Ecosystem Water Use Efficiency Research Articles

Contributory factors of the secular trends to changes in ecosystem water-use efficiency in China

Ecosystem water-use efficiency (WUE) is an effective indicator of ecosystem functioning, which is widely used to characterize the coupling relationship between terrestrial carbon and the water cycle. Although the linear trends in WUE have been well reported, the nonlinearity of the secular trends in WUE and their relative contribution to the overall changes were still poorly investigated. In this study, we used ensemble empirical mode decomposition (EEMD) to extract the secular trends of the WUE and their relative contribution (RC) to the WUE changes in China, which represents the stability of the ecosystem functioning. We found that although increasing trends dominated the secular trends of the WUE, nearly half of the study areas had experienced trend shifts. Except that deciduous forests and mixed forests were dominated by trend shifts, most of the land cover types were dominated by increasing trends. The RC exhibited a bimodal pattern along the elevation gradient, with peaks at 20 m and 5000 m. Similarly, the RC peaked at the east and south aspects, and was the lowest for the west and north aspects. In comparison, the RC exhibited a U-shaped pattern with the slope, and was the lowest at 80°. For the monotonically increasing trends and decreasing-to-increasing trends, the high RC mainly occurred on the North China Plain and Loess Plateau, indicating ecosystem improvement and strong stability. Meanwhile, it was low in northeastern China and southwestern China, indicating unsustainableecosystem improvement. The RC was always very low for increasing-to-decreasing trends and monotonically decreasing trends, indicating low ecosystem stability with a high possibility for improvement. Temperature and the atmospheric carbon dioxide (CO2) concentration were the two main climatic factors driving the WUE changes. The climatic mean state rather than the climatic variability affected the ecosystem stability. The results of this study highlight the relative contribution of the secular trends to WUE changes and provide a better understanding of the structure and the stability of ecosystem functioning.

Recent Seasonal Variations in Ecosystem Water Use Efficiency in China's Key Tropical‐Subtropical Transitional Zones in Response to Climate Change

AbstractUnderstanding the response of terrestrial ecosystem water use efficiency (WUE) to climate change is critical to accurately represent the carbon‐water cycle processes. However, how WUE dynamics are evolving under seasonal climate variations and biome‐specific characteristics remains still unclear. In this study, we integrated two state‐of‐the‐art retrieval algorithms to estimate gross primary productivity (GPP) and evapotranspiration (ET) from satellite MODIS records. Such metrics served as input to quantify ecosystem WUE, expressed as the ratio of GPP to ET, and explore its dynamics during the dry and wet season from 2001 to 2018 in China's key tropical to subtropical transitional zones, that is, Yunnan Province. Results show large spatial and seasonal variability in WUE over the observational period. During the dry season, the increasing trends in GPP and ET have led to contrasting WUE patterns in forest and non‐forest biomes, leading to positive and negative WUE trends, respectively. During the wet season, the declining trends in GPP occurring in combination with opposite trends in ET, have caused decreasing WUE consistently across all biomes except croplands, likely further modulated by human factors. The observed changes in WUE appear primarily driven by variations in air temperature (Ta) and vapor pressure deficit (VPD) during both dry and wet seasons. Overall, these results contribute to a better understanding of the carbon‐water interplay in tropical‐subtropical transitional zones and provide new insights to improve our capacity to predict the terrestrial ecosystem's response to climate change.

Rainfed cotton crop in central India is a strong net CO2 sink: An eddy covariance-based analysis of ecosystem fluxes

