Variability Of Evapotranspiration Research Articles

Evapotranspiration and carbon exchange of the main agroecosystems and their responses to agricultural land use change in North China Plain

The North China Plain (NCP) plays an important role in food production while there is serious groundwater overexploitation in this area. The quantified relationship between the variation in evapotranspiration (ET) and net ecosystem CO2 exchange (NEE) and agricultural land use change in NCP remains unclear. In this study, based on the eddy covariance method, water balance method, and remote sensing data, ET and NEE variations were quantified and the impacts of agricultural land use change from 2002 to 2012 on regional ET and NEE variation were analyzed. The average ET and NEE of the winter wheat – summer maize cropland ecosystem were 714.2 mm yr-1 and − 534.3 gC m-2 yr-1, respectively. Correspondingly, the above values were 786.2 mm yr-1 and 667.5 gC m-2 yr-1 for the pear orchard ecosystem, and 778.6 mm yr-1 and 814.8 gC m-2 yr-1 for the vegetable field ecosystem, which had relatively high ET and ecosystem productivity. The average annual ET and NEE of the cotton field ecosystem were only 592.0 mm and − 351.3 gC m-2, respectively. Agricultural land area increased by 331.2 × 103 ha over 2002–2012, which was caused by the increasing land area of spring maize, forest/fruit trees, and vegetables. As a result, the ET and NEE of the agroecosystems in NCP increased by 25.6 × 108 m3 and − 257.9 × 104 tonC from 2002 to 2012, which are beneficial for carbon sequestration. However, the increase in carbon sink was at the cost of greater groundwater overexploitation due to increased regional water consumption. The planting of crops with high water consumption should be reduced as the surpluses of these agricultural products compared to the food demand in this region. These results are meaningful for reasonable planting structure adjustment of water-adaptive sustainable agricultural development in the NCP.

Vegetation greening and climate change promote an increase in evapotranspiration across Siberia

• Evapotranspiration (ET) increased with a trend of 0.54 ± 1.38 mm/a from 2000 to 2020. • Greening is the predominant driver of ET variations. • Surface net radiation and wind speed are vital climate factors of ET variations. The greening of the Arctic and pan-Arctic regions in recent decades has been widely confirmed, while the details regarding the greening feedback effects involving the water and energy cycles are still vague. Evapotranspiration (ET), a vital process in the water and energy cycles, strongly corresponds to vegetation activities. Hence, in this study, we chose Siberia as the study area and, based on the Penman–Monteith–Leuning (PML) model, revealed the contribution of greening to ET. Moreover, the effects of the water vapour pressure deficit, surface net radiation ( R n ) and wind speed ( U m ) on ET were evaluated. The results indicated that from 2000 to 2020, the annual ET in Siberia was 248.2 ± 94.1 mm, and the trend was 0.54 ± 1.38 mm/a. Greening was the major driver of ET variations; its contribution was 0.79 ± 0.76 mm/a, and its relative contribution was 37%. Among the other analysed climate factors, ET was sensitive to R n and U m ; these factors contributed 0.51 ± 0.85 mm/a and −0.38 ± 0.54 mm/a, respectively, to ET variation, and their relative contributions were 33% and 19%, respectively. The effect of the water vapour pressure deficit was slight (0.29 ± 0.22 mm/a, 11%), indicating that ET was hardly constrained by the water supply in Siberia. These results quantify the importance of greening on ET variations and highlight the important effects of R n and U m on ET in cold region terrestrial ecosystems. Furthermore, this study improves our understanding of the mechanism by which evapotranspiration varies and is valuable for predicting and evaluating the Arctic water cycle in “Arctic amplification”.

Improved global evapotranspiration estimates using proportionality hypothesis-based water balance constraints

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

Spatiotemporal variation of potential evapotranspiration and its dominant factors during 1970-2020 across the Sichuan-Chongqing region, China.

