## 4 Discussion
### Uncertainties and Limitations  
A potential uncertainty matter in the dynamic change in carbon-water fluxes and the effects of drought stress on WUE may exist, particularly when the uncertainties are linked with uncertain values from AET and GPP estimates (remote sensing-based GPP-Glass and AET-GLEAM estimation) due to inputs and/or algorithm structure, ultimately influencing WUE. These limitations in the GPP/AET dataset may cause uncertain interpretations when assessing the effects of drought stress on WUE using a few or different GPP/AET products. For instance, the origins of uncertainty may have originated from the satellite's orbital drift and spectral coverage. As such, remotely sensed NDVI and PAR data may suffer from limitations contributing to the excess variation of trained parameters and output suboptimal representation in model parameter modification, which may impact the GPP model [40]. However, the cross-validation reliably illustrated that the EC-LUE could reproduce the variability of GPP spatially and temporally for numerous ecosystem types, and several model comparisons also showed that the EC-LUE model performs better than other methods and algorithms [59; 61; 105; 124]. Also, the GLEAM AET dataset performed better than other algorithms' evaporation datasets in several regions and has demonstrated better agreement with the estimates of the Budyko, LSA-SAF, and Makkink datasets [55; 56; 57]. However, either GPP/AET estimates from remote sensing, process-based models, or machine learning algorithms should be cautiously utilized when assessing WUE's response to drought. The use of the DS method and SPIE-6 for detecting and assessing responses of ecosystem carbon-water fluxes to drought intensification may come with certain uncertainties and limitations. Some of these are as follows: (i) Spatial and Temporal Resolution: The spatial and temporal resolution of the DS method and SPIE-6 calculations can influence the accuracy of drought detection and its impacts on carbon-water fluxes. Coarser resolution data may not capture localized drought events, while finer resolution data may introduce noise and uncertainty in the analysis. (ii) Drought Definitions: The choice of drought definition can vary among studies and can impact the results. Different drought definitions may yield different DS classifications, leading to varied responses in ecosystem carbon-water fluxes. Also, the DS method and SPEI-6 require the selection of threshold values to classify DS. The choice of these thresholds can influence the detected drought events and responses of ecosystem carbon-water fluxes. (iii) Lag Effects: The response of ecosystem carbon-water fluxes to drought may exhibit lag effects, where the impact of drought on these fluxes may not be immediate. The DS method and SPEI-6 may not fully account for such lag effects, leading to uncertainties in the analysis. (iv) Ecosystem Heterogeneity: Ecosystems within the study area may exhibit varying sensitivities to drought, depending on their composition, structure, and adaptation to climate conditions. The DS method and SPEI-6 may not fully capture the heterogeneity of ecosystem responses, leading to uncertainties in generalizing the results. (v) Nonclimate Factors: Ecosystem carbon-water fluxes can also be influenced by nonclimate factors such as land use changes, land management practices, and disturbances like wildfires [26; 125; 126]. The DS method and SPEI-6 may not consider these factors, introducing uncertainties in attributing changes solely to drought. Addressing these uncertainties and limitations requires careful consideration of data sources, methodologies, and assumptions when using the DS method and SPEI-6 for studying ecosystem responses to drought intensification. Sensitivity analyses and validation against ground-based observations can help assess the reliability of the results and provide a more robust understanding of ecosystem responses to drought [126]. It is necessary to further enhance the models' capacity to estimate GPP and AET.