The present study reports ecosystem CO2, H2O exchanges (at diurnal, daily, and seasonal-scale) from a rainfed (dryland) cotton crop (Gossypium sp) grown in deep black soil of tropical central India during 2019–20 and 2020–21, using an Eddy Covariance (EC) system. The half-hourly CO2 fluxes were found to vary significantly across the growth stages of cotton crop with peak values during first boll opening to first picking stage [mean Net Ecosystem CO2 Exchange (NEE), −16 to −18 µmol m-2 s-1; mean Gross Primary production (GPP), 20–22 µmol m-2 s-1; and mean Ecosystem Respiration (Reco), 5–6 µmol m-2 s-1]. The response of daytime GPP to Photosynthetic Photon Flux Density (PPFD) was found to be poor during 1–30 days after sowing (DAS) with R2∼0.4. It improved during 30–150 DAS (R2∼0.6), while deteriorating further as the crop started senescing. The rainfed cotton crop was found to be a strong net CO2 sink, with seasonal NEE, GPP, Reco, and evapotranspiration (ET) of − 333.66 gC m-2, 990.16 gC m-2, 656.50 gC m-2 and 468.32 mm during 2019–20, respectively. The cotton season of 2020–21 had seasonal NEE, GPP, Reco, and ET of − 391.85 gC m-2, 1063.89 gC m-2, 672.04 gC m-2, and 545.25 mm respectively. The seasonal Ecosystem Water Use Efficiency (EWUE) was estimated to be 1.9 and 2.1 gC kg-1 H2O during 2019–20 and 2020–21, respectively. Maximum values of crop coefficient (Kc) were observed to be 1–1.2 during 30–75 DAS, signifying favourable conditions for crop growth. The inter-seasonal differences in the CO2 and H2O exchanges were mainly driven by the differences in incoming PPFD due to cloudiness. Results of the present study will help in understanding the ecosystem processes and exchanges from a rainfed cotton-based cropping system, which in turn can significantly contribute to the global carbon and moisture budget.

Impact of planting date on CO2 fluxes, evapotranspiration and water-use efficiency in peanut using the eddy-covariance technique

The combination of increased climate variability with changing weather patterns heightens the importance of the role of planting date on water use in peanut production. Experiments addressing different peanut planting dates were conducted over three years and daytime ecosystem water-use efficiency was determined using the eddy-covaraince method. Results suggest that during the early growing stage in all three planting dates, a late planting date presented by far the highest CO2 fluxes and daytime ecosystem water-use efficiency. Of immediate significance, the latest planting date was also found to exhibit the highest yield in two of the three years in the experiment. Furthermore, an examination of the last 20 years (2001-2020) of climatological records suggests that a late planting date carries a risk of 40% chance of sub-optimal low temperatures in early fall, potentially reducing the yield in those years. In addition, the late planting date carries 15% probability of ten or more consecutive rainy days in the narrow harvest window, potentially impacting a timely harvest.

Sustained vegetation greening enhanced ecosystem water-use efficiency in the Loess Plateau of China in recent 20 years

Global vegetation greenness has already been confirmed to increase substantially since the early 1980s, and this phenomenon is extremely heterogeneous in both time and space. The Loess Plateau (LP) serves as an ecological barrier of northern China, where the ecological environment is characterized by exceptional fragility and sensitivity. However, the spatial pattern of vegetation dynamics in both greenness and productivity and their relationship with vegetation ecosystem water-use efficiency (WUE) are still poorly documented, especially in recent 20 years. In this study, the leaf area index (LAI) and gross primary productivity (GPP) were employed to characterize vegetation canopy leaf area and productivity, respectively. The time series data of ecosystem WUE in the LP were obtained by the ratio of GPP and ET during the period of 2000–2018, the dynamic trend of vegetation greenness and productivity in the LP was detected by using a linear fitting and nonlinear method, then, the extent to which the sustained greening in the LP revealed an increase of ecosystem WUE in recent 20 years was quantified. The results showed that sustained vegetation greening and increased GPP elucidated a great improvement in ecosystem WUE in the LP during 2000–2018, in which the sustained greening potentially reveals an increase of ecosystem WUE in >69.33% regions of the LP in recent 20 years. >93% regions of the LP have been experiencing a vegetation greening progress derived by both the annually averaged LAI (LAIAN) and growing season (April to October) maximum LAI (LAIGS). Furthermore, precipitation and soil moisture positively contributed to an interannual increase in ecosystem WUE; however, increasing temperature greatly resulted in a decrease in ecosystem WUE in the LP in recent 20 years. Importantly, this work also implied that ecological water is significantly consumed by the increasing vegetation photosynthesis derived from the significant growth of ET in the LP since the 2000s. These findings implied that the increasing ecological water consumption should be incorporated into indigenized ecosystem process models.