Analyzing the primary factors of potential evapotranspiration (PET) dynamic is fundamental to accurately estimating crop yield, evaluating environmental impacts, and understanding water and carbon cycles. Previous studies have focused on regionally average regional PET and its dominant factors. Spatial distributions of PET trends and their main causes have not been fully investigated. The Mann–Kendall test was used to determine the significance of long-term trends in PET and five meteorological factors (net radiation, wind speed, air temperature, vapor pressure deficit, relative humidity) at 56 meteorological stations in the Sichuan-Chongqing region from 1970 to 2020. Furthermore, this present study combining and quantitatively illustrated sensitivities and contributions of the meteorological factors to change in annual and seasonal PET. There was a positive trend in PET for approximately 58%, 68%, 38%, 73% and 73% of all surveyed stations at annual, spring, summer, autumn and winter, respectively. Contribution analysis exhibited that the driving factors for the PET variation varied spatially and seasonally. For stations with an upward PET trend, vapor pressure deficit was a dominant factor at all time scales. For stations with a downward PET trend, annual changes in PET mainly resulted from decreased wind speed, as did changes in spring, autumn and winter; decreasing net radiation was the dominant factor in summer. The positive effect of the vapor pressure deficit offset the negative effects of wind speed and net radiation, leading to the increasing PET in this area as a whole. Sensitivity analysis showed that net radiation and relative humidity were the two most sensitive variables for PET, followed by vapor pressure deficit in this study area. Results from the two mathematical approaches were not perfect match, because the change magnitude of the meteorological factors is also responsible for the effects of meteorological factors on PET variation to some extent. However, conducting sensitivity and contribution analysis in this study can avoid the uncertainties from using a single method and provides detailed and well-understood information for interpreting the influence of global climate change on the water cycle and improving local water management.

Spatial and temporal estimation of actual evapotranspiration of lower Bhavani basin, Tamil Nadu using Surface Energy Balance Algorithm for Land Model

Estimating evapotranspiration's spatiotemporal variance is critical for regional water resource management and allocation, including irrigation scheduling, drought monitoring, and forecasting. The Surface Energy Balance Algorithm for Land (SEBAL) method can be used to estimate spatio-temporal variations in evapotranspiration (ET) using remote sensing-based variables like Land Surface Temperature (LST), Normalized Difference Vegetation Index (NDVI), surface albedo, transmittance, and surface emissivity. The main aim of the study was to evaluate the actual evapotranspiration for the lower Bhavani basin, Tamil Nadu based on remote sensing methods using Landsat 8 data for the years 2018 to 2020. The actual evapotranspiration was estimated using SEBAL model and its spatial variation was compared over different land covers. The estimated values of daily actual evapotranspiration in the lower Bhavani basin ranged from 0 to 4.72 mm day-1. Thus it is evident that SEBAL model can be used to predict ET with limited ground base hydrological data. The spatially estimated ET values will help in managing the crop water requirement at each stage of crop and irrigation scheduling, which will ensure the efficient use of available water resources.

The altered drivers of evapotranspiration trends around the recent warming hiatus in China

AbstractThis study focuses on the trends and the causes of variation in actual evapotranspiration (AET) around the warming hiatus over China by a comprehensive analysis applying various temporal–spatial methods. It is observed that the annual AET showed a different trend around 2000 for China as a whole. By employing segmented regression analysis for detecting warming hiatus points, high temporal inconsistency can be found in eight climatic regions of China. The impacts of meteorological variables on AET were further identified by affecting the intensity and relative change of meteorological factors. AET was highly correlated (p < .01) with solar radiation in the southeast (R = 0.80) and air specific humidity in the northwest areas (R = 0.83). AET changes presented the highest sensitivity to specific humidity in Northwest before 2006 and in north central China after 2003, with sensitivity coefficients of 1.48 and 1.74, respectively. Three variables, including air specific humidity (with an average contribution rate of ~17% in the northwest), short‐wave radiation and air temperature, can be the main factors that lead to the changes in AET. The specific meteorological factors varied from region to region: the changes in AET can be ascribed to the increased wind and short‐wave radiation in north central China and east China, the decreased air temperature in Tibetan Plateau, the increased specific humidity in southeast China during warming hiatus, and so on. After the warming hiatus occurred, the dominant factor of AET trends changed from air specific humidity to short‐wave radiation and other factors. Generally, air specific humidity and air temperature have played leading roles in AET trends during the past 30 years.

