D, with the implementation of a dynamic root scheme to analyze the role of the distribution and dynamics of roots in simulating evapotranspiration (ET) of phreatophytes with declining groundwater levels (GWLs). The results showed that the root mean square error (RMSE) between simulated and observed ET was reduced by approximately 39% using a model with a more authentic root distribution than the generic root profile. Static root schemes cannot portray the adaptation of phreatophytes to GWL changes well, but their performance can be enhanced by introducing a water compensation scheme, which, however, has a weakness related to the difficulties in parameter calibration. The ET was well retrieved by the model with the dynamic root scheme. Its RMSE was 40% to 70% less than those of the static models, and the Nash–Sutcliffe efficiency reached 0.94, demonstrating the importance of root dynamics in simulating phreatophytic root water uptake. In addition, multiscenario estimations showed nonlinear responses of phreatophyte ET to the rate of GWL decrease (RGWD); that is, as the RGWD accelerated, plant adaptation showed three different stages: no water stress, adaptable and unadaptable. The resistance of plants to water stress decreases with decreasing root growth rate. The identified key ecological thresholds for the RGWD provide a reference for ecological protection in arid areas. We highlight the importance of root processes in the plant response to water stress and suggest that more attention should be given to the root adaptation process in Earth system models.

Spatiotemporal variations in evapotranspiration and its influencing factors in the semiarid Hailar river basin, Northern China

Evapotranspiration (ET) is a critical variable in the world's water cycle, and plays a significant role in estimating the impact of environmental change on the regional hydrothermal cycle. Moreover, as an essential of eco-hydrological processes, changes in ET may exceptionally impact the local climate and provide indicative information on the eco-system's functioning. The Hailar River Basin (HRB), located in northern China, is one of the most sensitive areas to climate warming. Under the influence of climate change in recent years, the vegetation dynamics of the basin have been significant and have had profound effects on the regional water cycle conditions and hydrological processes. The HRB is located in a semiarid region and ET is the main mode of water consumption. The ET response to climate change and vegetation dynamics is the focus of research on ecohydrological processes in this basin. In this study, a distributed hydrological model, the BTOPMC model, is used to evaluate the actual ET in the HRB from 1981 to 2020, based on in situ meteorological data as well as LAI data obtained by satellite remote sensing. The seasonal, interannual and spatial dynamics of ET were characterized. The contribution of meteorological factors to ET was calculated by sensitivity analysis and multiple linear regression analysis, and the predominant elements influencing the difference in ET in the HRB were also discussed. The results show that: (1) estimated ET values can clarify over 85% of the seasonal variation in the observed values (R2= 0.79, P < 0.001; R2= 0.84, P < 0.001), which demonstrates that the model has a high precision. (2) Over the past 40 years, the annual ET has shown a clear increasing trend and a large spatial heterogeneity in its spatial distribution, which is consistent with the trend of vegetation. It mainly shows that the eastern forest area is larger than the central forest–grass transition area and the western meadow steppe area. (3) Sensitivity and influential factor contribution analyses show that the main factor driving interannual variability in ET is climate warming, followed by precipitation. At the same time, vegetation dynamics also play a crucial role in ET, especially in areas with different vegetation types and high coverage, while climatic factors also have a strong influence on ET indirectly through vegetation. Due to its special geographic location, the HRB is more sensitive to global climate change and is a typical ecologically fragile area. Therefore, this study has important scientific value and social significance for maintaining ecological security and the sustainable use of water resources.

Towards hybrid modeling of the global hydrological cycle

Abstract. State-of-the-art global hydrological models (GHMs) exhibit large uncertainties in hydrological simulations due to the complexity, diversity, and heterogeneity of the land surface and subsurface processes, as well as the scale dependency of these processes and associated parameters. Recent progress in machine learning, fueled by relevant Earth observation data streams, may help overcome these challenges. But machine learning methods are not bound by physical laws, and their interpretability is limited by design. In this study, we exemplify a hybrid approach to global hydrological modeling that exploits the data adaptivity of neural networks for representing uncertain processes within a model structure based on physical principles (e.g., mass conservation) that form the basis of GHMs. This combination of machine learning and physical knowledge can potentially lead to data-driven, yet physically consistent and partially interpretable hybrid models. The hybrid hydrological model (H2M), extended from Kraft et al. (2020), simulates the dynamics of snow, soil moisture, and groundwater storage globally at 1∘ spatial resolution and daily time step. Water fluxes are simulated by an embedded recurrent neural network. We trained the model simultaneously against observational products of terrestrial water storage variations (TWS), grid cell runoff (Q), evapotranspiration (ET), and snow water equivalent (SWE) with a multi-task learning approach. We find that the H2M is capable of reproducing key patterns of global water cycle components, with model performances being at least on par with four state-of-the-art GHMs which provide a necessary benchmark for H2M. The neural-network-learned hydrological responses of evapotranspiration and grid cell runoff to antecedent soil moisture states are qualitatively consistent with our understanding and theory. The simulated contributions of groundwater, soil moisture, and snowpack variability to TWS variations are plausible and within the ranges of traditional GHMs. H2M identifies a somewhat stronger role of soil moisture for TWS variations in transitional and tropical regions compared to GHMs. With the findings and analysis, we conclude that H2M provides a new data-driven perspective on modeling the global hydrological cycle and physical responses with machine-learned parameters that is consistent with and complementary to existing global modeling frameworks. The hybrid modeling approaches have a large potential to better leverage ever-increasing Earth observation data streams to advance our understandings of the Earth system and capabilities to monitor and model it.

