Mean Annual Evapotranspiration Research Articles

Elevation Influences Belowground Biomass Proportion in Forests by Affecting Climatic Factors, Soil Nutrients and Key Leaf Traits.

Forest biomass allocation is a direct manifestation of biological adaptation to environmental changes. Studying the distribution patterns of forest biomass along elevational gradients is ecologically significant for understanding the specific impacts of global change on plant resource allocation strategies. While aboveground biomass has been extensively studied, research on belowground biomass remains relatively limited. Furthermore, the patterns and driving factors of the belowground biomass proportion (BGBP) along elevational gradients are still unclear. In this study, we investigated the specific influences of climatic factors, soil nutrients, and key leaf traits on the elevational pattern of BGBP using data from 926 forests at 94 sites across China. In this study, BGBP data were calculated from the root biomass to the depth of 50 cm. Our findings indicate considerable variability in forest BGBP at a macro scale, showing a significant increasing trend along elevational gradients (p < 0.01). BGBP significantly decreases with increasing temperature and precipitation and increases with annual mean evapotranspiration (MAE) (p < 0.01). It decreases significantly with increasing soil phosphorus content and increases with soil pH (p < 0.01). Key leaf traits (leaf nitrogen (LN) and leaf phosphorus (LP)) are positively correlated with BGBP. Climatic factors (R2 = 0.46) have the strongest explanatory power for the variation in BGBP along elevations, while soil factors (R2 = 0.10) and key leaf traits (R2 = 0.08) also play significant roles. Elevation impacts BGBP directly and also indirectly through influencing such as climate conditions, soil nutrient availability, and key leaf traits, with direct effects being more pronounced than indirect effects. This study reveals the patterns and controlling factors of forests' BGBP along elevational gradients, providing vital ecological insights into the impact of global change on plant resource allocation strategies and offering scientific guidance for ecosystem management and conservation.

Assessing Groundwater Recharge in the Wabe River Catchment, Central Ethiopia, through a GIS-Based Distributed Water Balance Model

The utilization of groundwater has emerged as an indispensable asset in facilitating economic advancement, preserving ecological integrity, and responding to the challenges posed by climate change, especially in regions characterized by aridity and semi-aridity. The sustainable management of water resources requires an assessment of the geographical and temporal patterns of groundwater recharge. The present study employed the GIS-based WetSpass-M model to model the water balance components by utilizing hydro-meteorological and biophysical data from the Wabe catchment, which spans an area of 1840 km2 in central Ethiopia, for a long time. The objective of this study was to assess the long-term average annual and seasonal groundwater recharge for the catchment area utilizing the WetSpass-M model. The input data were collected through remote sensing data and surveys in the field. The model was employed to gain insights into the process of groundwater recharge in a particular region and to facilitate effective management, prudent utilization, and sustainable planning of water resources in the long run. Water balance components were estimated using seasonal fluctuations in evapotranspiration, surface runoff, and groundwater recharge. The Wabe catchment’s summer, winter, and mean long-term yearly groundwater recharge were determined to be 125.5 mm, 78.98 mm, and 204.51 mm, respectively. The model indicates that summer seasons account for 86.5% of the mean annual precipitation, while winter seasons account for 13.5%. On the other hand, the groundwater system percolates 14.8% of the total annual rainfall (1374.26 mm). While evapotranspiration accounts for 51% of total precipitation and surface runoff accounts for 34.1%, the Wabe catchment’s mean annual evapotranspiration and surface runoff values are simulated at 701.11 mm and 485.58 mm, respectively. The findings suggest the use of the WetSpass-M model to precisely calculate the water balance components within the Wabe catchment.

Energy partitioning and controlling factors of evapotranspiration in an alpine meadow in the permafrost region of the Qinghai-Tibet Plateau