Quantifying the impacts of agricultural alteration and climate change on the water cycle dynamics in a headwater catchment of Lake Urmia Basin

The rapid shrinkage of Lake Urmia over the past two decades has raised serious environmental concerns. Several restoration plans have been proposed to reduce agricultural water consumption to supply a remarkable fraction of the lake’s environmental flow requirement. However, an accurate and realistic evaluation of the effectiveness of these plans in reducing agricultural water consumption across the basin is still poorly understood. This study assessed the water-saving potential of agricultural alteration in a snow-dominated catchment in the Lake Urmia Basin. In particular, we explored the impact of crop pattern alteration via reducing the cultivation area and employing different irrigation systems. We further studied the degree of extra stress on the agricultural water demand under the future trends of air temperature and precipitation due to climate change. To this end, we developed a three-dimensional physically-based hydrological model that simulated the main processes involved in the water and energy cycles. In this setting, a surface-groundwater model was coupled to a land-surface model that integratively computed the spatiotemporal variability of evapotranspiration, snowmelt, groundwater, and soil moisture in the root zone, among other processes. Pumping from wells and different irrigation systems were also included in the model. The results showed that a reduction in the cultivation area does not necessarily lead to a significant saving in the total water loss due to evapotranspiration, which strongly depends on the trade-off between the decrease in transpiration and the increase in soil evaporation. We further found that saving in agricultural water consumption is partly controlled by topography and the vertical distribution of soil moisture. Also, the drip irrigation system showed lower evapotranspiration rates than the spray irrigation system. Finally, the study of the future impacts of climate change revealed that the agriculture sector would be under more intensive stress, which poses a severe threat to the long-term restoration of Lake Urmia if effective agricultural management practices are not taken into consideration. The findings of this study highlight important issues for the Lake Urmia restoration plans on agricultural water management. Given the fragility of the lake ecosystem, this study suggests a reliable modeling framework for water and agricultural managers to identify the most appropriate regions in the basin that would yield the highest saving in agricultural water consumption by improving the current crop patterns and irrigation systems.

Potential Variation of Evapotranspiration Induced by Typical Vegetation Changes in Northwest China

Evapotranspiration (ET), as a key eco-hydrological parameter, plays an important role in understanding sustainable ecosystem development. Each plant category has a unique functional trait on transpiration and photosynthesis, with ET implying that water cycle and energy transformation is linked with vegetation type. Changes in surface vegetation directly alter biophysical land surface properties, hence affecting energy and ET transfer. With the rapid increase in land surface changes, there is a need to further understand and quantify the effects of vegetation change on ET, especially over the vulnerable water-cycle region in the arid and semi-arid regions of Northwest China. We adopted the GlobalLand30 land cover and MOD16A2 in 2010 and 2020 to investigate, discuss the spatio-temporal characteristics of annual and seasonal ET of cultivated land, grassland, and forests in Northwest China, and quantify the impact on vegetation changes with absolute and relative changes from different climatic subecoregions on ET. Our results show the following: (1) Forest ET was generally the highest at 688 mm, followed by cultivated land and grassland with 200–400 mm in arid climatic subecoregions. (2) Returning cultivated land to forests and cultivated land expansion potentially enhances ET by 90–110 mm/10a, with the relative rate of change increasing by 22.1% and 45.8%, respectively, away from unchanged vegetation within identical subecoregions. (3) The ET of most investigated areas gains the highest value in summer, followed by spring, autumn, and winter. This study provides reference for sustainable ecosystem development and the reasonable utilization of limited water resources in Northwest China.

Spatio-Temporal Characteristics of the Evapotranspiration in the Lower Mekong River Basin during 2008–2017

Droughts and floods have occurred frequently in the Lower Mekong River Basin in recent years. Obtaining the evapotranspiration (ET) in the basin helps people to better understand water cycle and water resources. In this study, we retrieved and validated ET in the Lower Mekong Basin over multiple years (from 2008 to 2017) using remote sensing products. Based on the retrieval ET, we analyzed the spatial-temporal variation of ET and influencing factors at the monthly, seasonal, and inter-annual scale respectively. The results revealed that the overall variation trend of ET at annual scale slightly increased during 2008 to 2017, with the highest annual ET being 1198 mm/year in 2015 and the lowest annual ET being 949 mm/year in 2008. At the seasonal scale, ET in the rainy season was lower than the dry season; at the monthly scale, March had the highest monthly ET (101 mm/month) while July had the lowest monthly ET (73 mm/month). Spatial analyzing showed that ET in the margin of this region was higher (with on average about 1250 mm/year) and lower in the middle (with on average about 840 mm/year), and monthly ET changed mostly in forest areas with the difference of 60 mm/month. Influencing analyzing results showed that ET was mainly driven by solar radiation and near-surface temperature, and precipitation had an inhibitory effect on ET in the rainy season months. The study also showed that forests in the basin are very sensitive to solar radiation, with a correlation coefficient of 0.89 in March (the month with the highest ET) and 0.45 in July (the month with the lowest ET).

