## 4 Discussion
### Implications and Future Work  
This study first evaluated the performance of the coupled PMLv2 and uncoupled MODIS products in the eight-day and annual GPP, ET, and eWUE estimations for C4 maize and C3 soybean and subsequently analyzed the uncertainty in the RS-based GPP, ET, and eWUE estimates. To acquire high-accuracy eWUE based on the widely used RS-based GPP and ET products over cropland, some modifications for the RS-based products should be implemented to eliminate those systematic biases of eWUE: (1) the PMLv2 and MODIS products are expected to separately calibrate the model free parameter over cropland by differentiating C4 and C3 crop species to better reflect their different photosynthesis and transpiration; (2) the MOD17 product should be re-parameterized to eliminate the severe systematic underestimations of observed GPP; and (3) both two products should be calibrated and validated at more cropland sites to enhance their feasibility.  
The coupled product can effectively avoid internal inconsistencies between ET and GPP from the uncoupled product and therefore could provide higher-accuracy eWUE estimates [13]. Over the years, a large amount of eWUE-related research (e.g., the spatial and temporal variation of eWUE, the response of eWUE to drought and natural and human activities, and the driver of eWUE changes [1; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48]) has been conducted for various PFTs (including but not limited to cropland); however, most of these studies calculated eWUE based on the MODIS product, whose accuracy is strongly queried by this study as well as previous studies [2; 13; 49]. This study indicates that the coupled product is more suitable and effective to be applied in the eWUE-related research for different crop species to reduce the uncertainty from the inaccurate RS-based eWUE estimates from the uncoupled products. In future work, more coupled products (e.g., BESS and BEPS) and those high-accuracy uncoupled products (e.g., GLASS and FLUXCOM) could be further validated for more various crop species (e.g., barley, rape, and wheat) to explore the best combination of GPP and ET products for eWUE estimations. Note that some uncertainties may be caused by the imperfect match between 500 m remote sensing pixels and in-situ observations. For the sites with homogeneous landscapes (such as the US-Ne1, US-Ne2, and US-Ne3 sites, please see Figure 1), the in-situ observations could be treated as the relative truth to validate the remote sensing data collected at the collocated pixels, and the evaluation is more reliable. However, for the sites with less homogeneous landscapes (such as the US-Ro1 and US-Ro5 sites), the mismatch issue may not be negligible and reduce the reliability of the evaluation. Take the US-Ro5 site for example, a few trees that fall within 500 m of the site may lead to a certain degree of errors in eWUE estimates from the MODIS and PMLv2 products, since the remote sensing products treat the underlying surface of the 500 m pixel as the homogeneous cropland. However, it is difficult to quantitatively explore the uncertainty without a specific and sophisticated C4 and C3 species map. In future work, more attempts should be implemented to address this issue.