Abstract Energy partitioning and evapotranspiration (ET) of alpine meadows in permafrost areas are crucial for water cycle on the Qinghai-Tibet Plateau. However, seasonal (freeze–thaw cycle) variations in energy partitioning and ET and their driving factors must be clarified. Therefore, 4-year energy fluxes [i.e. latent heat (LE) and sensible heat (H)] were observed, and bulk parameters [i.e. surface conductance, decoupling coefficient (Ω), and Priestley–Taylor coefficient (α)] were estimated in an alpine meadow in the Qinghai-Tibet Plateau. Mean daily LE (27.45 ± 23.89 W/m2) and H (32.51 ± 16.72 W/m2) accounted for 31.71% and 50.14% of available energy, respectively. More available energy was allocated to LE during the rainfall period, while 67.54 ± 28.44% was allocated to H during the frozen period. H was half the LE during rainfall period and seven times the LE during frozen period due to low soil water content and vegetation coverage during the frozen season. Mean annual ET was 347.34 ± 8.39 mm/year, close to mean annual precipitation. Low mean daily Ω (0.45 ± 0.23) and α (0.60 ± 0.29) throughout the year suggested that ET in the alpine meadow was limited by water availability. However, ET was constrained by available energy because of sufficient water supply from precipitation during rainfall season. In contrast, large differences between ET and precipitation indicated that soil water was supplied via lateral flow from melting upstream glaciers and snow during the transition season. The results suggest that seasonal variations in bulk parameters should be considered when simulating water and energy fluxes in permafrost regions.

The Spatial and Temporal Variability of the Blue–Green Spatial Structures of the South Dongting Lake Wetland Areas Amidst Climate Change, including Its Relationship with Meteorological Factors

In recent years, the water level of the Dongting Lake (DTL) has been continuously low, and the wetland area and landscape pattern have changed significantly. Considering the obvious spatial heterogeneity of water regime changes in different waters of the DTL, this paper takes two core areas of the South Dongting Lake Nature Reserve (SDLNR) as study areas and analyzes the spatial distribution characteristics of the wetland blue–green landscape patterns by using remote sensing image data and hydrological and meteorological data. The multi-scale correlation between runoff, precipitation, temperature, and evapotranspiration in the SDLNR was studied via cross-wavelet transform analysis. The results show the following: (1) The change in the blue–green spatial patterns in different regions in different periods is inconsistent, and this inconsistency is related to the topography, climate, and human activities in each region; (2) there are seasonal fluctuations in precipitation, air temperature, and evapotranspiration in the SDLNR. Among them, the annual mean temperature shows a rising trend and passes the significance test with 95% confidence, while the annual mean precipitation and annual mean evapotranspiration show no significant change trend; and (3) our Pearson correlation analysis and cross-wavelet change results show that precipitation and temperature are strongly correlated with runoff, with a resonance period of 8–16 months, while the correlation between evapotranspiration and runoff is not significant. We recommend that policymakers establish an effective early warning system and make plans to store water through micro-terrain transformation in possible climate change treatments and strategies.

Variability in evapotranspiration in the Andean páramo as influenced by fog and rainfall

AbstractTropical mountains such as the páramos of the Andes, which serve as ‘water towers’ for local communities and downstream cities, are important areas for early detection of climate change. Here, fog and low‐intensity rainfall are very common and play a key role in ecohydrological processes. Although evapotranspiration (ET) represents an important part of the water cycle, how ET and fog processes interact and how they affect páramo vegetation and water resources availability are poorly understood. This study investigated the effects of foggy (fog only) and mixed (fog and rainfall) conditions on ET. To determine whether fog significantly reduces ET, we compared ET and meteorological data under these two conditions with those during dry days. We found that on foggy days, when fog was most prevalent in the early morning, ET declined on average by 4% and net radiation (Rn) by 9.2%. Under mixed conditions, daily ET declined by 42% and Rn by 33%. In the páramo, where mean annual precipitation and ET are 1210 and 635 mm, respectively, the estimated annual reduction in ET due to fog and rainfall combined is between 77 and 174 mm. We found that during fog and rainfall mixed conditions, solar radiation was reduced, consequently constraining the energy available for ET while sustaining high relative humidity, ultimately reducing water loss. Our findings, which suggest that the presence of fog and low‐intensity rainfall restricts water losses by evaporative demand, contribute to a better understanding of the ecohydrological importance of these water inputs in the Andes.

Dynamics and variability of microclimate in an unmanaged mountain forest after a bark beetle outbreak