Long-term reference evapotranspiration trend and causative factors analysis in the sugarbelt area of the midlands of KwaZulu-Natal, South Africa

Global warming is widely recognised, and its effects are becoming apparent throughout the world. Evaporation and evapotranspiration, the key components of the hydrological cycle, are generally expected to increase due to the rise in air and surface temperatures. However, previous studies suggest a decrease in these phenomena despite the observed global warming. The decreasing evaporation and evapotranspiration have been attributed to various factors. In this study, reference evapotranspiration (ETo) trends estimated using the Penman-Monteith method were studied over the KwaZulu-Natal midlands area of South Africa for the period 1997–2017. This study employed the Mann–Kendall test and linear regression model to analyse annual and seasonal ETo trends. In addition, the trends of climate parameters and their contribution towards ETo variation were analysed. The results indicate a generally decreasing ETo trend for most weather stations studied over the study period. The climate variables analysed indicate an average decreasing trend in wind speed, solar irradiance, and relative humidity while average air temperature exhibited no significant change. Relative humidity and solar irradiance were found to greatly influence ETo variation in the study area. We therefore conclude that atmospheric condition studies should consider both local and global climate phenomena to understand the actual drivers of change in any atmospheric factor.

Global Evapotranspiration Datasets Assessment Using Water Balance in South America

Evapotranspiration (ET) connects the land to the atmosphere, linking water, energy, and carbon cycles. ET is an essential climate variable with a fundamental importance, and accurate assessments of the spatiotemporal trends and variability in ET are needed from regional to continental scales. This study compared eight global actual ET datasets (ETgl) and the average actual ET ensemble (ETens) based on remote sensing, climate reanalysis, land-surface, and biophysical models to ET computed from basin-scale water balance (ETwb) in South America on monthly time scale. The 50 small-to-large basins covered major rivers and different biomes and climate types. We also examined the magnitude, seasonality, and interannual variability of ET, comparing ETgl and ETens with ETwb. Global ET datasets were evaluated between 2003 and 2014 from the following datasets: Breathing Earth System Simulator (BESS), ECMWF Reanalysis 5 (ERA5), Global Land Data Assimilation System (GLDAS), Global Land Evaporation Amsterdam Model (GLEAM), MOD16, Penman–Monteith–Leuning (PML), Operational Simplified Surface Energy Balance (SSEBop) and Terra Climate. By using ETwb as a basis for comparison, correlation coefficients ranged from 0.45 (SSEBop) to 0.60 (ETens), and RMSE ranged from 35.6 (ETens) to 40.5 mm·month−1 (MOD16). Overall, ETgl estimates ranged from 0 to 150 mm·month−1 in most basins in South America, while ETwb estimates showed maximum rates up to 250 mm·month−1. ETgl varied by hydroclimatic regions: (i) basins located in humid climates with low seasonality in precipitation, including the Amazon, Uruguay, and South Atlantic basins, yielded weak correlation coefficients between monthly ETgl and ETwb, and (ii) tropical and semiarid basins (areas where precipitation demonstrates a strong seasonality, as in the São Francisco, Northeast Atlantic, Paraná/Paraguay, and Tocantins basins) yielded moderate-to-strong correlation coefficients. An assessment of the interannual variability demonstrated a disagreement between ETgl and ETwb in the humid tropics (in the Amazon), with ETgl showing a wide range of interannual variability. However, in tropical, subtropical, and semiarid climates, including the Tocantins, São Francisco, Paraná, Paraguay, Uruguay, and Atlantic basins (Northeast, East, and South), we found a stronger agreement between ETgl and ETwb for interannual variability. Assessing ET datasets enables the understanding of land–atmosphere exchanges in South America, to improvement of ET estimation and monitoring for water management.

