The net physiological effect of rising atmospheric carbon dioxide (aCO₂) on terrestrial evaporation (ET) is highly uncertain. While increased CO₂ fertilization elevates ET through more biomass production, the reduction in stomatal conductance (g_s) that it downregulates ET. Here, using satellite-based estimates of ET and dynamic vegetation models, we investigate the physiological influence of aCO₂ on ET, and isolate the respective contribution of biomass increase and g_s reduction. Our results indicate that the CO₂ fertilization had a net negative effect of –4.4±0.3×10^–2 mm ppm^–1 on ET over 1982–2018. The negative physiological effect tends to intensify with increasing aCO₂, particularly in warm and humid forests. The high sensitivity of ET to g_s may attenuate the expected water cycle acceleration over land, although the future evolution of these two competing physiological processes remains uncertain.

How to cite: Qian, H., Wang, W., Chen, Z., Koppa, A., Liu, G., and Miralles, D.: Recent intensification of the negative physiological effect of CO2 on terrestrial evaporation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3593, https://doi.org/10.5194/egusphere-egu24-3593, 2024.

Water use efficiency (WUE) and carbon use efficiency (CUE) in dryland ecosystems are highly sensitive to complex climate and CO₂ changes, which may cause imbalance between carbon and water cycles in terrestrial ecosystems. However, the mechanism of the systematic effects of multiple factors on WUE and CUE remains unclear. Here, we examined the trends in WUE and CUE in China’s Loess Plateau during 2001-2020 and assessed the underlying drivers using PML_V2 products and satellite-based data by employing the spatial random forest (SRF) method. Our analysis identified a significantly increasing trend in WUE and a slightly downward trend in CUE. In space, NDVI was the most important factor affecting the spatial variation of WUE and CUE, but WUE had a significant positive response to NDVI, while CUE had a significant negative response to NDVI. Precipitation and CO₂ concentration were the most important environmental factors driving spatial variability in WUE and CUE, respectively. However, vapor pressure deficit was the most important factor driving CUE annual variation controlling most areas of the greening region. Our research revealed that despite the improvement in water utilization, the greening of vegetation did not enhance carbon sequestration potential in the Loess Plateau. Furthermore, we demonstrated that vegetation was the most important factor causing WUE spatiotemporal variation and CUE spatial variation, while atmospheric drought inhibiting vegetation growth was the most important factor causing CUE temporal variation, reflecting the interactivity and complexity of the driving factors behind the spatial and temporal variability of WUE and CUE. Our study provides new insights into the driving characteristics of WUE and CUE spatiotemporal variability and enhances the knowledge of how the carbon-water coupling process induced by vegetation greening responds to environmental changes in arid and semi-arid regions in the backdrop of climate change, contributing to ecological restoration practices and sustainable management in the dryland.

How to cite: Wang, Y., Gao, G., Huang, Y., and Wang, Z.: Water use efficiency and carbon use efficiency response differently to greening on the Loess Plateau in China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5887, https://doi.org/10.5194/egusphere-egu24-5887, 2024.

Ecological Restoration (ER) measures can achieve considerable carbon benefits and reduce sediment loads, concurrently resulting in unintended hydrological consequences. The Middle Yellow River Basin (MYRB), with intensive large-scale ER implementation during the past decades, serves as an excellent case to investigate the concomitant water-carbon-sediment synergies and trade-offs. This study combined a vegetation dynamics simulation scheme and a distributed hydrological model with explicit ER representation to investigate the water-sediment-carbon changes in response to ER in the MYRB. According to the results, ER promoted synergies between carbon sequestration and sediment control and led to improved water use efficiency (WUE). The actual Leaf Area Index and Gross Primary Productivity (GPP) showed improvements in region-averaged values by +0.56 m² m^-2 yr^-1 (+7.4%) and +52 gC m^-2 yr^-1 (+10.9%) compared to those under natural conditions. In the Toudaoguai-Tongguan section which suffered the most serious soil erosion, ER decreased the sediment loads by 11.3×10⁸ ton yr^-1 (71.1% of the natural level). Furthermore, WUE changes indicated higher GPP gain per unit evapotranspiration. Meanwhile, trade-offs were also found when taking account of the water yield reduction. During 1982-2019, ER led to significant increases in actual evapotranspiration (+8.3 mm yr^-1; +2.2%) and decreases in runoff (-7.6 mm yr^-1; -12.7%). Two indicators evaluating the cost-effectiveness of ER, i.e., carbon sequestration and sediment settlement at the cost of per unit runoff decline, remained positive with the average values of 6.12 kgC m^-3H₂O yr^-1 and 0.22 ton m^-3H₂O yr^-1 during 2000-2019, respectively. Nevertheless, both indicators showed downward trends, indicating decreasing marginal benefits brought by ER measures which could have approached the optimal scale in the MYRB.

