Trends In Actual Evapotranspiration Research Articles

Annual and seasonal trends in actual evapotranspiration over different meteorological sub-divisions in India using satellite-based data

Annual and seasonal trends in actual evapotranspiration over different meteorological sub-divisions in India using satellite-based data

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 contribution of terrestrial water storage to actual evapotranspiration trends by the extended Budyko model in Northwest China

Actual evapotranspiration (AE) is a key component of water cycle. The arid climate of northwest China makes water supply a major constraint on AE. Water source analysis is of great significance to study the causes of AE change. In this paper, we presented an assessment of AE trends in northwest China based on GLEAM (AE_GLEAM), an ensemble of six reanalyses (AE_reanalyses), and a complementary-relationship-based AE dataset (AE_CR). Both AE_GLEAM and AE_reanalyses increased significantly (p < 0.001) during 1981–2017. AE_CR also increased to a certain extent, but the rate was slower. In general, the increasing trends of AE were mainly found in mountainous areas, such as the Altai, Tianshan, Karakoram, Kunlun, Altun, and Qilian Mountains. For these regions, AE was even higher than precipitation in terms of both the climatological mean and the linear trends. It demonstrated that terrestrial water storage (S) could give supplementary supply for AE. Therefore, we extended the Budyko framework to evaluate the effect of S on AE. Taking the landscape parameter n = 1.8 as a reference, S contributed approximately 27% of total water availability for AE. Overall, S seemed to be a major contributor to the increasing AE in the Altai, Tianshan, Qilian, and Altun Mountains. Precipitation was the main source of evaporated water in the western Tianshan to Karakoram Mountains, where potential evapotranspiration also contributed to the increase in AE. In the case of different n values, the contribution from S had a deviation range of −16.9% to 8.5%. This model provides an opportunity to assess the response of AE to S in the absence of observational data.

Long‐term mean changes in actual evapotranspiration over China under climate warming and the attribution analysis within the Budyko framework

AbstractIn 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.

Global Trends in Evapotranspiration Dominated by Increases across Large Cropland Regions

Irrigated croplands require large annual water inputs and are critical to global food production. Actual evapotranspiration (AET) is a main index of water use in croplands, and several remote-sensing products have been developed to quantify AET at the global scale. In this study, we estimate global trends in actual AET, potential ET (PET), and precipitation rate (PP) utilizing the MODIS Evapotranspiration product (2001–2018) within the Google Earth Engine cloud-computing environment. We then introduce a new index based on a combination of AET, PET, and PP estimates—the evapotranspiration warning index (ETWI)—which we use to evaluate the sustainability of observed AET trends. We show that while AET has not considerably changed across global natural lands, it has significantly increased across global croplands (+14% ± 5%). The average ETWI for global croplands is −0.40 ± 0.25, which is largely driven by an extreme trend in AET, exceeding both PET and PP trends. Furthermore, the trends in water and energy limited areas demonstrate, on a global scale, while AET and PET do not have significant trends in both water and energy limited areas, the increasing trend of PP in energy-limited areas is more than water-limited areas. Averaging cropland ETWI trends at the country level further revealed nonsustainable trends in cropland water consumptions in Thailand, Brazil, and China. These regions were also found to experiencing some of the largest increases in net primary production (NPP) and solar-induced fluorescence (SIF), suggesting that recent increases in food production may be dependent on unsustainable water inputs. Globally, irrigated maize was found to be associated with nonsustainable AET trends relative to other crop types. We present an online open access application designed to enable near real-time monitoring and improve the understanding of global water consumption and availability.

Assessment of actual evapotranspiration variability over global land derived from seven reanalysis datasets

With the motivation to identify whether global warming would lead to an increase in the rate of actual evapotranspiration (AE) over land, an assessment is made of AE variability from an ensemble of seven reanalyses (NCEP‐NCAR, NCEP‐DOE, MERRA, MERRA V2, ERA‐Interim, JRA‐55, and CFSR). By subdividing global land into nine climatic regions using the aridity index (AI), we examine the variability and long‐term trend of regional AE and various related factors. Results indicate that AE shows a significant increasing trend during 1979–2015 in the humid, wet, and arctic regions. Overall, three main parameters, namely, precipitation, surface net solar radiation, and wet‐day frequency, strongly influence the temporal variations of AE over most regions. The possible causes of trends in AE are examined in view of the linear trends of the related meteorological variables. Accordingly, increasing potential evapotranspiration and increasing vapour pressure are the main contributors to increasing AE in humid regions. Increasing temperature is the main contributor to increasing AE in wet regions. Both increasing energy supply and water supply on AE largely explain the significant increasing trend of AE in arctic regions. In view of data availability, a correlation analysis was conducted between AE in individual reanalyses and the results from the Mezentsev–Choudhury–Yang equation and the ensemble of the seven reanalyses. On average, NCEP‐NCAR is the worst among the seven reanalysis datasets in revealing temporal characteristics of AE.