Evapotranspiration Seasonality over Tropical Ecosystems in Mato Grosso, Brazil

Brazilian tropical ecosystems in the state of Mato Grosso have experienced significant land use and cover changes during the past few decades due to deforestation and wildfire. These changes can directly affect the mass and energy exchange near the surface and, consequently, evapotranspiration (ET). Characterization of the seasonal patterns of ET can help in understanding how these tropical ecosystems function with a changing climate. The goal of this study was to characterize temporal (seasonal-to-decadal) and spatial patterns in ET over Mato Grosso using remotely sensed products. Ecosystems over areas with limited to no flux towers can be performed using remote sensing products such as NASA’s MOD16A2 ET (MOD16 ET). As the accuracy of this product in tropical ecosystems is unknown, a secondary objective of this study was to evaluate the ability of the MOD16 ET (ETMODIS) to appropriately represent the spatial and seasonal ET patterns in Mato Grosso, Brazil. Actual ET was measured (ETMeasured) using eight flux towers, three in the Amazon, three in the Cerrado, and two in the Pantanal of Mato Grosso. In general, the ETMODIS of all sites had no significant difference from ETMeasured during all analyzed periods, and ETMODIS had a significant moderate to strong correlation with the ETMeasured. The spatial variation of ET had some similarity to the climatology of Mato Grosso, with higher ET in the mid to southern parts of Mato Grosso (Cerrado and Pantanal) during the wet period compared to the dry period. The ET in the Amazon had three seasonal patterns, a higher and lower ET in the wet season compared to the dry season, and minimal to insignificant variation in ET during the wet and dry seasons. The wet season ET in Amazon decreased from the first and second decades, but the ET during the wet and dry season increased in Cerrado and Pantanal in the same period. This study highlights the importance of deepening the study of ET in the state of Mato Grosso due to the land cover and climate change.

Development of a simple Budyko-based framework for the simulation and attribution of ET variability in dry regions

Global changes have largely altered the magnitude and distribution of evapotranspiration (ET), which in turn significantly impacts regional water availability and water partitioning, particularly in semi-arid areas, such as the Agricultural-Pastoral Ecotone in Northwest China (APENC). In this study, the Budyko framework was revised and applied at a finer spatio-temporal scale for the simulation and attribution of monthly ET variability in dry regions. As a holistic approach for modeling water partitioning, the revised Budyko framework (RBF) is expected to achieve parsimony of data input and model parameters. Application of the Budyko-based framework in the APENC showed an accurate ET simulation (R2 = 0.96, RMSE = 3.45 mm/mon, NSE = 0.95), which is a better performance compared with two classical models, FAO crop coefficient model and Complimentary Relationship (CR) model with in situ observations. For the attribution of monthly ET variability, vegetation has the highest sensitivity coefficient, while the precipitation shows the largest actual contribution because of its larger variability. Subsequently, the RBF was assessed with two different groups of in situ observations in semi-arid areas with different environment and vegetation conditions, and showed credible results on ET simulation. This shows that the RBF with parsimony of model parameters is highly applicable in data lacking dry regions.

A calibration free radiation driven model for estimating actual evapotranspiration of mountain grasslands (CLIME-MG)

Ecosystems in the Alps are considered hotspots of climate and land use change. In addition, alpine regions are usually characterized by complex morphologies, which make measurement (especially in the long term) of states and fluxes of water, energy and matter particularly challenging. Therefore, there is a limited availability of information and modelling tools to characterize actual ecosystem conditions, and to simulate future scenarios.Despite the fact that in high altitude areas meteorological forcing is extremely variable in space and time, much of the variability of actual evapotranspiration (AET) in the above-mentioned regions is largely related to land surface properties such as aspect, shadowing and slope. Therefore, a simple, radiation driven, calibration free, bucket hydrological model for predicting AET and estimating the soil–water balance is proposed here (i.e. CLIME-MG). Conventional meteorological data from a network of automatic weather stations together with a 10 m digital terrain model (aggregated at 30 m), and a land cover map are used to inform the model. All the parameters and values required are obtained or calculated from data provided in literature.CLIME-MG has proved to perform well for AET modelling of mountain grassland. The model is validated both temporally and spatially. Temporal validation of AET is performed using eddy-covariance datasets from two different high mountain sites: a sunny and steep abandoned pasture facing S-E at an altitude of 1730 m, and a meadow with a S-SE aspect located at an altitude of 2555 m. Spatial validation is performed by comparing CLIME-MG simulations with the Landsat-based METRIC model evapotranspiration output. Results show good daily temporal performance, especially in wetter periods with recurring rainfall events. Sensitivity of the correlation coefficient between measured and modeled AET values to some key parameters such as effective porosity, and the vegetation and stress coefficients was found to be quite low. Spatial validation of hourly results shows SPAEF values in the range 0.21–0.34 between the outputs of the two models (with a similar spatial structure ruled by the DTM). Boxplots of deviations between CLIME and METRIC with respect to morphological characteristics has highlighted some dependency on elevation and slope (but not on aspect and soil depth); this suggests an opportunity to refine the modelisation of the grassland AET processes. Finally, spatial results demonstrated the non-sensitivity of the proposed model to local elevation and to the distance from the meteorological stations.

