## 5 Discussion  
Rice is the major crop type in our study area. Compared to other crops, the WUE of rice is usually relatively low. For example, [PERSON] et al. (2016) reported that, among the staple crops, the maize cropland has the highest WUE with 2.48 \(\pm\) 0.69 gC/kgH\({}_{2}\)O, followed by winter wheat (2.00 \(\pm\) 0.39 gC/kgH\({}_{2}\)O), soybean (1.92 \(\pm\) 0.52 gC/kgH\({}_{2}\)O), and paddy rice (1.88 \(\pm\) 0.63 gC/kgH\({}_{2}\)O). Our results showed that WUE for rice at the Xiangtang station ranges from 1.41 gC/kgH\({}_{2}\)O to 1.86 gC/kgH\({}_{2}\)O for the early rice and late rice, which was consistent with the review of [PERSON] et al. (2016). The relatively low WUE for rice is partly because the ET of rice is usually large due to the (nearly) constant saturation condition of the paddy fields. The Bowen ratio of the paddy field in our study was about only 0.1 for  
Figure 10: Correlation coefficients r between the predicted GPP, ET, WUE, and the environmental variables, including the NDVI, downward shortwave radiation (R\({}_{\rm sj}\)), air temperature (T\({}_{\rm a}\)), wind speed (u), specific humidity (SH), and precipitation (P) for the early rice (**a**) and late rice (**b**). both the early rice and late rice (Table 3), indicating that ET consumes most of the net radiation of the land surface, which is consistent with other studies, e.g., the Bowen ratio of the flooded rice paddy fields in the Philippines is also only 0.14 on average [63; 64]. Among the total ET, transpiration takes up \(\sim\)50% on average in the growing stages of the early rice, and over 80% for the late rice (Figure 11). Therefore, the ecosystem WUE (WUE\({}_{\rm e}\)) is close to the canopy WUE (WUE\({}_{\rm c}\)) for the late rice, whereas the WUE\({}_{\rm c}\) is much larger than WUE\({}_{\rm e}\) for the early rice, especially in the stages from jointing to yellow ripen (Figure 11). The accuracy of the PML-V2 model in simulating GPP and ET of rice is reasonably good compared to other modeling studies. For example, [PERSON] et al. (2020) estimated the photosynthetic rate of rice using a light-response curve model and found that R\({}^{2}\) ranges from 0.77 to 0.96 at a daily scale when the multi-spectral reflectance data from an unmanned aerial vehicle were used. R\({}^{2}\) from the PML-V2 GPP simulations was also larger than 0.90 in our study case if parameters were optimized in each growing stage. R\({}^{2}\) was 0.72 for GPP during the validation period (2018) if the parameters in 2017 were used. As for ET estimation for paddy fields, [PERSON] et al. (2017) proposed a Javis-Stewart type conductance model and successfully simulated rice ET using the conductance-based PM model at the Kunshan irrigation and drainage experiment station (31\({}^{\circ}\)15\({}^{\prime}\)15\({}^{\prime\prime}\)N, 120\({}^{\circ}\)57\({}^{\prime}\)43\({}^{\prime\prime}\)E), which is located to the northeast of the Xiangtang station (28\({}^{\circ}\)26\({}^{\prime}\)25\({}^{\prime\prime}\)N, 116\({}^{\circ}\)00\({}^{\prime}\)01\({}^{\prime\prime}\)E) and is also subject to a subtropical monsoon climate in the Taihu Lake region of China. After calibration, the model performed well with RMSE of 0.24 mm/d and 0.30 mm/d for 2014 and 2015, respectively, which is relatively small compared to the mean ET (3.52 mm/d) during the rice growth period ([PERSON] et al., 2017). In comparison, the RMSE in ET predictions from the PML-V2 model is also less than 15% of the mean ET during the validation period (2018) if the parameters in 2017 were used. Although similar modeling accuracy for GPP or ET has been achieved in previous studies, the advantage of the PML-V2 model is that the rates of photosynthesis and transpiration of vegetation are interconnected in the calculation process, which helps to correctly assess the WUE of an ecosystem and the canopy. However, we want to note that the reasonable performance of the model here relies on local calibration. In the paper of [PERSON] et al. (2019), parameters are fixed for all crops across the globe.