Spatiotemporal Variability in Water-Use Efficiency in Tianshan Mountains (Xinjiang, China) and the Influencing Factors

Water-use efficiency (WUE) is a crucial physiological index in carbon–water interactions and is defined as the ratio of vegetation productivity to water loss. The variation in climatic variables and drought have the most significant effects on WUE and evapotranspiration (ET). Nevertheless, how WUE varies with climate factors and drought processes in the Tianshan Mountains (TMS) is still poorly understood. In the present work, we analyzed the spatiotemporal variations in WUE, and investigated the correlations between WUE, climate factors, and drought, in the study area. The results showed that, in the TMS during 2000–2020, annual net primary productivity (NPP) ranged from 147.9 to 189.4 gC·m−2, annual ET was in the range of 212.5–285.8 mm, and annual WUE ranged from 0.66 to 0.78 gC·kg−1·H2O. Both NPP and ET exhibited an increasing trend with some fluctuation, whereas WUE showed the opposite tendency during the study period. The obtained results demonstrated that the decrease in WUE was primarily because of the increase in ET. There were obvious differences in WUE, under different land-use types, caused by NPP and ET. However, the interannual variation in WUE showed small fluctuations and the dynamic process of WUE in each land-use type showed good consistency. Temperature and wind speed had a positive influence on WUE in the middle and eastern regions of the TMS. Precipitation also played a mainly positive role in enhancing WUE, especially on the northern slope of the TMS. There was strong spatial heterogeneity of the correlation coefficient (0.68, p < 0.05) between WUE and the temperature vegetation drought index (TVDI). Moreover, the slopes of WUE and TVDI showed good consistency in terms of spatial distribution, suggesting that drought had a significant impact on ecosystem WUE. This work will enhance the understanding of WUE variation, and provide scientific evidence for water resource management and sustainable utilization in the study area.

Effects of Vegetation Phenology on Ecosystem Water Use Efficiency in a Semiarid Region of Northern China.

Water use efficiency (WUE) is an important ecosystem functional property for measuring coupled relationships of the carbon-water cycle. Both biotic and environmental factors significantly impact WUE in terrestrial ecosystems. Relationships between environmental factors and WUE have been well discussed in previous studies. Although the crucial role of vegetation phenology, a common indicator of climate-vegetation interactions, in regulating the WUE has been widely reported, the underlying mechanism has not yet to be fully elucidated. Here, we utilized multiple long-term remote sensing datasets to analyze the interannual changes in seasonal WUE, and discussed the potential associations between phenology and WUE in the Luanhe River basin, which is a typical semiarid region of China, from 1988 to 2015. Most of the pixels across Luanhe River basin showed increasing spring WUE and decreasing autumn WUE. The start of the growing season (SOS) was slightly advanced by an average of 0.06 days per year (d/y) in the whole study area, with a delayed trend (0.04 d/y) in the upper reaches region (UR) and an advanced trend (0.20 d/y) in the middle-lower reaches region (MLR). The end of the growing season (EOS) showed a generally delayed trend (0.11 d/y) across the basin. Furthermore, negative correlations were detected between spring WUE and SOS in the UR, while positive correlations were detected in the MLR. The opposite patterns of the relationship of autumn WUE and EOS were also found between UR and MLR. The results were attributed to the balance and compensation of biotic and abiotic factors in the regulation of ecosystem WUE. Our findings provide new insights into the interaction between vegetation dynamics and carbon-water cycle coupling.

Spatiotemporal tendency of agricultural water use efficiency in the northernmost Yellow River: Indicator comparison and interactive driving factors

Water use efficiency (WUE) is calculated as the ratio between the gross primary productivity (GPP) and the water consumed by evapotranspiration (ET) or vegetation transpiration (Et). The evapotranspiration of Hetao irrigation District is very large, and the WUE of this area is affected by many factors. At present, the correlation analysis of multi factor interaction has not been carried out. A comparison of the spatiotemporal patterns as well as the impact of meteorological factors on the WUE of ecosystems (WUEe) and the WUE of canopies (WUEc) in the Hetao Irrigation District can provide a fundamental basis for water management at the basin level, especially in dryland irrigation areas. Trend analysis indicated that the trends of WUEe and WUEc had opposite directions, namely, the increase in WUEe concealed the decrease in WUEc based on the application of remotely sensed GPP and ET products in the Hetao Irrigation District. In addition, WUEe and WUEc had different responses to meteorological factors. The interactive influence of multiple climate factor interactions on WUE was greater than the influence of single factors. The identification of the response of WUE to meteorological factors can provide a scientific basis for farmland water replenishment in arid areas.