Systematic variation in evapotranspiration trends and drivers across the Northeastern United States

AbstractThe direction and magnitude of responses of evapotranspiration (ET) to climate change are important to understand, as ET represents a major water and energy flux from terrestrial ecosystems, with consequences that feed back to the climate system. We inferred multidecadal trends in water balance in 11 river basins (1940–2012) and eight smaller watersheds (with records ranging from 18 to 61 years in length) in the Northeastern United States. Trends in river basin actual ET (AET) varied across the region, with an apparent latitudinal pattern: AET increased in the cooler northern part of the region (Maine) but decreased in some warmer regions to the southwest (Pennsylvania–Ohio). Of the four small watersheds with records longer than 45 years, two fit this geographic pattern in AET trends. The differential effects of the warming climate on AET across the region may indicate different mechanisms of change in more‐ vs. less‐energy‐limited watersheds, even though annual precipitation greatly exceeds potential ET across the entire region. Correlations between AET and time series of temperature and precipitation also indicate differences in limiting factors for AET across the Northeastern U.S. climate gradient. At many sites across the climate gradient, water‐year AET correlated with summer precipitation, implying that water limitation is at least transiently important in some years, whereas correlations with temperature indices were more prominent in northern than southern sites within the region.

Analysis of Dynamic Spatiotemporal Changes in Actual Evapotranspiration and Its Associated Factors in the Pearl River Basin Based on MOD16

Evapotranspiration is an important part of the hydrological cycle, surface energy balance and global climate system. Due to spatial heterogeneity, the trends in actual evapotranspiration (ET) and its associated factors vary in different regions. Because direct measurements of ET are limited over large areas, remote sensing provides an efficient method of ET spatial analysis, and standard data products are available at the global scale. This study uses the monthly MOD16 ET dataset and daily meteorological data to analyze the dynamic spatiotemporal changes in ET and its associated factors in the Pearl River Basin (PRB) from 2000 to 2014. The results of the study are as follows. (1) Over time and space, annual ET exhibited a slight increasing trend from 2000 to 2014, with an average value of approximately 946.56 mm/a. ET considerably varied at the monthly and seasonal scales, and in July displayed the highest monthly ET of approximately 119.57 mm, accounting for 36.37% of the annual ET. (2) ET displayed obvious spatial heterogeneity. Specifically, the west was a low-ET region, and moderate and high ET values were interspersed in the central and eastern PRB. Moreover, the rate of change of ET ranged from −13.99 mm/a to 12.81 mm/a in space, and 46.25% of the basin exhibited an increasing trend. (3) Dynamic changes in ET were mainly associated with temperature and relative humidity (RH). Additionally, energy-related elements and wind speed were positively correlated with ET, and temperature was the most influential factor of ET in some months (February–March and September–November). RH was the most important factor in other months but negatively correlated with ET in June and July. Affected by the actual environmental condition, qualitative changes were observed in the correlation between RH and ET in different months. The positive and negative spatial correlations between ET and its associated factors changed in different regions and in different months, and the changes mainly occurred from northwest to southwest.

On the divergence of potential and actual evapotranspiration trends: An assessment across alternate global datasets

AbstractEvapotranspiration is a key variable in hydrology, playing an important role in water and energy balance of the land surface. There has been speculation on the direction of trend in potential and actual evapotranspiration (PET and AET) resulting from rising global temperatures, in both observational and derived records representing the historical climate. In this study, PET and AET trends of eight different global model datasets where analyzed over two time periods: from 2003 to 2012 (short term) and from 1980 to 2012 (multidecadal), to identify regions where the trends coincide or differ and to study the reasons behind these changes. The short‐term analysis showed considerable uncertainty exists on the detection and direction of significant trends on both PET and AET. There was little agreement among the datasets about the direction of the global trends. The multidecadal study showed much more consistent trends throughout the datasets, particularly in relation to positive significant PET trends. During this period, the global PET mean increased 0.091 mm month−1 yr−1, while the global AET rose at 0.045 mm month−1 yr−1. Much of the opposite PET/AET trends can be attributed to changes in the precipitation. Most of the regions which present these trends are water‐limited and present strong correlations between AET and precipitation trends. Some energy‐limited regions showed an increasing gap between PET and AET, suggesting the influence of additional variables controlling AET.