Dissecting the Mutual Response of Potential Evapotranspiration with Vegetation Cover/Land Use over Heilongjiang River Basin, China

Spatiotemporal changes in potential evapotranspiration (PET) are important referential data for monitoring the variation in ecohydrology under climate change. As the key area of agricultural/forestry reserves and trade, research on the interactions between vegetation cover and land use with accurate PET among the Heilongjiang River basin, China is vital for the sustainability of this cross-border region. To obtain high-suitability PET estimation based on the proven Penman–Monteith (PM) principle, two schemes were adopted using 36-year in situ meteorological data (1984–2019) and derived remote-sensing product MOD16A2 from 2001 to 2019. At meteorological sites with heterogeneous underlying surfaces, the coefficients of the evaluation factors between MOD_PET and the referrable observations were better. This suggests that remotely sensed estimation was preferentially chosen as the input to study the mutual responses. The results indicated that the annual PET and fractional vegetation cover (FVC) changes differed significantly depending on the land use. The proportions of PET according to equal intervals initially increased and then decreased with increasing vegetation coverage, and changing trends indicated a negative correlation. The strongest correlations appeared in the middle to northern sub-basins, which were affected by their distinctive climatic characteristics. The response of PET was reverse-related with changes in the area of each selected land type, but could change with the expected climatic conditions.

Forest water use is increasingly decoupled from water availability even during severe drought

ContextKey to understanding forest water balances is the role of tree species regulating evapotranspiration (ET), but the synergistic impact of forest species composition, topography, and water availability on ET and how this shapes drought sensitivity across the landscape remains unclear.ObjectivesOur aims were to quantify (1) the effect of forest composition and topography including elevation and hillslope gradients on the relationship between ET and water availability, and (2) whether the relationship has changed over time.MethodsWe used remotely sensed Landsat and MODIS ET to quantify forest ET across the Blue Ridge ecoregion of the southeastern USA. Then quantified metrics describing ET responses to water availability and trends in responses over time and assessed how these metrics varied across elevation, hillslope, and forest composition gradients.ResultsWe demonstrated forest ET is becoming less constrained by water availability at the expense of lateral flow. Drought impacts on ET diverged along elevation and hillslope gradients, and that divergence was more pronounced with increasingly severe drought, indicating high elevation and drier, upslope regions tend to maintain ET rates even during extreme drought. We identified a decoupling of ET from water availability over time, and found this process was accelerated at higher elevations and in areas with more diffuse-porous trees.ConclusionsGiven the large proportion of forests on the landscape distributed across high elevation and upslope positions, reductions in downslope water availability could be widespread, amplifying vulnerability of runoff, the health of downslope vegetation, and aquatic biodiversity.

Diurnal Evapotranspiration and Its Controlling Factors of Alpine Ecosystems during the Growing Season in Northeast Qinghai-Tibet Plateau

It is generally believed that evapotranspiration at night is too miniscule to be considered. Thus, few studies focus on the nocturnal evapotranspiration (ETN) in alpine region. In this study, based on the half-hour eddy and meteorological data of the growing season (from May to September) in 2019, we quantified the ETN of alpine desert (AD), alpine meadow (AM), alpine meadow steppe (AMS), and alpine steppe (AS) in the Qinghai Lake Basin and clarified the different response of evapotranspiration to climate variables in daytime and nighttime with the variation of elevation. The results show that: (1) ETN accounts for 9.88~15.08% of total daily evapotranspiration and is relatively higher in AMS (15.08%) and AD (12.13%); (2) in the daytime, net radiation (Rn), temperature difference (TD), vapor pressure difference (VPD), and soil moisture have remarkable influence on evapotranspiration, and Rn and VPD are more important at high altitudes, while TD is the main factor at low altitudes; (3) in the nighttime, VPD and wind speed (WS) control ETN at high altitudes, and TD and WS drive ETN at low altitudes. Our results are of great significance in understanding ETN in the alpine regions and provide reference for further improving in the evapotranspiration estimation model.