Spatial responses of ecosystem water-use efficiency to hydrothermal and vegetative gradients in alpine grassland ecosystem in drylands

The carbon–water cycle is critical to ecosystem function and sustainability in dryland region. Ecosystem water use efficiency (WUE) is an important indicator reflecting the interaction between carbon and water, and investigating its spatial responses to hydrothermal and vegetative gradients is vital to understand the underlying mechanism of carbon–water coupling. The source region of the Yellow River (SRYR) is a typical alpine grassland ecosystem in drylands characterized by cold and arid climate, which is ecologically fragile and sensitive to environmental factors. In this study, we investigated the spatial variations of ecosystem WUE in the SRYR during 2000–2014 and examined its underlying drivers. The mean annual WUE showed significantly increasing trends along hydrothermal and vegetative gradients from semi-arid zone (SA) to dry sub-humid zone (DSH), with the average value of 0.59 ± 0.35 gC kg−1 H2O. The spatial variations of WUE were mainly controlled by gross primary productivity (GPP) rather than evapotranspiration (ET). Compared to precipitation (P), temperature (Ta) better explained the spatial variations of WUE. And a temperature threshold of 0 °C was found, which indicated a shift of sensitivity of WUE to thermal conditions from permafrost to seasonally frozen ground due to the effects of low temperature on photosynthetic process in alpine grassland ecosystem. The responses of WUE to LAI gradients reflected the indirect effects of hydrothermal conditions on WUE by altering LAI. This study improves our understanding of the spatial responses of the regional carbon and water interactions to environmental conditions.

Contributions of climatic and social factors to two water use efficiency indicators in the Yellow River

AbstractTracing the spatiotemporal tendency and driving forces of water use efficiency (WUE) is of great significance for understanding the carbon‐water balance, especially in large river basins. The lack of differential comparisons regarding the tendencies and driving forces of WUE indicators between the upper and lower reaches of rivers often affects the collaborative allocation and utilization of water resources at the basin scale, especially social factors such as population and economy distributions. Using remote sensing data and hierarchical decomposition methods, we identified the spatiotemporal distribution of the WUE of ecosystems (WUEe) and the WUE of canopies (WUEc) in the Yellow River Basin from 2003 to 2017 as well as the contribution rates of climatic and social factors to WUEe and WUEc. Our results show that WUEe had a different distribution pattern than WUEc. An increase in WUEe was accompanied by a decrease in WUEc, and these changes varied among the upper, middle and lower reaches. WUEc was most negatively correlated with pressure (| r | = 0.65), while WUEe was most positively correlated with the precipitation rate (| r | = 0.68). The contribution rates of climatic factors to both WUEc and WUEe in the upper reaches were higher than 95%. The middle reaches were mainly driven by climatic factors, with contribution rates higher than 90%. In the lower reaches, the contribution rates of social factors to WUEe were higher than 30%. These differentiated contributions indicated the importance of considering social factors in the lower reaches than in the upper reaches when managing WUEe under the dominance of climatic factors on WUEc.

Impacts of Vegetation Changes on Land Evapotranspiration in China During 1982–2015

Evapotranspiration (ET) bridges the hydrological and energy cycle through vegetation transpiration (T), soil evaporation (ES), and canopy interception evaporation (EI). Transpiration to evapotranspiration ratio (T/ET) quantifies the water use efficiency of terrestrial ecosystems explaining the mechanism of vegetation water transport and water–carbon interactions. This study employed GIMMS LAI3g data to improve the CLM4.5 land surface scheme of RegCM4.6. We designed two simulation experiments, each with control (CTL) and sensitivity (SEN), simulating the interannual variability of vegetation on ET and T/ET in China from 1982 to 2015. Studies show China has experienced a greening trend, especially in mid-south China and South China. Leaf area index (LAI) increased significantly (0.002 m2m−2yr−1). ∆LAI (SEN input LAI data minus CTL input LAI data) and ∆T/ET (T/ET data output by SEN minus T/ET data output by CTL) have shown significant positive correlations. The impacts of LAI on T/ET are more prominent during spring and winter than in autumn and summer. Compared with T/ET and LAI (R = 0.70), the correlation between ET and LAI is moderate (R < 0.5), indicating that vegetation has a higher impact on T/ET than ET. The impact of vegetation anomalies (positive and negative LAI anomalies) on T/ET and ET is spatially different, mainly due to dominant factors affecting ET and T/ET changes. In spring, summer, and autumn, &T (transpiration changes corresponding to vegetation anomalies) is the leading factor affecting both ET and T/ET regionally, and &T has a stronger influence on T/ET than ET, especially in summer. Vegetation anomalies have a stronger influence on T/ET than ET; and the influence of positive vegetation anomalies on ET and T/ET is greater than that of negative vegetation anomalies, especially in spring and autumn. This study reveals the mechanisms behind vegetation processes and their influences on the water and heat fluxes at the land–atmosphere interface and provides a strong scientific basis for studying the water cycle under climate warming.