How to cite: Yan, Z., Wang, T., and Yang, D.: Water-carbon-sediment synergies and trade-offs: multi-faceted impacts of large-scale ecological restoration in the Middle Yellow River Basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6713, https://doi.org/10.5194/egusphere-egu24-6713, 2024.

Phreatic groundwater hydrology has a well-documented influence on the land water/energy/carbon cycles. To capture the resilience of the biosphere to dry spells in land surface models, it is particularly crucial to incorporate groundwater dynamics. With the ISBA-CTRIP land surface system, it is possible to perform a coupled simulation of the land surface fluxes and groundwater hydrology. Here, we evaluate this model configuration over Belgium, and focus on the quality of the simulated groundwater dynamics, soil moisture and resulting surface fluxes. A network of piezometer and eddy covariance towers is used to validate the model outcomes. Furthermore, the sensitivity of the model parametrization is analyzed (considering different pedotransfer functions), and the impact of groundwater coupling on the surface fluxes is quantified.

How to cite: De Pue, J., Munier, S., Barrios, J. M., Arboleda, A., Baguis, P., Hamdi, R., and Meulenberghs, F.: Coupled simulation of phreatic groundwater and surface fluxes from the terrestrial biosphere in Belgium, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10072, https://doi.org/10.5194/egusphere-egu24-10072, 2024.

Tropical forests account for approximately one-fourth of the global terrestrial carbon sink, playing an important role in the Earth’s carbon cycle. Importantly, mainland Southeast Asia has the densest vegetation surface but its ecohydrology is historically understudied due to the paucity of field observations and modelling studies. Leveraging on existing flux tower data, remote sensing products, and the mechanistic ecohydrological model T&C, we provide an enhanced understanding of carbon and water exchanges in mainland Southeast Asia. The T&C model is tested to reproduce various ecosystem types of Southeast Asia, including tropical evergreen forests, subtropical deciduous forests, savannas, rubber plantations, and rice fields. The flux tower data including gross primary productivity (GPP) and evapotranspiration (ET) along with remote sensing data of leaf area index and other vegetation indexes, allow us to better refine and constrain model simulations.  With the integration of data and model, we provide a comprehensive picture of spatiotemporal patterns and key drivers of carbon and water fluxes in mainland Southeast Asia. Our findings highlight a strong latitudinal gradient in carbon fluxes and ET associated with seasonality of rainfall as well as an important role of vapour pressure deficit (VPD) and soil moisture content with different responses in wet and dry years. Direct effects of temperature and precipitation are relatively smaller when compared to VPD and soil moisture in driving changes of carbon and water fluxes. These findings, combined with our model framework, pave the road to more accurate predictions of ecohydrological variables in the relatively understudied region of mainland Southeast Asia.

How to cite: Ren, J., Luo, Z., Galelli, S., and Fatichi, S.: Combining mechanistic modelling and observations to characterize carbon and water fluxes in mainland Southeast Asia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14210, https://doi.org/10.5194/egusphere-egu24-14210, 2024.

Connecting environmental conditions with plant growth and stress is an important part of ecosystem management in the context of a rapidly changing climate. Our understanding of how varying growing conditions (e.g., soil water availability, meteorological conditions) translate into plant stress and recovery continues to be thwarted by technical limitations in the monitoring of environmental conditions at the appropriate spatio-temporal scale and signs of stress and recovery at the plant and ecosystem scale.

One of the most limiting factors to plant growth is water availability and an important stressor is drought. During drought, physiological changes induce a reduction in photosynthesis and thus plant growth. However, intensity and duration of water stress conditions determine the plant’s physiological response. Under mild water stress, plant regulation of water loss and uptake still allows the plant to maintain its water status with little change in photosynthetic efficiency. However, severe water stress leads to effects ranging from inhibition of photosynthesis and growth to xylem embolism, leaf wilting and loss of key pigments and thus irreversible damage to the photosynthetic and water transport machinery.

Several in situ measurements and remote sensing technologies have been developed to quantify plant stress and ecophysiological response to drought, each with their own strengths and limitations. For example, dendrometers can measure very small changes in stem diameter and thus record daily growth rates and water status variations , while sap flux measurements help quantifying the amount of transpired water. While these techniques are useful for quantifying individual tree responses to stress in terms of mass fluxes and plant water status, they are difficult to apply to whole forests or agricultural fields. Quantifying radiation budgets is another approach for measuring plant stress and response to droughts. Thermal infrared (TIR) and hyperspectral (visible, near-, and shortwave infrared reflectance (VNIR)/SWIR) approaches (besides sun-induced fluorescence) are widely used remote sensing techniques for the detection of plant water stress. An important advantage of remote sensing is that it can be applied to a broader spatial scale. However, the spatial resolution is often coarse and the interpretation in relation to in-situ processes can be complicated by phenological dynamics.

