## 3 Results
### Time Series of Eight-Day GPP and ET  
Figure 4 shows the time series of the eight-day observed GPP and RS-based GPP from the PMLv2 and MOD17 products at five cropland sites. Overall, both the PMLv2 and MOD17 products could effectively capture the seasonal variation of observed GPP. Not surprisingly, C4 maize (with a mean of annual maximum observed GPP of 192.93 \(\pm\) 14.80 g C m\({}^{-2}\) sd\({}^{-1}\) (mean \(\pm\) standard deviation)) showed higher photosynthesis capacity than C3 soybean  
Figure 3: Scatter plots of the eight-day observed ET and RS-based ET from the PMLv2 and MOD16 products for C4 maize and C3 soybean at the five cropland sites. The statistical metrics with green, orange, and black colors represent the performance of the PMLv2 and MOD17 products for maize, soybean, and both species, respectively.  
Figure 2: Scatter plots of the eight-day observed GPP and RS-based GPP from the PMLv2 and MOD17 products for C4 maize and C3 soybean at the five cropland sites. The statistical metrics with green, orange, and black colors represent the performance of the PMLv2 and MOD17 products for maize, soybean, and both species, respectively.  
(with a mean of annual maximum observed GPP of 116.14 \(\pm\) 16.66 g C m\({}^{-2}\) 8d\({}^{-1}\)). However, both the PMLv2 and MOD17 products cannot reflect the distinct photosynthesis capacity between C4 and C3 species, where the means of annual maximum GPP from the PMLv2 and MOD17 products for maize (105.98 \(\pm\) 13.42 g C m\({}^{-2}\) 8d\({}^{-1}\) and 56.74 \(\pm\) 7.61 g C m\({}^{-2}\) 8d\({}^{-1}\), respectively) were consistent with those for soybean (92.95 \(\pm\) 16.28 g C m\({}^{-2}\) 8d\({}^{-1}\) and 53.24 \(\pm\) 10.56 g C m\({}^{-2}\) 8d\({}^{-1}\), respectively). Even worse, in most cases, the severe underestimations for soybean from both the PMLv2 and MOD17 products and the severe underestimations for maize from the PMLv2 product were observed during the peak of growing seasons, while the underestimations for soybean from the PMLv2 product were insignificant. In addition, these underestimations of observed GPP from the MOD17 product were more remarkable than those from the PMLv2 product.  
Figure 4: Time series of the eight-day observed GPP and RS-based GPP from the PMLv2 and MOD16 products at the five cropland sites. The years with white backgrounds represent that C4 maize was planted this year. The years with grey backgrounds represent that C3 soybean was planted this year.  
Figure 5 shows the time series of the eight-day observed ET and RS-based ET from the PMLv2 and MOD16 products at the five cropland sites. Overall, both the PMLv2 and MOD16 products also effectively captured the seasonal variation of the observed ET and showed no systematic underestimations or overestimations of observed ET. Unlike the results for GPP, there were marginal differences in the magnitude of observed ET between C4 maize (with a mean of annual maximum observed ET of 45.15 \(\pm\) 5.64 mm 8d\({}^{-1}\)) and C3 soybean (with a mean of annual maximum observed ET of 42.23 \(\pm\) 5.11 mm 8d\({}^{-1}\)). The PMLv2 and MOD16 products provided relatively consistent ET estimates at all five sites. Additionally, the main disparity between the PMLv2 and MOD16 products was that the PMLv2 product tended to generate higher ET estimates than the MOD16 product at the green-up stage (the period that GPP and ET continuously increased during each growing season) at some growing seasons, while both two products generated consistent ET estimates during the senescence stage (the period that GPP and ET continuously decreased during each growing season).  
Figure 5: Time series of the eight-day observed ET and RS-based ET from the PMLv2 and MOD16 products at the five cropland sites. The years with white backgrounds represent that C4 maize was planted this year. The years with grey backgrounds represent that C3 soybean was planted this year.