Forest disturbances alter hydrological, energy, and microclimatic characteristics and their extents vary with the landscape characteristics of the terrain. We evaluated changes in evapotranspiration and latent heat rate in an unmanaged, central European mountain forest (Norway spruce) after a bark beetle-induced mortality of >75 % trees, and the spatial variability in microclimate during its natural regeneration. We observed that (1) the annual mean incident solar radiation to the treeless plots was 8.2 MJ m−2 day−1 (95 W m−2), being reduced by ∼45 % compared to its theoretical flux by high average cloudiness (70.6%) and short daily sunshine (4.3 hour day−1). The observed energy input to the forest floor beneath survived canopies (1.2 MJ m−2 day−1) was an order of magnitude lower than solar radiation to the canopy surface. (2) The mean annual evapotranspiration and the average annual latent heat rate decreased by 155 mm yr−1 and 385 MJ m−2 yr−1 (12 W m−2), respectively, after tree dieback. (3) The soil and air temperatures at individual plots correlated positively with the incident solar radiation and negatively with elevation. Mean soil and air temperatures exhibited similar lapse rates of 5.9–8.0 °C km−1 during the growing season (May-October). The energy input (positive relationship) and elevation (negative relationship) explained 74–83 % of the between-plot variability in mean soil and air temperatures and 51–77 % of their daily amplitudes during the growing season. (4) On sunny days, the observed soil and air temperatures were significantly lower (by ∼50 and 15 %, respectively) than ground surface temperature based on thermal satellite data (Landsat-8). The ground surface temperature explained 80–92 % of the observed variability in the soil and air temperatures. (5) The between-plot differences in daily soil and air temperatures and their amplitudes were from 24 to 70 % explained by solar radiation (positive correlation) and relative air humidity and wind speed (negative correlations).

Impacts of heat and drought on the dynamics of water fluxes in a temperate deciduous forest from 2012 to 2020

Temperate deciduous forests play a significant role in the regional and global water cycles. However, climate change and associated extreme weather events are affecting the growth, water use, and survival of these forests. This study examined nine years of water flux measurements made using the eddy covariance (EC) technique in a temperate deciduous forest in eastern North America from 2012 to 2020 and assessed the impacts of heat and drought events on forest evapotranspiration (ET) and water use efficiency (WUE). Study results showed that the highest annual evapotranspiration (ET) of 521 mm yr−1 was observed in 2020, while the lowest annual ET of 359 mm yr−1 was recorded in 2014. Photosynthetically active radiation (PAR, lmg = 0.33) and air temperature (Tair, lmg = 0.33) were the primary drivers of ET, while vapor pressure deficit (VPD, lmg = 0.41) was the primary regulator of WUE. Over the nine-year study period, observed mean annual ET and precipitation (P) values were 367 ± 40 mm yr−1 and 1148 ± 262 mm yr−1, respectively, while mean annual P-ET value was 729 ± 272 mm yr−1. Annual ET accounted for 38% of P over the study period. Mean growing season WUE, which is an indicator of water utilization by the forest for growth and carbon assimilation, was 3.8 ± 0.4 g C Kg H2O−1, with the highest WUE of 4.4 g C Kg H2O−1 observed in 2014 (cool year), and the lowest values of 3.0 and 3.1 g C Kg H2O−1observed in 2012 (hot-dry year) and 2020 (dry year), respectively. We found an increasing trend in WUE values from 2012 to 2014 and then a decreasing trend from 2015 to 2020, when every year experienced extreme weather conditions (e.g. hot, dry or hot-dry), except 2019. This long-term flux observation study will help to enhance our understanding of water exchanges and water use in temperate deciduous forests in Eastern North America and develop strategies for managing forest water resources and ensuring water security in the region in changing climate.

Trade-offs and synergies relationships of ecosystem services and their socio-ecological driving factors under different spatial scales in Shaoguan City, Guangdong, China.

Ecosystem services refer to the benefits that human obtain from natural ecosystems. Different ecosystem services are generated by the combination of social-ecological driving factors, and exhibit different spatial patterns across scales. The complex relationships and driving mechanisms among ecosystem services under different spatial scales remain unclear. With Shaoguan City from Guangdong Province as the study area, we analyzed the spatial patterns and relationships of four ecosystem services and their trade-offs/synergies (TOSs), quantified their responses to seven social-ecological drivers at the kilometer grid scale and sub-watershed scale, and proposed regional ecologi-cal management and planning strategies for cross-scale sustainable development. The results showed that the spatial distribution of ecosystem services in Shaoguan City exhibited spatial clustering and cross-scale variations. Habitat quality, water yield, and carbon storage exhibited similar spatial distribution pattern. High supply was mainly distributed in mountainous areas in the east, north, west, and south, while weak supply was distributed in plain areas in the central, northwest, south and northeast. In addition, the spatial clustering of these services intensified with increasing spatial scale. Ecosystem services displayed synergistic relationships at both spatial scales, and the intensity of the synergy changed with scale. At both the kilometer grid and sub-watershed scale, the primary drivers for ecosystem services were the normalized vegetation index and digital elevation model. The main driver for TOSs was the mean annual temperature at the kilometer grid scale, while it was the mean annual evapotranspiration at the sub-watershed scale. Based on the supply levels of ecosystem services, the study area could be divided into five distinct ecosystem service bundles, i.e., mountain ecological balance zone, forest ecological conservation zone, urban forest maintenance zone, ecologically sensitive zone, and ecological risk zone. All bundles exhibited both spatial heterogeneity and cross-scale variations. We integrated the cross-scale variations of four representative ecosystem services and their complex interactions and driving mechanisms in Shaoguan City into spatial planning to facilitate the sustainable ecosystem management across multiple scales, which could offer valuable references for the construction of ecological civilization in other regions.