Here we present results from a European beech stand in Luxembourg, where we analysed continuous in situ measurements of dendrometer, sap flux, TIR canopy temperature, meteorological variables and soil moisture. We compare water stress indices derived from sap flux and dendrometer data with a TIR-based crop water stress index (CWSI) recently developed for crops (Ekinzog et al. 2022). Results are put into context with a leaf and canopy energy balance model and implications of drought stress for short and long-term carbon and water fluxes are discussed.

Literature:

Ekinzog, E. K., Schlerf, M., Kraft, M., Werner, F., Riedel, A., Rock, G., and Mallick, K.: Revisiting crop water stress index based on potato field experiments in Northern Germany, Agricultural Water Management, 269, 107664, https://doi.org/10.1016/j.agwat.2022.107664, 2022.

How to cite: Schymanski, S., Schlerf, M., Keim, R., and Iffly, J. F.: Detecting forest drought stress from above and from below, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16625, https://doi.org/10.5194/egusphere-egu24-16625, 2024.

Hydrology has been guided by establishing empirical relationships between the movement of water through landscapes and the application of the conservation of mass law in catchments. This has resulted in models with complex calibration frameworks that often overlook the physical and biochemical water-related processes linking plants to hydrological cycles. Studies have revealed that some of the empirical relationships in catchments might also reflect a potential ecosystem's coevolution with climate, driving catchments to optimise their supply and demand limits. This agrees with the eco-evolutionary optimality principles used in vegetation modelling that are based on the hypothesis that canopy conductance acclimates to environmental variations by balancing the costs of carbon assimilation and maintenance of transpiration rates. Here, we developed meaningful interfaces between simple models and approaches based on the use of optimality principles in vegetation modelling and hydrology. Our work is based on the application of the P-model to estimate to quantify gross primary productivity and transpiration and the use of a mass-balance approach to quantify the root zone storage. These integrations not only provide a more nuanced understanding of hydrological processes but also pave the way for more accurate and physically-informed models in hydrology. Our findings underscore the potential of using eco-evolutionary principles as a unifying framework in hydrological research, offering new insights for understanding and predicting water movement in catchments under varying climatic and ecological conditions.

How to cite: Nóbrega, R., Miranda, R., Sandoval, D., Tan, S., and Prentice, I. C.: Using optimality principles to couple terrestrial carbon and water cycles in hydrological models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18435, https://doi.org/10.5194/egusphere-egu24-18435, 2024.

Vegetation water content varies in response to the shifting balance between transpiration loss and water supply through the soil--plant--atmosphere continuum. These variations are coupled to carbon dynamics by stomatal regulation of gas exchange, linking transpiration and photosynthesis, and through rootzone soil moisture, determined in part by the allocation and turnover of carbon to roots. Microwave sensors have been demonstrated to be sensitive to variations in vegetation water content and related measures of plant hydraulic status, such as plant water potential (PWP). We use synthetic experiments representative of a European deciduous forest to explore whether time series observations of PWP can constrain an intermediate complexity terrestrial ecosystem model (DALEC) with fully coupled carbon and water balances using a Bayesian model-data fusion framework (CARDAMOM). To generate a synthetic truth, we calibrated DALEC using detailed site-specific inventory data from the Hainich ICOS site (DE-Hai), spanning 2006-2011, from which we generated a synthetic time series of average daily mean PWP. The Hainich forest is a temperate forest dominated by beech and established on clay-rich soil. We used the calibrated model as the basis for a series of synthetic data assimilation experiments under conditions of reduced data availability to represent information typically available from satellites and/or global products (e.g. Leaf Area Index, aboveground biomass, soil characteristics) to assess the potential to constrain C cycle dynamics using information on time varying PWP. We compared the diagnostics to a baseline experiment with no assimilated PWP information. Assimilation of PWP reduced the bias in estimates of GPP and ET relative to the synthetic “truth”, with a small reduction in the width of the 90% confidence range, compared to the baseline experiment. PWP observations provided more notable constraints on model parameters that were connected to plant hydraulics and water supply, including root dynamics. The emergent constraint on root dynamics is significant, because below-ground processes are inherently challenging to observe remotely. Assimilating PWP also constrained within-ensemble covariance between certain parameter pairings, and between fluxes, particularly pairings linked to the water balance, and between the water balance and productivity, highlighting the potential for enhanced constraint through the addition of complementary information. Once the signal noise exceeded 0.20 MPa, there was very limited information transfer into either the model parameters retrieved during the inversion, or the resultant fluxes. Our synthetic experiments demonstrate the potential for satellite estimates of PWP (e.g. through microwave VOD) to provide constraints on carbon-water coupling, that these constraints extend to both fast processes (GPP, ET), and slower processes (root dynamics), and that such observations would be highly complementary to C-cycle information from other EO data streams.

How to cite: Milodowski, D., Williams, M., Smallman, L., and Steele-Dunne, S.: Can time series of plant water potential constrain carbon cycle dynamics using the CARDAMOM model-data fusion framework?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20457, https://doi.org/10.5194/egusphere-egu24-20457, 2024.