Inherent Water-Use Efficiency of Different Forest Ecosystems and Its Relations to Climatic Variables

Inherent water-use efficiency (IWUE) is a vital parameter connecting the carbon and water cycles. However, the factors influencing the IWUE in different forest ecosystems are still a subject of debate. In this work, FLUXNET platform measurements of 67 forest sites were used to detect trends of the IWUE of four forest ecosystems, namely deciduous broadleaf forests (DBF), evergreen broadleaf forests (EBF), needle-leaf forests (ENF), and mixed forests (MF). The IWUE differed significantly among different forest ecosystems and positively correlated with temperature and solar radiation. The IWUE of EBF was the highest at 32.02 g·C·Kg·H2O−1. The values of DBF and MF were similar and higher than that of ENF. With increasing latitude, the IWUE increased first and then decreased, with a maximum of 35° N. The IWUE of EBF was negatively correlated with precipitation and leaf area index. Temperature and solar radiation were the main factors controlling the IWUE of forest ecosystems, whereas precipitation was the major factor controlling the inter-annual variation in the ΔIWUE of forest ecosystems. Our results provide a scientific basis for the study of forest carbon sinks, forest eco-hydrological processes, and forest ecosystem responses to global climatic changes.

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.

CO2 fertilization is spatially distinct from stomatal conductance reduction in controlling ecosystem water-use efficiency increase

It is well known that global ecosystem water-use efficiency (EWUE) has noticeably increased over the last several decades. However, it remains unclear how individual environmental drivers contribute to EWUE changes, particularly from CO2 fertilization and stomatal suppression effects. Using a satellite-driven water–carbon coupling model—Penman–Monteith–Leuning version 2 (PML-V2), we quantified individual contributions from the observational drivers (atmospheric CO2, climate forcing, leaf area index (LAI), albedo and emissivity) across the globe over 1982–2014. The PML-V2 was well-calibrated and showed a good performance for simulating EWUE (with a determination coefficient (R 2) of 0.56) compared to observational annual EWUE over 1982–2014 derived from global 95 eddy flux sites from the FLUXNET2015 dataset. Our results showed that global EWUE increasing trend (0.04 ± 0.004 gC mm−1 H2O decade−1) was largely contributed by increasing CO2 (51%) and LAI (20%), but counteracted by climate forcing (−26%). Globally, the CO2 fertilization effect on photosynthesis (23%) was similar to the CO2 suppression effect on stomatal conductance (28%). Spatially, the fertilization effect dominated EWUE trend over semi-arid regions while the stomatal suppression effect controlled over tropical forests. These findings improve understanding of how environmental factors affect the long-term change of EWUE, and can help policymakers for water use planning and ecosystem management.

Pre- and post-harvest evapotranspiration, carbon exchange and water use efficiency of a mature peach orchard in semi-arid climate