Patterns of Water Consumption in Longleaf Pine Restoration Areas and the Relationship with Cone Production

Monitoring water consumption dynamics across the geographic range of an ecosystem may indicate the possible variation and stress in a biome. Here, model output data based on remote sensing (1979–2022) were used to examine the water consumption dynamics and effects on cone production in three geographic margins in the longleaf pine’s range (i.e., Bladen Lake State Forest, Escambia Experimental Forest, and Kisatchie National Forest) under varying climatic conditions. Results indicated that the mean annual transpiration at Escambia was approximately 431 mm and that at Bladen and Kisatchie was 500 mm. Mean monthly transpiration peaked twice (June and October) at Escambia but only once (August) at Bladen and Kisatchie. The mean annual evapotranspiration ranged from approximately 900 mm at Kisatchie to about 791 mm at Escambia and Bladen. The mean annual transpiration/evapotranspiration ratio was about 0.65 at Bladen and 0.55 at Escambia and Kisatchie. A significant correlation existed between evapotranspiration and specific humidity across the sites on a monthly scale but not on a yearly scale. Significant negative relationships existed between precipitation and the ratios of transpiration/precipitation and evapotranspiration/precipitation on the yearly scale across the sites. Negative power relationships were observed between precipitation and the specific humidity/precipitation ratio on monthly and yearly scales. Cone production was generally highest in years with moderate water consumption. These results provide baseline information on how hydrological and ecological processes of longleaf pine forests interact with climate across broad spatial and temporal scales.

Calibrating a remote sensing evapotranspiration model using the Budyko framework

Remote sensing (RS) evapotranspiration (ET) models are commonly used to estimate ET at a large scale. These models often include spatially varying parameters that need to be determined. Typically, these parameters are either held as constants or obtained from a pre-specified lookup table, potentially introducing bias into ET estimates. This study aimed to evaluate the feasibility of calibrating a RS ET model using Budyko-derived mean annual ET. We applied this approach to calibrate the single parameter of the Penman-Monteith-Leuning (PML) model. Instead of using the default Budyko-Fu parameter ω (ω=2.6), we developed a data-driven model that relates the basin-calibrated ω and its primary controls. Subsequently, we mapped the spatial pattern of ω across China. The calibrated PML model was evaluated using flux observations from 12 ChinaFlux sites and water balance-based ET estimates from 80 basins in China. Results indicate that the Budyko-based calibration significantly improved the performance of the PML model at both site and basin scales compared to the uncalibrated model. This study highlights the potential of leveraging the Budyko framework to constrain the parameter estimates of RS ET models, thereby enhancing the accuracy of large-scale ET estimates.

Quantification of Spatiotemporal Variability of Evapotranspiration (ET) and the Contribution of Influencing Factors for Different Land Cover Types in the Yunnan Province

Evapotranspiration (ET) is an important component of terrestrial ecosystems and is sensitive to climate and land-use change due to its obvious link to ecohydrological processes. Therefore, understanding the spatiotemporal variability of evapotranspiration and its drivers under different land cover types plays an important role in estimating the impact of environmental change on the regional water cycle. In this study, we first estimated the spatiotemporal variations of ET for different land cover types in the Yunnan Province from 2001 to 2020 using the MODIS-Terra ET product (MOD16A2.06) and meteorological datasets, and quantified the contribution of six factors: namely, temperature (TEMP), precipitation (PRCP), relative humidity (RH), wind speed (WDSP), soil moisture (SLME), NDVI, elevation, and slope, to the ET under different land cover types by using a ridge regression model. We then discussed the main reasons for the differences in ET in the Yunnan Province under different land cover types. The conclusions are as follows: during the study period, the annual mean ET ranged from 27 to 1183 mm, and there was a large spatial heterogeneity in its spatial distribution, with the smallest increasing trend of 2.1 mm/year in agricultural land and the largest increasing trend of 4.7 mm/year in grassland. Except for cropland, the sum of the relative contributions of the three influence factors, precipitation (PRCP), NDVI, and elevation, to all land cover types exceeded 40%, making them the most dominant factors influencing ET changes in the Yunnan Province. This study provides a comprehensive assessment of the impacts of climate, vegetation, topography, and soils on ET, and contributes to the development of appropriate water resource management policies for different subsurface types in the context of climate warming and revegetation programs.

