## 4 Results
### Comparison of GPP Products in Rainfed and Irrigated Croplands  
In order to further evaluate the EF-LUE GPP, we cross-compared our estimated GPP for rainfed and irrigated croplands with the existing GPP products listed in Table 4 (Figure 9). As seen in Figure 9, the global average of GPP estimated by different models from 2002 to 2018 in the rainfed crop is generally higher than that in the irrigated crop for all the models/products. This is consistent with previous study that showed higher rainfed croplands GPP values than that of irrigated croplands [75]. Our GPP results in rainfed cropland are very close to the mean values of the six GPP products, while the GPP in irrigated cropland from our model is slightly higher than the average of the multiple GPP products of irrigated cropland.  
Figure 8: Mean annual GPP (in 2002–2018) difference between the EF-LUE model and (**a**) MOD17; (**b**) revised EC-LUE; (**c**) GOSIF GPP; (**d**) PML-V2; (**e**) NIRv GPP.  
We compared the average values of GPP estimated by different models for rainfed cropland and irrigated cropland in 5 climate zones (Figure 10). In tropical climate zone (A in Figure 10), the EF-LUE GPP (both rainfed and irrigated cropland) was closer to the GPP values from the GOSIF and the PML-V2 GPPs. Except MOD17, rainfed cropland GPP values by the other five GPP products were higher than the irrigated cropland GPP and significantly higher than the MOD17 rainfed cropland GPP in tropical climate zone. In arid climate zone (B in Figure 10), the EF-LUE GPP of rainfed cropland was closer to the rainfed cropland GPP by GOSIF, MOD17, and PLM-V2. In temperate climate zone (C in Figure 10), the EF-LUE GPP of rainfed cropland was close to that by MOD17, GOSIF, NIRv, and PML-V2, while the irrigated cropland GPP by the EF-LUE model was close to that by GOSIF and PLM-V2. In boreal climate zone (D in Figure 10), the irrigated cropland GPP by the EF-LUE model was close to that by GOSIF, NIRv, and PLM-V2, but the EF-LUE model showed lower GPP in rainfed cropland than MOD17, GOSIF, NIRv, and PLM-V2. In the polar climate zone (E in Figure 10), the GPP values from our EF-LUE model and from the other two LUE-based models were significantly lower than the values from GOSIF and PLM-V2, particularly for irrigated cropland.  
In summary, in most climate zones (except the polar zone), our GPP estimation for both rainfed and irrigated croplands were close to that of GOSIF. The previous study found strong positive correlations between SIF and root-zone soil moisture [76]. Proper consideration of the restriction of soil moisture availability as done in our method might help improve the estimation accuracy in cropland GPP. The PML-V2 GPP value in irrigated cropland in dry climate areas was lower than our results and GOSIF GPP, which might be partially due to that the PML-V2 model did not explicitly consider soil moisture constraint in GPP calculation. The PML-V2 GPP was generally higher, but the PML-V2 GPP value in irrigated cropland in dry climate areas was lower. In the arid and boreal climate zones, the revised EC-LUE was significantly lower than other GPP products. A previous study of product comparison also found that VPM and GOSIF showed similar results in cropland GPP estimations whereas MOD17 and the revised EC-LUE GPP had relatively lower values [77].  
Figure 9: Comparison of annual mean GPP simulated by the EF-LUE model with other five different GPP products in rainfed and irrigated croplands from 2002 to 2018. The blue and orange dashed lines are the average GPP values of all 6 GPP products over rainfed cropland and irrigated croplands, respectively.