Better knowledge of the evapotranspiration and carbon exchange of fruit trees is needed to optimize the trade-off between water use and carbon assimilation and to better understand the role of agriculture in the biogeochemical cycles. In this work, we measured water and carbon fluxes with eddy covariance and transpiration with sap flow in a drip-irrigated peach orchard of 70% ground cover located in southern Spain for 2 years. The empirically measured crop coefficient (Kc) under good watering conditions in the summer ranged from 1 to 1.1. The daytime net ecosystem exchange (NEE) flux of the orchard averaged 30 g CO2 m2 day−1 during the period of maximum activity in July. The daytime ecosystem water use efficiency (WUE) of the orchard reached a minimum in late June, flattened around 4 g CO2 L−1 throughout the summer, and increased in autumn, but was unaffected by fruit removal or post-harvest irrigation reduction imposed by the farm (30% reduction). The response of instantaneous peach ecosystem WUE to VPD was also investigated. Both Kc, NEE, leaf water potential and stomatal conductance decreased sharply after harvest. Transpiration data from some purposely over-irrigated experimental trees demonstrated that the post-harvest alterations we found were not caused by fruit removal, but are result of mild water stress originated by the irrigation reduction. Hence, the often-observed alterations in water relations after harvest in well-watered trees were not observed in this experiment. This work adds insight on peach irrigation efficiency and on the contribution of orchards to agricultural carbon budgets.

Vegetation in Arid Areas of the Loess Plateau Showed More Sensitivity of Water-Use Efficiency to Seasonal Drought

Studying the impact of regional or seasonal drought on vegetation water-use efficiency (WUE) can identify an effective theoretical basis by which vegetation can cope with future climate change. Based on remote sensing data and climate grid data, in this study, we calculated the ecosystem WUE and standardized precipitation evapotranspiration index (SPEI), analyzed the temporal and spatial divergence of seasonal drought and WUE, and explored the relationship between WUE and seasonal drought in the Loess Plateau. The results indicate that from 2001 to 2019, the humidity in spring and summer on the Loess Plateau shows an increasing trend, and the aridity in fall also shows an increasing trend. Averaged over four seasons, WUE presents distribution characteristics of “high in the southeast and low in the northwest”, with the highest WUE in summer. However, the geological distribution of the sensitivity of WUE to seasonal drought was significantly different. Spring drought increased WUE, whereas summer drought led to a decrease in WUE. When fall drought was less severe, the WUE increased; WUE response to winter SPEI was negative, but the sensitivity did not change with variation of drought degree. The sensitivity of WUE to the magnitude of seasonal drought was affected by regional dry and wet conditions. A clear seasonal divergence was found in four climate regions, along with increased drought intensity, and the sensitivity of WUE to drought magnitude in arid areas was generally higher than that in semi-arid, semi-humid areas, or humid areas. With this study, we deeply explored how ecosystems deal with the water supply strategy of seasonal drought, which is of great significance in the understanding of the coupling relationship between the carbon–water cycle and climate change.

Assessing Variations in Water Use Efficiency and Linkages with Land-Use Changes Using Three Different Data Sources: A Case Study of the Yellow River, China

The dependence of water use efficiency (WUE) on changes in land cover types is crucial for understanding of long-term water availability and assessment of water-saving strategies. Investigating the impact of land cover types on ecosystem WUE has important implications when revealing water dynamics and land management. However, the determination of WUE and its dominant factors have always been subject to high data dependency and large calculation consumption within large basins. This paper proposes a framework for processing actual evapotranspiration (AET) and WUE calculation by coupling the Maximum Entropy Production (MEP) method with the Google Earth Engine (GEE). By employing the proposed framework and three data sources available in the GEE platform, results for actual ET and WUE from 2001 to 2020 were obtained in the Yellow River Basin (YRB). The results show that the proposed framework provides an acceptable estimation of actual ET via validation with Eddy Covariance flux sites in the YRB. The calculated WUE values varied greatly in different sub-basins within the YRB, indicating a cumulative growth rate of about 56% during the past 20 years. The dominant factor that led to these changes was the transition from Grasslands into other land-use types. Our results suggest that the use of the GEE platform coupled with the MEP method offers new possibilities for advancing understanding of water exchange and water resource management.

Increased Interception Induced by Vegetation Restoration Counters Ecosystem Carbon and Water Exchange Efficiency in China