Interannual variation in evapotranspiration in an urban forest reserve with respect to drought

IntroductionA warming global climate is expected to perturb the hydrological cycle, resulting in deviations in both frequency and duration of drought and thus being hypothesized to lead to interannual variation in evapotranspiration (ET). Interannual variation in ET in urban forest ecosystems in response to drought remains poorly understood.MethodsHere, ET in an urban forest reserve in the megalopolis of Beijing was investigated using eddy-covariance measurements collected over six consecutive years (2012–2017).ResultsThe mean annual cumulative ET was 462 ± 83mm (±first standard deviation), with a coefficient of variation of 18%. Interannual variation in both annual and monthly ET was shown to be largely controlled by canopy conductance (gs), affected by environmental factors. The main factors affecting interannual variation in monthly ET varied seasonally, namely, soil volumetric water content (VWC) and normalized difference vegetation index (NDVI) in spring, precipitation and soil temperature in summer, and VWC and net radiation (Rn) in autumn. Interannual variation in annual ET was driven largely by spring and mid-summer droughts induced by insufficient precipitation during the non-growing and mid-growing seasons, respectively. Spring drought reduced annual ET by restricting leafing out, shortening growing season length (GSL), and reducing the normalized difference vegetation index (NDVI). The summer drought reduced annual ET by reducing stomatal conductance.Discussion and conclusionResults from this study point to the importance of precipitation timing and volume and the soil moisture carry-over effect in controlling interannual variation in ecosystem ET. Irrigation during the early spring and mid-summer is viewed as a practical management measure for sustaining growth and better ecosystem services in urban forests in Northern China.

Controlling Factors of the Spatial‐Temporal Fluctuations in Evapotranspiration Along an Elevation Gradient Across Humid Montane Ecosystems

AbstractDiverse meteorological and vegetation conditions at different elevations and years lead to varied contributions of evapotranspiration (ET) to water yield in montane areas. However, how physical and biological factors affect interannual and elevational ET fluctuations in energy‐limited montane ecosystems remains unclear. Therefore, we quantified the spatial‐temporal patterns of ET and separated physical and biological controls on ET fluctuations using 5‐year (2015–2019) eddy‐covariance data and perturbation analysis on Mount Gongga, Southwest China. Coefficients of variations of annual ET were 0.85%–4.33%, suggesting the remarkable interannual consistencies of ET in all studied zones. In forest zones, decreased precipitation (P) contributed more than 85% of ET reduction in the severe dry year, while the decreased vapor pressure deficit and available energy accounted for 88%, 78%, and 87% of transpiration reductions in broadleaf forest, broadleaf and coniferous forest and coniferous forest, respectively, in the wet year. In shrubland, the positive influence from biological factors (41.37 mm) surpassed the negative impact from physical conditions (−16.46 mm), resulting in an increase in ET in the wet year. Mean annual ET was highest at 2,200 m a.s.l., 15.04% higher than that at 3,996 m a.s.l. From perturbation analysis, elevational ET fluctuations were smaller in low‐elevation (12.31 mm) and middle‐elevation (−31.18 mm) zones than that in high‐elevation zone (−99.86 mm). Large elevational ET reduction in high‐elevation zone mainly resulted from decreased P and leaf area index. In humid montane areas, although ET is predicted to increase, how interannual and elevational ET respond to climate changes should be considered separately.