Assessing the ability of potential evapotranspiration models in capturing dynamics of evaporative demand across various biomes and climatic regimes with ChinaFLUX measurements

Estimates of atmospheric evaporative demand have been widely required for a variety of hydrological analyses, with potential evapotranspiration (PET) being an important measure representing evaporative demand of actual vegetated surfaces under given metrological conditions. In this study, we assessed the ability of various PET models in capturing long-term (typically 2003–2011) dynamics of evaporative demand at eight ecosystems across various biomes and climatic regimes in China. Prior to assessing PET dynamics, we first examined the reasonability of fourteen PET models in representing the magnitudes of evaporative demand using eddy-covariance actual evapotranspiration (AET) as an indicator. Results showed that the robustness of the fourteen PET models differed somewhat across the sites, and only three PET models could produce reasonable magnitudes of evaporative demand (i.e., PET≥AET on average) for all eight sites, including the: (i) Penman; (ii) Priestly-Taylor and (iii) Linacre models. Then, we assessed the ability of these three PET models in capturing dynamics of evaporative demand by comparing the annual and seasonal trends in PET against the equivalent trends in AET and precipitation (P) for particular sites. Results indicated that nearly all the three PET models could faithfully reproduce the dynamics in evaporative demand for the energy-limited conditions at both annual and seasonal scales, while only the Penman and Linacre models could represent dynamics in evaporative demand for the water-limited conditions. However, the Linacre model was unable to reproduce the seasonal switches between water- and energy-limited states for some sites. Our findings demonstrated that the choice of PET models would be essential for the evaporative demand analyses and other related hydrological analyses at different temporal and spatial scales.

Understanding hydrological trends by combining the Budyko hypothesis and a stochastic soil moisture model

The actual evapotranspiration and runoff trends of five major basins in China from 1956 to 2000 are investigated by combining the Budyko hypothesis and a stochastic soil moisture model. Based on the equations of Choudhury and Porporato, the actual evapotranspiration trends and the runoff trends are attributed to changes in precipitation, potential evapotranspiration, rainfall depth and water storage capacity which depends on the soil water holding capacity and the root depth. It was found that the rainfall depth increased significantly in China during the past 50 years, especially in southern basins. Contributions from changes in the water storage capacity were significant in basins where land surface characteristics have changed substantially due to human activities. It was also observed that the actual evapotranspiration trends are more sensitive to precipitation trends in water-limited basins, but more sensitive to potential evapotranspiration trends in energy-limited basins.Editor D. Koutsoyiannis; Associate editor A. Porporato

Data‐driven regionalization of river discharges and emergent land cover–evapotranspiration relationships across Sweden

AbstractChanges in river discharge and river water quality, due to climate change and other drivers such as land cover change, pose both societal and ecosystem threats. Analyses of measured terrestrial river fluxes are key for identifying the drivers and quantifying the magnitudes of such riverine changes. In this paper, we develop and apply a data‐driven regionalization approach using the dense network of discharge measurements in Sweden. The developed regionalization approach facilitates detailed mapping of discharges (Q) and change trends in Q across Sweden. Combining these with estimates of precipitation (P) and change trends in P, we estimated actual evapotranspiration (AET) and change trends in AET via catchment‐scale water balance constraints. We identified characteristic land cover–evapotranspiration relationships by plotting water use efficiency (AET/P) against energy use efficiency (AET/potential ET) for areas with unique land cover across Sweden. Our results show that wetlands have clearly lower water and energy use efficiencies compared to open waters, forests, and agriculture, and that agriculture has water and energy use efficiencies closest to those of open waters. We further compared the data‐driven regionalization estimates of different water balance components with estimates of regional climate models (RCMs). The RCMs do not describe well the observed change trends in Sweden. In particular, for evapotranspiration, the observed change trends are not reproduced by any of the investigated 24 RCMs.

