# A Multimodal Data-Driven Framework for Enhanced Wheat Carbon Flux Monitoring
## 3 Results
### Importance of Winter Wheat Carbon Flux Estimation Feature  
The estimation of wheat carbon flux across different growth stages using the MCFEW model was coupled with an analysis of the model's post hoc interpretability. The feature importance was determined by calculating the SHAP values for each feature channel, as shown in Figure 5. The interpretability analysis revealed vegetation indices--particularly LAI, NDVI, and EVI--as primary contributors to model predictions, with climatic variables demonstrating a secondary influence on carbon flux estimation accuracy. The feature importance ranking for each stage is shown in Table 3. For four growth stages, the top five ranked features are (1) LAI, (2) NDVI, (3) EVI, (4) vapor pressure (Vap), and (5) the Palmer Drought Severity Index (PDSI). This highlights that satellite remote sensing variables, such as LAI, NDVI, and EVI, alongside meteorological variables, including PDSI and Vap, are the most influential in predicting wheat carbon flux across all growth stages.  
LAI represents the ratio of plant leaf area to ground area, which is crucial in assessing the density of vegetation. A higher LAI typically reflects greater photosynthetic capacity, directly influencing carbon flux. In Figure 5, LAI consistently shows the highest SHAP values across all growth stages, underscoring its critical role in determining wheat's carbon assimilation. EVI, by reducing the impact of soil background and atmospheric conditions, provides a clearer and more accurate reflection of vegetation growth. This is particularly relevant for crops like wheat, which are sensitive to soil moisture and environmental conditions. The SHAP values for EVI are substantial, particularly in the heading-filling and jointing stages, indicating its importance in reflecting seasonal changes in vegetation health and biomass. NDVI, a widely used vegetation index, is a measure of vegetation coverage and health. As seen in Figure 5, NDVI shows substantial importance during the early to mid-growth stages, with the highest SHAP values observed during the green-up-to-jointing stage, when wheat biomass and chlorophyll content are at their peak. However, NDVI values tend to decrease in the later stages, particularly during the milk maturity stage, reflecting reduced photosynthetic activity and leaf senescence.  
Among the meteorological parameters, PDSI integrates soil moisture and climate conditions, and it significantly influences wheat growth. PDSI is particularly important during periods of water stress, and its SHAP values are consistent across all growth stages, indicating its persistent role in predicting carbon flux. Similarly, Vap is a key indicator of atmospheric humidity, directly influencing plant transpiration and photosynthesis. Vap is especially important during the jointing and heading-filling stages, as indicated by its elevated SHAP values, suggesting that atmospheric humidity plays a critical role in determining the carbon flux during periods of rapid growth.  
In addition to these key features, the comparison of feature importance across different growth stages reveals that the importance of the Pr feature varies significantly. At the green-up stage, Pr's importance exceeds that of Pet and PDSI, highlighting the crop's higher dependence on precipitation early in the growth cycle. However, at the milk maturity stage, the importance of Pr almost drops to zero, indicating that wheat's reliance on precipitation decreases as it matures. This variation in Pr's importance is likely related to the changing dependence of wheat on precipitation at different growth phases, as well as its interaction with other environmental factors such as soil moisture and temperature.