Vegetation optimality explains the convergence of catchments on the Budyko curve

Abstract. The Budyko framework puts the long-term mean annual evapotranspiration (ET) of a catchment in relation to its maximum possible value determined by the conservation of mass (ET cannot exceed mean annual precipitation) and energy (ET can not exceed mean annual net radiation) in the absence of significant storage contributions. Most catchments plot relatively close to this physical limit, which allowed the development of an empirical equation (often referred to as the Budyko curve) for estimating mean annual evaporation and runoff from observed net radiation and precipitation. Parametric forms of the curve often use a shape parameter, n, that is seen as a catchment characteristic. However, a satisfying explanation for the convergence and self-organization of catchments around such an empirical curve is still lacking. In this study, we explore if vegetation optimality can explain the convergence of catchments along a Budyko curve and in how far can n be seen as a catchment characteristic. The Vegetation Optimality Model (VOM) optimizes vegetation properties and behavior (e.g., rooting depths, vegetation cover, stomatal control) to maximize the difference between the total carbon taken up from the atmosphere and the carbon used for maintenance of plant tissues involved in its uptake, i.e., the long-term net carbon profit (NCP). This optimization is entirely independent of observed ET and hence the VOM does not require calibration for predicting ET. In a first step, the VOM was fully optimized for the observed atmospheric forcing at five flux tower sites along the North Australian Tropical Transect, as well as 36 additional locations near the transect and six Australian catchments. In addition, the VOM was run without vegetation for all sites, meaning that all precipitation was partitioned into soil evaporation and runoff. For comparison, three conceptual hydrological models (TUWmodel, GR4J, and FLEX) were calibrated for the Australian catchments using the observed precipitation and runoff. Subsequently, we emulated step changes in climate by multiplying precipitation (P) by factors ranging between 0.2 and 2 before running the VOM and hydrological models without changing the vegetation properties or model parameters, emulating invariant catchment characteristics under a changed climate. In a last step, the VOM was re-optimized for the different P amounts, allowing vegetation to adapt to the new situation. Eventually, Budyko curves were fit by adapting the parameter n to the model results. This was carried out for both multiple sites simultaneously and for each individual study site, thereby assuming that n is a site-specific characteristic. The optimized VOM runs tracked relatively close to a Budyko curve with a realistic n value and close to observations, whereas the runs without vegetation led to significantly lower evaporative fractions and unrealistically low n values compared with literature. When fitting n to individual catchments, changes in P led to changes in n (increasing n for decreasing P) in all model runs (including the three conceptual models) except if the VOM was re-optimized for each change in P, which brought the value of n back close to its value for the unperturbed P in each catchment. For the re-optimized VOM runs, the variation in n between catchments was greater than within each catchment in response to multiplications of P with a factor 0.2 to 2. These findings suggest that optimality may explain the self-organization of catchments in Budyko space, and that the accompanying parameter n does not remain constant for constant catchment and vegetation conditions as hypothesized in the literature, but in fact emerges through the adaptation of vegetation to climatic conditions in a given hydrological setting. Moreover, the results suggest that n might initially increase in response to suddenly reduced P, and only slowly returns to its original, catchment-specific value, as vegetation re-adjusts to the new climate over decades and centuries. This may constitute a new basis for the evaluation and prediction of catchment responses to climatic shifts.

Long-term climatological and ecohydrological analysis of a paired catchment – flux tower observatory near Dresden (Germany). Is there evidence of climate change in local evapotranspiration?

Water budgets and climate are related in many ways and at all scales. Therefore, we expect climate change to trigger changes in all water budget components at any scale. For Central Europe observed and projected climate change indicates higher variability of precipitation, while evapotranspiration (ET) should increase due to higher temperatures, yielding lower and more variable infiltration and runoff. However, evidence in ET records is limited, as long-term measurements of ET are methodologically challenging and as factors other than climate are changing in parallel, like vegetation and land use.In this study, we take advantage of long-term hydro-meteorological data from the small research catchment Wernersbach (4.6 km2, dominated by Norway spruce) in operation since 1967 and from two eddy-covariance (EC) flux towers, all located in the Tharandt Forest, Germany. The tower DE-Tha is located a few kilometres east of the catchment, is spruce dominated and in operation since 1996. After a wind break of a spruce stand (situated inside the catchment) and planting of deciduous oaks, the tower DE-Hzd was set up in 2009. For the first time, we report systematically about observation, correction methods and metadata of the long data series of the observatory, represented by the Wernersbach catchment and the EC flux towers.Climate change signals in the region are mirrored in the Tharandt Forest records. They show rising air temperature with a breakpoint around 1988 and complex changes in solar radiation associated to a regional peak in air pollution around the same time. The catchment and both towers did not show any systematic differences in climate or meteorological data, allowing us to address observed changes in the water budget components as related to (i) climate change, (ii) change in vegetation, and (iii) different responses due to different soil and hydrogeological characteristics as well as methodological aspects. The catchment term ET plus storage, derived from precipitation minus runoff, showed the expected high variability with a significant increase over the more than 50 years of operation. The flux-tower DE-Tha showed much lower inter-annual variability in ET with an average annual total of 486 mm (1997 to 2019), but no significant trend. For the same period, average catchment ET was 734 mm/yr. The younger flux-tower DE-Hzd showed ET values in between, closer to catchment ET at the very dry end of the ten-year record (2010 to 2019). An analysis of decadal trends in a Budyko framework at catchment level revealed the dominating response of ET to land use or vegetation change until around 1990. The climate induced change of ET increased in the last decades, on the one hand directly due to an increased atmospheric demand. On the other hand, extreme weather events exerted harmful effects on vegetation, especially triggered by two dry years at the end of the record.Furthermore, we found that the mean annual tower ET was about 250 mm lower than catchment ET despite the careful correction for energy balance closure. We attribute this difference to soil and to a lesser extend to vegetation characteristics, but also to methodological uncertainties. There is evidence from interception and transpiration measurements at the flux tower as well as from water budget modelling that a major contribution of this difference is related to an insufficient EC closure correction during interception events. A careful consideration of rain events and evaporation from interception is recommended when addressing ET of similar evergreen forests in a humid climate, as EC records might be generally too low. This illustrates the necessity of redundant and complementary measurements when dealing with large system complexity.