Carbon–water coupling and its relationship with environmental and biological factors in a planted Caragana liouana shrub community in desert steppe, northwest China

Abstract The carbon and water cycle, an important biophysical process of terrestrial ecosystems, is changed by anthropogenic revegetation in arid and semiarid areas. However, there is still a lack of understanding of the mechanisms of carbon and water coupling in intrinsic ecosystems in the context of human activities. Based on the CO2 and H2O flux measurements of the desert steppe with the planted shrub Caragana liouana, this study explored the carbon and water flux coupling of the ecosystem by analyzing the variations in gross primary productivity (GPP), evapotranspiration (ET) and water use efficiency (WUE) and discussing the driving mechanisms of biological factors. The seasonal variation in climate factors induced a periodic variation pattern of biophysical traits and carbon and water fluxes. The GPP and ET fluctuated in seasons, but the WUE was relatively stable in the growing season. The GPP, ET and WUE were significantly driven by global radiation (Rg), temperature (Ta and Ts), water vapor pressure deficit, leaf area index and plant water stress index (PWSI). However, Rg, temperature and PWSI were the most important factors regulating WUE. Rg and temperature directly affected WUE with a positive effect but indirectly inhibited WUE by rising PWSI. Plant water stress inhibited photosynthesis and transpiration of the planted shrub community in the desert steppe. When the plant water stress exceeded a threshold (PWSI >0.54), the WUE would decrease since the GPP responded more quickly to the plant water stress than ET. Our findings suggest that policies related to large-scale carbon sequestration initiatives under afforestation must first fully consider the status of water consumption and WUE.

Re-assessment of the climatic controls on the carbon and water fluxes of a boreal aspen forest over 1996-2016: Changing sensitivity to long-term climatic conditions.

Recent evidence suggests that the relationships between climate and boreal tree growth are generally non-stationary; however, it remains uncertain whether the relationships between climate and carbon (C) fluxes of boreal forests are stationary or have changed over recent decades. In this study, we used continuous eddy-covariance and microclimate data over 21 years (1996-2016) from a 100-year-old trembling aspen stand in central Saskatchewan, Canada to assess the relationships between climate and ecosystem C and water fluxes. Over the study period, the most striking climatic event was a severe, 3-year drought (2001-2003). Gross ecosystem production (GEP) showed larger interannual variability than ecosystem respiration (Re ) over 1996-2016, but Re was the dominant component contributing to the interannual variation in net ecosystem production (NEP) during post-drought years. The interannual variations in evapotranspiration (ET) and C fluxes were primarily driven by temperature and secondarily by water availability. Two-factor linear models combining precipitation and temperature performed well in explaining the interannual variation in C and water fluxes (R2 > .5). The temperature sensitivities of all three C fluxes (NEP, GEP and Re ) declined over the study period (p < .05), and, as a result, the phenological controls on annual NEP weakened. The decreasing temperature sensitivity of the C fluxes may reflect changes in forest structure, related to the over-maturity of the aspen stand at 100 years of age, and exacerbated by high tree mortality following the severe 2001-2003 drought. These results may provide an early warning signal of driver shift or even an abrupt status shift of aspen forest dynamics. They may also imply a universal weakening in the relationship between temperature and GEP as forests become over-mature, associated with the structural and compositional changes that accompany forest ageing.

