## 4 Discussion
### Biochar Effects on Evaporative Fraction and Plant WUE  
We found that the rate of change of the evaporative fraction (EF; i.e., the fraction of net radiation energy converted to latent heat) with soil moisture content in BC1 was about half of that in the other groups, indicating that the rice in BC1 was less sensitive to soil moisture changes at a relatively higher moisture level. Extrapolating the EF-0 fitting curve to the line of EF = EFmax defines hypothetical soil moisture contents at which the available energy is maximally converted to latent heat, indicating the maximum transpiration and minimum sensible heat of the net radiation energy partition. These soil moisture values are higher in the two biochar amendment groups compared to the control (Figure 9), suggesting that the stomata of rice growing in biochar amended soil started closing (thus reducing the evaporative fraction) at higher soil water content than in the control. Moreover, the evaporative fraction remained lower in biochar amended soil at any given soil water level compared to the control, all the way to the wilting point. This finding provides empirical support to the model results in ([21] Figure 4B). This pattern can also be interpreted in a different way--that biochar-amended soils hold more water, but trigger stomatal closure and reduced evapotranspiration at relatively higher soil moisture compared to control plots. As a consequence, despite holding more water, the amount of plant-available water in biochar-amended soils was comparable to control plots. We found that EF increased nonlinearly with soil matric potential (\(\psi\)) in each group (Figure 9b) and may be fitted using sigmoidal curves [62]. The EF-\(\psi\) fitting curve of BC2 group shifted toward lower \(\psi\) compared to the control, suggesting a higher evaporative fraction at a given \(\psi\) in BC2. Indeed, BC2 had the highest evapotranspiration among all three experimental groups (Supplementary Table S1). This curvilinear relation between EF and \(\psi\) is a direct consequence of the nonlinear shape of the soil water retention curves [50]. However, the average EF in BC2 was slightly lower than the other two groups during the UAV campaign period, indicating that the rice in this group was relatively water-stressed. Correspondingly, the rice canopy surface temperature, an indicator for instantaneous plant water stress [63], was slightly higher in BC2 than the other two groups (Supplementary Table S1). Moreover, the leaf water content, an indicator for sustained water stress [14], was the lowest in BC2 among the three groups. Although these differences between the biochar groups and the control were generally small in magnitude during the campaign days, if the mild plant stress had been accumulated over time, it could lead to significantly inferior rice growth, explaining the lower LAI and canopy cover in BC2 compared with the control. How could the biochar amendment affect plant WUE? The answer to the question is very critical to crops in water-limited areas where the experiment locates. In the review by [PERSON] et al. [21], few studies reported a positive effect of biochar on crop WUE based on yield. Plant WUE is expected to increase if the biochar promotes plant productivity without a significant increase in evapotranspiration. This is the case for BC1, where the gross primary productivity (GPP) increased significantly and the evapotranspiration did not change. If the GPP increases at a significant cost of evaporative water loss, the plant WUE will change depending on which (GPP or evapotranspiration) increases more. This is the case for BC2, where the GPP increased along with the significant increase in evapotranspiration. The GPP increment largely surpassed the increment of evapotranspiration and the consequent WUE was increased in BC2 as well.