Climate and management impacts on the spatiotemporal dynamics of water-carbon fluxes in the North China Plain

Evapotranspiration (ET) and gross primary production (GPP) over agroecosystems could be noticeably altered by climate change and human activities. A better understanding of the impacts on water-carbon interactions and the underlying mechanisms, are crucial to improve water use efficiency (WUE). Comparing to 23 site-years of observations, regional ET dataset and statistical yields, the Remote Sensing-based cropland Ecohydrological Model (ReSEM) was validated to estimate the spatiotemporal dynamics of ET, GPP and WUE for dryland agroecosystems in the North China Plain (NCP) from 2001 to 2018. Spatially, the distributions of mean annual ET, GPP and WUE were comparable which changed obviously in different climatic conditions while varied slightly between various irrigation areas. The significantly increased WUE occurred in 38.6 % cropland, mainly located in the humid south regions with less irrigation requirement and the central sub-humid areas along the Yellow River with access to surface irrigation. The averaged increasing trend of WUE in the heavily irrigated semi-arid areas was relatively low. The temporal dynamic of regional mean WUE indicated a significantly increased rate of 0.026 gC kg−1H2O per year during 2001–2018. The spatiotemporal trends of WUE were mainly resulted from the significantly increased GPP rather than ET. The experimental simulations demonstrated that agricultural management dominated the interannual trend of WUE, whose relative contribution (79.5 %) was obviously larger than that of air CO2 concentration (40.2 %) and climate variable change (−19.7 %). Besides, the effects of agricultural management on WUE were further divided based on the classified six cropping systems, 82.4 % of which was attributed to the management of winter wheat-summer maize rotation system. This study sheds light on the improvement of WUE for dryland agroecosystem under changing environments and highlights the dominant impact of agricultural management.

Application of a multiple model integration framework for mapping evapotranspiration with high spatial–temporal resolution in the Haihe River Basin, China

Evapotranspiration (ET) is a key component of the water and carbon cycles. Because it cannot be observed directly on large regional scales at present, many satellite-based ET datasets have been widely used for different purposes. However, their use has been limited at regional and field scales because of their coarse spatial and temporal resolution. In this study, the Bayesian model averaging (BMA) method was used to simulate daily ET values with 500 m spatial resolution in the Haihe River Basin (HRB) from 2000 to 2019. Validation performed with the in-situ observations showed that the BMA ET values had higher accuracy (R2 = 0.69, RMSE = 1.15 mm/day) than those found with individual models. The BMA ET values also had higher accuracy and more credibility based on a water balance equation validation in the HRB. We used interpolated meteorological datasets, reanalysis net radiation products, and remote-sensing datasets to drive the BMA ET model. The mean annual BMA ET in the HRB from 2000 to 2019 was about 601.8 mm/year. The monthly change characteristics of this BMA ET product reflected water consumption characteristics and irrigation regularity of the winter wheat and summer maize rotation system. The BMA ET in the HRB showed a significant increasing trend from 2000 to 2019 of 3.39 mm/y2. The BMA ET values showed a strong positive trend at 80 % of HRB areas. The increasing trend of the BMA ET values was mainly attributed to an increase in the leaf area index. The high spatiotemporal resolution product showed variations in ET trends more accurately than the coarse resolution products. This high spatiotemporal product has great potential for applications in studies of regional microclimate change, interactions between human activities and climate change, drought disaster monitoring, agricultural policy making, and water security.