AbstractChanges in ecosystem water use efficiency (WUE) and evapotranspiration (ET) that are driven by vegetation change can be used as metrics to quantify ecosystem carbon and water exchange. The effect of vegetation on WUE has been explored in the past through consideration of total ET. However, interception (Ei) doesn't directly participate in ecosystem carbon and water exchange, and uncertainty of modeled Ei, resulting in great uncertainty of ET partitioning and ecosystem WUE. Here, we estimated Ei in China using Gerrits' model, replacing the algorithm for Ei in the Priestley‐Taylor Jet Propulsion Laboratory (PT‐JPL) model. The modified PT‐JPL model was then incorporated into the analytical WUE model to estimate WUE over China for 2003–2011, and the results were validated against literature data (WUELiterature). To evaluate the net effect of vegetation restoration on WUE, we simulated WUE in three ways: with vegetation change; without vegetation change; and with vegetation change, but without any Ei changes caused by vegetation changes. In the study presented here, we demonstrate the WUE simulated by the analytical WUE model is significantly correlated with the WUELiterature (R2 = 0.76, p‐value < 0.001), and leaf area index contributes the most to the spatial variability of Ei/ET, ranging from 10% to 55%. Increased Ei weakened the effect of vegetation change on WUE by ∼51.3% during vegetation restoration in China; a reduction that was particularly marked in southern China. These findings highlight the importance of considering Ei when assessing the effects of vegetation change on ecosystem carbon and water exchange, and provide insights that are important for the sustainable management of vegetation change in China.

Community Composition and Structure Affect Ecosystem and Canopy Water Use Efficiency Across Three Typical Alpine Ecosystems.

Unique ecosystems distributed in alpine areas of the Qinghai–Tibetan Plateau play important roles in climate change mitigation, local food supply, and conservation of species diversity. To understand the water use efficiency (WUE) of this fragile and sensitive region, this study combined observed data from the eddy covariance system and the Shuttleworth–Wallace (S-W) model to measure the continuous mass exchange, including gross primary productivity (GPP), evapotranspiration (ET), and canopy transpiration (T) throughout 2 or 3 years (2016–2018) in three common alpine ecosystems (i.e., alpine steppe, alpine meadow, and alpine swamp). These ecosystems represent a water availability gradient and thus provide the opportunity to quantify environmental and biological controls on WUE at various spatiotemporal scales. We analyzed the ecosystem WUE (WUEe; defined as the ratio of GPP to ET) and canopy WUE (WUEc; defined as the ratio of GPP and canopy T). It was found that the yearly WUEe was 1.40, 1.63, and 2.16 g C kg–1 H2O, and the yearly WUEc was 8.93, 2.46, and 5.19 g C kg–1 H2O in the three typical ecosystems, respectively. The controlling factors of yearly WUE diverged between WUEe and WUEc. We found that plant functional group proportion (e.g., gramineous and Cyperaceae) highly explained the yearly WUEe variation across sites, and a good correlation was observed between community species diversity and WUEc. These findings suggest that community composition and trait change are critical in regulating WUEe and WUEc across different alpine ecosystems and that the regulation mechanisms may differ fundamentally between WUEe and WUEc.

The role of groundwater in the spatio-temporal variations of vegetation water use efficiency in the Ordos Plateau, China

Groundwater influences the water and carbon cycle by supplying moisture to plants in the semi-arid and arid zones. However, little is known about the response of ecosystem water use efficiency (WUE) to climate change in different groundwater depth (GD) sections. Recent research has shown that plant photosynthesis and growth are closely related to GD via field experiments but the wider recognition of GD effect on regional-scale ecosystems has not been yet established. In this study, we test whether the GD has an impact on ecosystem WUE and its variability to climate change at the regional scale. Based on the observed data of nearly 3000 wells, meteorological data (precipitation and pan evaporation), and the 0.01°-resolution remote sensing datasets including gross primary production (GPP), evapotranspiration (ET), and normalized difference vegetation index (NDVI), we explored the spatio-temporal variations of WUE and its composites (i.e., GPP and ET), and their characteristics depending on GD under different aridity conditions and biomes across the Ordos Plateau, a semi-arid to arid area in northern China. Results show that WUE increases with decreasing GD due to water availability in the semi-arid lands where WUE variability is mainly regulated by biological processes (i.e., GPP), while WUE is insensitive to the changes in GD across the arid zone where the physical processes (i.e., ET) control WUE change. However, when drought happens the groundwater-independent vegetation in the arid zone can also utilize groundwater, characterized by lower reductions of GPP with the decrease in GD. A dense vegetation condition (i.e., large NDVI) is more vulnerable to climatic disturbance over the semi-arid zone because it tends to decrease GPP and WUE, especially in the large GD regions. These findings have important implications for reasonable land use and groundwater management over the semi-arid and arid regions.