Long-term observed evapotranspiration and its variation caused by anthropogenic controls in an ecofragile region

Arid and semiarid regions are the focus of terrestrial ecological vulnerability as a consequence of their distinctive geography and rapid evolution of anthropogenic controls. To understand the evapotranspiration (ET) variation in ecofragile regions, this study obtained seven years of eddy covariance (EC) observations from the Yulin site in Mu Us Sandy Land, and further proposed a framework to separate the impact of meteorological and anthropogenic controls on ET based on EC observations and Simple Biosphere Model-V2 (SiB2). The observed annual ET ranged from 336 mm·year−1 to 430 mm·year−1. Anthropogenic controls, namely, land cover change into bare soil, alfalfa, and potato, had obvious impacts on ET, which led to − 16.0%, 26.1% and 14.3% of the change in ET during the vegetation-growing period (April-October), respectively. At the annual scale, if alfalfa is planted, ET rises to ~120% that of natural vegetation; if potato is planted, ET remains largely the same. Remarkably, during the crop growing period of potato (June-September), ET of potato rises to ~150% that of natural vegetation. At this ecofragile region, it will take ~ 3–4 years to recover to the natural situation. This study provided a framework for delineating the contribution of meteorological and anthropogenic impacts on ET and a theoretical basis for ecological restoration and vegetation amelioration in semiarid regions.

Towards effective drought monitoring in the Middle East and North Africa (MENA) region: implications from assimilating leaf area index and soil moisture into the Noah-MP land surface model for Morocco

Abstract. The Middle East and North Africa (MENA) region has experienced more frequent and severe drought events in recent decades, leading to increasingly pressing concerns over already strained food and water security. An effective drought monitoring and early warning system is thus critical to support risk mitigation and management by countries in the region. Here we investigate the potential for assimilation of leaf area index (LAI) and soil moisture observations to improve the representation of the overall hydrological and carbon cycles and drought by an advanced land surface model. The results reveal that assimilating soil moisture does not meaningfully improve model representation of the hydrological and biospheric processes for this region, but instead it degrades the simulation of the interannual variation in evapotranspiration (ET) and carbon fluxes, mainly due to model weaknesses in representing prognostic phenology. However, assimilating LAI leads to greater improvement, especially for transpiration and carbon fluxes, by constraining the timing of simulated vegetation growth response to evolving climate conditions. LAI assimilation also helps to correct for the erroneous interaction between the prognostic phenology and irrigation during summertime, effectively reducing a large positive bias in ET and carbon fluxes. Independently assimilating LAI or soil moisture alters the categorization of drought, with the differences being greater for more severe drought categories. We highlight the vegetation representation in response to changing land use and hydroclimate as one of the key processes to be captured for building a successful drought early warning system for the MENA region.

How Has the Recent Climate Change Affected the Spatiotemporal Variation of Reference Evapotranspiration in a Climate Transitional Zone of Eastern China?

Reference evapotranspiration (ET0) is essential for agricultural production and crop water management. The recent climate change affecting the spatiotemporal variation of ET0 in eastern China continues to still be less understood. For this purpose, the latest observed data from 77 meteorological stations in Anhui province were utilized to determine the spatiotemporal variations of ET0 by the use of the Penman–Monteith FAO 56 (PMF-56) model. Furthermore, the Theil–Sen estimator and the Mann–Kendall (M–K) test were adopted to analyze the trends of ET0 and meteorological factors. Moreover, the differential method was employed to explore the sensitivity of ET0 to meteorological factors and the contributions of meteorological factors to ET0 trends. Results show that the ET0 decreased significantly before 1990, and then increased slowly. The ET0 is commonly higher in the north and lower in the south. ET0 is most sensitive to relative humidity (RH), except in summer. However, in summer, net radiation (Rn) is the most sensitive factor. During 1961–1990, Rn was the leading factor annually, during the growing season and summer, while wind speed (u2) played a leading role in others. All meteorological factors provide negative contributions to ET0 trends, which ultimately lead to decreasing ET0 trends. During 1991–2019, the leading factor of ET0 trends changed to the mean temperature (Ta) annually, during the growing season, spring and summer, and then to Rn in others. Overall, the negative contributions from u2 and Rn cannot offset the positive contributions from Ta and RH, which ultimately lead to slow upward ET0 trends. The dramatic drop in the amount of u2 that contributes to the changes in ET0 in Region III is also worth noting.

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.