Faster increase in evapotranspiration in permafrost-dominated basins in the warming Pan-Arctic

Land evapotranspiration (ET) is a key factor in the hydrological cycle and is particularly sensitive to climate change. In the past few decades, the Pan-Arctic region has witnessed a strong warming trend, causing substantial changes in ground surface conditions. However, it remains poorly understood how ET varies across such a region. Here, we investigated the spatial–temporal changes in ET over six large Pan-Arctic river basins using corrected ET estimates from GLEAM and ERA5-Land during 1981–2020. The 40-year mean annual ET in the Ob, Yenisei, Lena, Kolyma, Yukon, and Mackenzie River basins were approximately 366 mm·yr−1, 271 mm·yr−1, 228 mm·yr−1, 239 mm·yr−1, 278 mm·yr−1, and 295 mm·yr−1, respectively. Climatologically, permafrost-dominated basins tend to have lower ET than those with less permafrost. Temporally, the annual ET in permafrost-dominated basins has increased faster than that in less permafrost-developed basins over the past four decades. This increase is particularly prominent in the Kolyma River basin (completely covered by permafrost), where the significant trend in ET (7.5 mm·decade−1, p < 0.001) is the highest among the six basins. Further analyses show that intensified permafrost thawing is likely the key factor contributing to increased ET in permafrost-dominated basins, as seen from the positive relationship between the thickening in the active layer (ALT) and the change in ET. Under an amplified warming background, ET in permafrost-dominated regions is expected to continue to increase. The results reported in this study improve our understanding of the variations in ET and the associated mechanisms in the changing Arctic.

Assessing the synergistic modulation of evapotranspiration by global impervious surface and vegetation changes

The different characteristics of impervious urban surfaces and natural underlying surfaces affect water and heat balances, with consequent impacts on evapotranspiration (ET). The global impervious surface area in 1982 was only 34% of that in 2015, motivating an assessment of the net effect of impervious surface changes on ET under rapid impervious surface expansion. In this study, we incorporated the van Dijk model's interception evaporation (Ei) algorithm into the PT-JPL (Priestley-Taylor Jet Propulsion Laboratory) model, and added an impervious surface evaporation (Eu) module, to enable urban ecosystem simulations. This improved model (named the PT-JPLim model) was validated against annual mean ET calculated by P-Q in 8,205 catchments, and against ET measured at 203 flux towers in natural ecosystems and 11 flux towers in urban ecosystems. Calibration showed that the PT-JPLim model performed well (catchments: R = 0.36; natural ecosystems: R = 0.78, RMSE = 13.17 mm; urban ecosystems: R = 0.79, RMSE = 11.74 mm). Additionally, we examined the reliability of modelled ET and its partitioning, by comparison with observed soil evaporation (Eb), Ei, and satellite-based products. By controlling for the impervious surface fraction in the model, it was concluded that the net increase in Eu due to increased impervious surface changes was 0.23 mm/yr, with larger increases in eastern China, India, Europe and southeastern North America than in other regions. The effects of impervious surface and vegetation changes on ET were then analyzed for global impervious surface, with a ratio of 1/4.1 for the effects of impervious surfaces to vegetated surfaces on ET. The contributions of transpiration (Et), Eu, and Eb changes to ET changes on impervious surfaces were 65%, 27% and 8% respectively. These results indicate that synergistic modulation of ET by impervious surface changes and vegetation changes should not be ignored when studying urban eco-hydrological processes.

Quantifying the effect of vegetation greening on evapotranspiration and its components on the Loess Plateau

As one of the largest revegetation projects in China, the Grain for Green Project has dramatically increased vegetation greenness (greening) on the Loess Plateau. Large-scale greening inevitably changed the spatiotemporal pattern of water availability on the Loess Plateau by increasing evapotranspiration (ET). Quantifying the effect of greening on ET is critical for regional water resources management and planning. This study uses the maximum entropy production (MEP) model and two attribution methods (the detrending and extending methods) to quantify the effect of greening on ET and its components (soil evaporation (Es) and transpiration (Et)) on the Loess Plateau from 2001 to 2018. The ensemble-averaged results of two attribution methods are used to reduce the potential uncertainty caused by the choice of attribution methods. The MEP model performs well in capturing the spatiotemporal variability of ET on both the site and basin scales. Greening significantly (p < 0.05) increases Et and decreases Es for the Loess Plateau, and the magnitude of increase in Et exceeds the magnitude of decrease in Es, leading to a nonsignificant (p > 0.05) increase in ET. The contribution of greening to mean annual Es, Et, and ET for the Loess Plateau is −16.6, 20.7, and 4.1 mm/yr, respectively. An increase in ET may further exacerbate water scarcity for the Loess Plateau.