Land‐atmosphere coupling explains the link between pan evaporation and actual evapotranspiration trends in a changing climate

Decreasing trends in pan evaporation are widely observed across the world as a response of the climate system to changes in temperature, precipitation, incoming radiation and wind speed. Nevertheless, we only partially understand how trends in actual evapotranspiration are linked to those trends. Here, we use a model to show that regulation of the near‐surface temperature and humidity by land‐atmosphere feedbacks results in a strong connection between pan evaporation, actual evapotranspiration and vapor pressure deficit (VPD) depending on the climate forcings. When climate change occurs, the feedbacks direct the system towards a different combination of the three variables. If we know the trends in pan evaporation, VPD and wind speed, we can therefore infer the change in the forcings and estimate the trend in actual evapotranspiration.

Evaluating the spatiotemporal variations of water budget across China over 1951–2006 using IBIS model

AbstractThe Integrated Biosphere Simulator is used to evaluate the spatial and temporal patterns of the crucial hydrological variables [run‐off and actual evapotranspiration (AET)] of the water balance across China for the period 1951–2006 including a precipitation analysis. Results suggest three major findings. First, simulated run‐off captured 85% of the spatial variability and 80% of the temporal variability for 85 hydrological gauges across China. The mean relative errors were within 20% for 66% of the studied stations and within 30% for 86% of the stations. The Nash–Sutcliffe coefficients indicated that the quantity pattern of run‐off was also captured acceptably except for some watersheds in southwestern and northwestern China. The possible reasons for underestimation of run‐off in the Tibetan plateau include underestimation of precipitation and uncertainties in other meteorological data due to complex topography, and simplified representations of the soil depth attribute and snow processes in the model. Second, simulated AET matched reasonably with estimated values calculated as the residual of precipitation and run‐off for watersheds controlled by the hydrological gauges. Finally, trend analysis based on the Mann–Kendall method indicated that significant increasing and decreasing patterns in precipitation appeared in the northwest part of China and the Yellow River region, respectively. Significant increasing and decreasing trends in AET were detected in the Southwest region and the Yangtze River region, respectively. In addition, the Southwest region, northern China (including the Heilongjiang, Liaohe, and Haihe Basins), and the Yellow River Basin showed significant decreasing trends in run‐off, and the Zhemin hydrological region showed a significant increasing trend. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Trends in Land Evapotranspiration over Canada for the Period 1960–2000 Based on In Situ Climate Observations and a Land Surface Model

Abstract An assessment of annual trends in actual evapotranspiration (AET) and associated meteorological inputs is performed at 101 locations across Canada with available long-term hourly surface climate observations to determine if AET in Canada is increasing in relation to observed increases in air temperature. AET was estimated for the dominant land cover class, with representative soil and leaf area index conditions, within a 50 km × 50 km window around each location for the period 1960–2000. The Ecological Assimilation of Land and Climate Observations (EALCO) land surface model, which simulates coupled carbon, energy, and water cycles, was applied to estimate AET on a half-hourly basis at each location using in situ meteorological measurements and ambient atmospheric CO2 concentrations. Increases in annual AET, of up to 0.73% yr−1, were identified at 81 locations, and decreases, of up to 0.25% yr−1, were found at the remaining 20 stations. Statistically significant increasing trends were detected in 35% of the locations with the majority corresponding to Atlantic and Pacific coastal regions. Increasing trends were generally related to increasing temperature and total downwelling surface radiation trends in eastern Canada and increasing temperature, surface radiation, and precipitation trends in western Canada. In sharp contrast to other studies based on simpler AET models, annual AET trends in the prairie climate zone were mixed in terms of increases and decreases with no locations showing statistically significant trends. Future studies focused on scaling AET model estimates to subbasins or basins are required both to account for this spatial variability in soil conditions and to permit water budget closure validation.

Response of the water balance to climate change in the United States over the 20th and 21st centuries: Results from the VEMAP Phase 2 model intercomparisons

Using the VEMAP Phase 2 data set, we tested the hypothesis that changes in climate would result in changes in the water balance as projected by four terrestrial ecosystem models: BIOME‐BGC, Century, LPJ, and MC1. We examined trends in runoff and actual evapotranspiration (AET), changes in runoff in relation to changes in precipitation, and differences in runoff ratios as produced by these models for 13 United States watersheds. Observed climate data were used as inputs for simulations covering 1895–1993. From 1994 to 2100, the Canadian Centre for Climate Modeling and Analysis (CGCM1) and the Hadley Centre for Climate Prediction and Research (HADCM2) general circulation models provided climate forcing. Runoff and AET trends were significantly positive in the majority of 13 watersheds examined. Percentage changes in runoff exceeded the underlying changes in precipitation and this amplification increased over time. Calculated runoff ratios showed model variability and differences based on the two GCM scenarios.