Response of Potential Evapotranspiration to Warming and Wetting in Northwest China

In the last few decades, the climate in Northwest China has exhibited a warming–wetting tendency, which has been particularly prominent since the beginning of the 21st century. In this context, we analyzed the change in potential evapotranspiration (PET)in the corresponding period and its response to warming and wetting, which revealed clear periodic changes. The most significant changes occurred in the 1970s and 1980s, when PET decreased in the humid climate zone and increased in the semi-arid climate zone. Factor effect analysis showed that PET had a positive response to temperature; the highest and lowest temperatures in the region continued to rise. Relative humidity reduced the overall PET in the region, especially in the humid zone. Sunshine duration has continued to decrease rapidly since the 1980s, especially in humid and arid zones, resulting in a corresponding decrease in PET. Similarly, corresponding to the consistent wind speed decrease, there has also been a significant decrease in PET, with the largest decrease in the arid zone, followed by the humid zone. In general, PET in the central and eastern parts of Northwest China has mainly been affected by the temperature, whereas wind speed has been the main factor in the western part of the region. Relative humidity and sunshine duration have had relatively little effect on the PET (below 20% in most places). The reasons and processes that affect PET are very complicated. Owing to the unique climate characteristics and underlying surface energy mechanisms in Northwest China, it is still difficult to offer a scientific explanation for its warming and wetting. Therefore, the extent to which PET impacts climate change in this region is currently unclear, and systematic and scientific research on this is needed.

Intra-Annual Variability of Evapotranspiration in Response to Climate and Vegetation Change across the Poyang Lake Basin, China

Improving understanding of changes in intra-annual variability (IAV) of evapotranspiration (ET) and the underlying drivers is an essential step for modeling hydrological processes in response to global change. Previous studies paid special attention to climatic regulations of IAV of ET. However, ignoring the role of landscape characteristics (e.g., vegetation coverage) can introduce great uncertainty in the explanation of ET variance. In this work, the Poyang Lake Basin, which is a typical humid basin in China, was taken as the study area. It has experienced an obvious climate change and revegetation since the 1980s. Here, trends of IAV of ET and their responses to four climatic variables (i.e., air temperature, precipitation, downward shortwave radiation and wind speed) and vegetation coverage were explored from 1983 to 2014. The results show that IAV of ET exhibited contrary trends during the past decades. It significantly (p &lt; 0.05) declined with a significant linear slope of −0.52 mm/year before 2000, and then slightly increased (slope = 0.06 mm/year, p &gt; 0.05) over the basin, which was generally consistent with the IAV of temperature and radiation. The proposed variables could well capture the change in IAV of ET, while their dominators were different during the two contrasting phases mentioned above. The IAV of radiation and temperature dominated the change of the IAV of ET over 77.82% and 35.14% of the basin, respectively, before and after the turning point. Meanwhile, the rapid increase in vegetation coverage, which was associated with afforestation, significantly (p &lt; 0.05) reduced IAV of ET over about 35% of the study area. The achievements of this study should be beneficial for a sophisticated awareness of responses of intra-annual variability of ET to climate and land cover changes at the basin scale.

Responses of evapotranspiration to changing climate for the past six decades in the upper catchment of the Olifants River basin

In river basins such as the Olifants, the evapotranspiration has been documented as the biggest water user and therefore one of the most important hydrological processes in the basin. Thus, using the Soil and Water Assessment Tool (SWAT) model, this study attempted to assess the hydrological responses of evapotranspiration (ET) to climate change in the Olifants River basin over a period of six decades, focusing mainly on the upper catchment (UC) of the basin. The study showed, on average, a general decline of seasonal ET with a decrease of 10.5% in spring, 29.8% in summer and 17.3% in winter. However, autumn ET showed an average increase of 12.5%. With (1960 – 1970) decade considered as the baseline, the study also found that annual ET in the UC, has been on the decline consistently since the 1960s with the most recent decade (2010 – 2018) taking the biggest share of the drop (12.8%). Here, the average annual decline of ET of 1.75 mm/yr was observed with most of the decline happening in the last decade. This general decadal trend of annual ET mortality in the area coincides with the temperature rise and precipitation decrease of the area, signifying the impact climate change may have had on the evapotranspiration for the past six decades.