## 3 Results and discussion
### Comparisons of Tower-Based WUE and the MODIS Estimates from GPP and ET  
Figure 3 shows the performance of MODIS-derived WUE estimates (WUERs) from GPP and ET in explaining the variability in EC measurements (WUEEC) at the crop flux site. Changes in WUEEC were tracked by WUERs with reasonable consistency for both corn and soybean. However, there existed two pronounced discrepancies. MODIS WUE estimates greatly overpredicted tower-based WUE at the beginning and ending of crop-growth periods whereas ecosystem WUE during the peak periods were severely underestimated, especially for the C4 plant-corn. This comparison reveals large uncertainties in the coupling processes between carbon and water cycles as represented in the MODIS GPP and ET products. These uncertainties may stem from the algorithm, various upstream inputs, as well as the specific parameters stored in a Biome Properties Look-Up Table (BPLUT). A single value for agricultural lands without considering the differences between C3 and C4 must be problematic. Thus, further analysis is needed to diagnose the errors.  
Seasonal patterns of GPP and ET from flux observations and the corresponding MODIS estimates are compared to explore the potential reasons for differences between WUEEC and WUERs. As shown in Figure 4, the MODIS ET product performed reasonably well with measured ET during both corn years and soybean years except several 8-day periods. Overall, the percentage deviation of multiyear mean annual ETEC and ETRS was less than 5%. However, large discrepancies existed in the seasonal dynamics between GPPEC and GPPRS. Similar to the variability in ecosystem WUE, crop GPP was overestimated at the emergence stage and after harvest, and was severely underestimated during the peak growth periods. The percentage difference during the corn planting years was up to 30% in contrast to less than 10% for the soybean years. In addition, we also found that 8-day patterns of MODIS GPP product during 2001 to 2006 were nearly identical without considering the differences between C3 and C4 photosynthetic pathways for corn and soybean crops. It suggests that improvement of the MODIS GPP product needs to consider this point for the purpose of monitoring crop WUE accurately.  
Ecosystem WUE of corn is generally higher than that of soybean. Statistical analyses for the flux tower site US-Bo1 during 2001-2006 found that tower-based WUE of corn during the crop-growth period ranged from 2.22 g C kg-1 H2O to 3.10 g C kg-1 H2O with the mean value of 2.81 g C kg-1 H2O, while soybean WUEEC spanned between 1.58 g C kg-1 H2O and 2.83 g C kg-1 H2O with the average value of 2.16 g C kg-1 H2O. These differences are related to the plant physiological pathways, as well as the leaf structure and architecture that differ between corn (C4 plant) and soybean (C3 plant) [48, 49]. C4 crops are generally found to have high phosphoenolpyruvate carboxylase activity [50, 51]. Theoretically, water use efficiency of plants is affected by the leaf-to-air CO2 concentration difference and VPD [52]. Recent studies [53, 54] have shown the existence of further internal resistances into the leaf (called mesophyll conductance) that may constrain as much as stomata the CO2 diffusion pathway and are also key actors in controlling and determining plant WUE. Under the same background and environmental conditions, WUE of a C3 crop is apparently lower than C4 crop because CO2 concentration is still higher in C4 leaves than in C3 leaves [51, 55]. Therefore, the characteristics of species determine a larger WUE for C4 plant than for C3 plant. In addition, many studies have found that C\({}_{4}\) vegetation exhibited increasing WUE in response to rising atmospheric CO\({}_{2}\) concentration by reducing the stomatal aperture as well as the transpiration rate compared with C\({}_{3}\) species [6,56,57]. We hypothesize that that the lack of this C\({}_{3}\)/C\({}_{4}\) distinction in the biome-specific parameters in BPLUT causes substantial uncertainty in the MODIS GPP product and the estimation of WUE.