# A Multimodal Data-Driven Framework for Enhanced Wheat Carbon Flux Monitoring
## 3 Results
### Spatial Distribution of Winter Wheat Carbon Flux  
A predictive model for wheat carbon flux in the Guanzhong Plain was developed and monitored based on multimodal data. Remote sensing data from 2023, including NDVI, EVI, and LAI, alongside meteorological and soil data provided by TerraClimate, were utilized. Data extraction, integration, and model training were performed using the GEE platform. Figure 4 presents the carbon flux estimation distribution across the four growth stages of 2023, offering an overview of the carbon flux dynamics in the region and revealing significant spatial heterogeneity.  
In the green-up stage, carbon flux values ranged from below 50 gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\) to 50 gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\), with substantial variations in the carbon fixation capacity among crops within the region. The relatively flat topography of the Guanzhong Plain makes climatic factors--such as precipitation, temperature, and solar radiation--the primary determinants of the spatial distribution of carbon flux. In particular, areas with better drainage conditions promote crop growth, enhancing carbon fixation. During the green-up stage, wheat vegetation is in the early stages of development, resulting in relatively low carbon flux  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Period** & **Adjusted** & **MAE** & **RMSE** & \\  & **R\({}^{2}\)** & **(gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\))** & **(gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\))** & **nRMSE** \\ \hline Green-up stage & 0.89 & 2.15 & 2.87 & 0.05 \\ Jointing stage & 0.94 & 4.57 & 5.90 & 0.04 \\ Heading–filing stage & 0.90 & 4.21 & 5.55 & 0.05 \\ Milk maturity stage & 0.79 & 5.33 & 6.90 & 0.06 \\ Average value & 0.88 & 4.07 & 5.31 & 0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Model accuracy evaluation for each phenological period in 2023.  
Figure 4: Distribution of winter wheat carbon flux estimated by the MCFEW model ((a) green-up stage, (b) jointing stage, (c) heading–filling stage, and (d) milk maturity stage).  
values. However, the rapid growth of vegetation during this period leads to significant carbon assimilation activity, although the overall flux remains low. The spatial distribution of carbon flux within the region is uneven, reflecting variations in planting density, crop health, and local environmental conditions.  
From the jointing stage onward, a marked shift occurs in the spatial distribution of carbon flux. Areas with higher flux values--represented by yellow and red zones--expand significantly, while the extent of lower flux areas (blue zones) decreases. By the heading-filling stage, nearly all agricultural regions in the central-southern and western parts of the Guanzhong Plain are predominantly red, indicating high carbon flux. More than half of these areas exceed 150 gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\), corresponding to the period when wheat reaches its peak biomass. This growth phase is characterized by high carbon assimilation, as the crops actively fix carbon to support rapid growth.  
As wheat transitions into the milk maturity stage, a noticeable decline in overall carbon flux levels occurs. This decline is likely due to the gradual reduction in photosynthetic efficiency as the crops mature and growth slows. However, certain regions continue to exhibit relatively high carbon flux values, ranging from 100 gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\) to 150 gC\(\cdot\)m\({}^{-2}\cdot\)8d\({}^{-1}\), reflecting the continued carbon fixation potential in areas with high crop biomass. This sustained carbon flux can be attributed to the high biomass of mature wheat, which helps to maintain significant carbon assimilation during the later growth stages, despite the decline in photosynthetic efficiency. The reduction in carbon flux at the milk maturity stage reflects the slowing down of biomass accumulation, a key feature of wheat as it nears harvest and its growth rate diminishes.