## 3 Results and Discussion
### Responses of ELUE and GPP to Bio-Physical and Environmental Factors
#### 3.2.3 Responses of ELUE and GPP to Water Availability  
ELUE was significantly correlated to EF in the soybean (r = 0.83, \(p<0.01\)), maize (r = 0.74, \(p<0.01\)) and winter wheat (r = 0.37, \(p<0.05\)) fields (Figure 6a-c). It was noted that ELUE had a negative relationship with EF in the rice fields (r = \(-0.07\), Figure 6d). A similar relationship between GPP and EF was shown in Table 3. Such results indicated that water availability might not be a limiting factor in the photosynthetic process in long-term flooded paddy rice croplands. ELUE and GPP were negatively related to VPD, while the relationship between VPD and ELUE in soybean (r = \(-0.36\), \(p<0.01\)), as well as VPD and GPP in winter wheat (r = \(-0.28\), \(p<0.05\)) was significant, respectively. [PERSON] et al. (2015) found there was a negative correlation between VPD and GPP in the maize and soybean fields [53], since increased VPD may trigger stomatal closure to avoid excess water loss due to the high evaporative demand of the air, leading to a negative carbon balance [70]. Besides, we also found that VPD did not fully characterize the impacts of water availability on vegetation production except for winter wheat. Conversely, EF can better explain the water availability effects on the variability of ELUE and GPP for all but paddy rice in our analysis. EF is estimated by the evaporative fraction (EF = LE/(LE + H)), since increasing the amounts of energy partitioned to evaporated water indicates a greater potential of soil in the water supply [7; 71]. Therefore, EF has a more significant correlation with vegetation production  
Figure 5: The relationship between daily ecosystem light use efficiency (ELUE) and canopy conductance (g\({}_{c}\)) on a daily basis on non-rainy days at the **(a)** paddy rice, **(b)** soybean, **(c)** summer maize and **(d)** winter wheat fields. The color of the cross symbol showed the level of daily gross primary produc-tion (GPP). than VPD in the agroecosystems, except for paddy rice. VPD tends to exert the effect of atmospheric water restraint but ignore water supply from the soil [72]. [PERSON] et al. (2015) also indicated that VPD easily decouples with GPP in crop fields [53]. Therefore, VPD might not be a suitable indicator to represent the effect of water availability on photosynthesis [71], though there might be a strong covariance between VPD and soil moisture in semi-arid areas [73]. The relationship between ELUE, GPP, PAR and total precipitation during the crop-growing season was investigated (Figure 7). An upward trend was found between ELUE and precipitation in summer maize and winter wheat, while a downward trend was found in paddy rice and soybean sites (Figure 7a). GPP significantly decreased with increasing total precipitation in paddy rice (r = 0.77, \(p\) < 0.05), while GPP increased with increasing total precipitation in the other three agroecosystems (Figure 7b). Increasing precipitation was thought to reduce VPD, which increases GPP by enhancing gc [74]. Therefore, ELUE rises when precipitation increases and VPD decreases. However, the negative relationship between GPP and precipitation was found in paddy rice in the current study, which was contrary to the other three agroecosystems as well as previously published reports on arid and semiarid regions [75]. Presumably because the effect of precipitation on soil water content (SWC) was weak in long-term flooded paddy rice croplands, more precipitation may indicate additional clouds and the downward trend of PAR; therefore, vegetation photosynthesis may be restricted due to the lack of PAR [76; 77; 7  
Figure 6: The relationship between ecosystem light use efficiency (ELUE) and evaporation fraction (EF) on a daily basis on non-rainy day at the (**a**) paddy rice, (**b**) soybean, (**c**) summer maize and (**d**) winter wheat fields. The color of the cross symbol showed the level of daily gross primary production (GPP).