# Upscaling Gross Primary Production in Corn-Soybean Rotation Systems in the Midwest
## 3 Results
### Upscaling GPP to County Level  
The total corn and soybean area from the ACPF land use maps corresponded well with the official production area in 2016 [1] (y = 1.01x; R\({}^{2}\) = 0.99, p \(<\) 0.05, n = 8). The land use maps also corresponded well with the MODIS land cover in 2016, and \(>\)99% of the ACPF map area used was classified as cropland in the MODIS land cover map. The average (\(\pm\) SE) ACPF land use area of corn and soybean in 2016 made up 55% \(\pm\) 2% and 26% \(\pm\) 1% of the total land area in four counties, respectively.  
The average within-county fPAR range (difference between maximum and minimum fPAR) was 0.65 at DOY 145, decreased to 0.32 at DOY 201 and increased again to 0.65 at DOY 265 (Figure 4, Video S1). The county median fPAR ranged from 0.25-0.9 with little variation among crops and counties during the 2016 growing season (Figure 5A). Calculated daily corn and soybean GPP followed a seasonal pattern with highest values at 22.1 and 11.7 g C m\({}^{-2}\) d\({}^{-1}\), respectively (Figure 5B). A linear regression between EC-derived GPP and GPPcalc within the 90% footprint was significant at p \(<\) 0.05, with a coefficient of determination (R\({}^{2}\)) of 0.96 and 0.86 and a root mean square error (RMSE) of 2.24 and 2.81 g C m\({}^{-2}\) d\({}^{-1}\), for corn and soybean, respectively (Figure 5C). A linear regression between EC-derived GPP and daily GPPMODIS within the 90% footprint was significant at p \(<\) 0.05, with a R\({}^{2}\) of 0.89 and 0.21 and a RMSE of 2.44 and 3.30 g C m\({}^{-2}\) d\({}^{-1}\) for corn and soybean, respectively. GPPMODIS was not substantially different between crops, overestimated GPP at the onset and end of the season, and underestimated corn GPP (Figures A2 and A3).  
The average within-county corn and soybean GPPcalc range (difference between maximum and minimum GPPcalc) was 13.2 and 6.0 g C m\({}^{-2}\) d\({}^{-1}\) at DOY 145, decreased to 1.5 and 0.9 g C m\({}^{-2}\) d\({}^{-1}\) at DOY 201 and increased again to 5.1 and 3.6 g C m\({}^{-2}\) d\({}^{-1}\) at DOY 265 (Figure 6). The YieldGFP was 12.82 \(\pm\) 0.65 and 6.73 \(\pm\) 0.27 Mg ha\({}^{-1}\), for corn and soybean, respectively (Table 2). The official county-wide corn and soybean yield in 2016 was 13.09 \(\pm\) 0.09 and 4.03 \(\pm\) 0.04 Mg ha\({}^{-1}\), respectively [1].  
Figure 4: MODIS fPAR product (MCD15A2H) for DOY 193 in 2016 in four Iowa counties for corn and soybean fields. The red triangles and yellow circles show the location of the eddy covariance (EC) stations on soybean and corn fields, respectively; fPAR: fraction of absorbed photosynthetic active radiation.  
Figure 5: \(\mathrm{GPP_{calc}}\) as calculated with MODIS fPAR product for corn (black circles) and soybean (red triangles) for 2016: (**A**) median county fPAR, averaged (\(\pm\) SE) over county (n = 4), (**B**) daily median county \(\mathrm{GPP_{calc}}\) (g C M\({}^{-2}\) d\({}^{-1}\)) averaged over county, and (**C**) linear regression analysis (p \(<\) 0.05) with daily EC-derived GPP as independent and 90% footprint \(\mathrm{GPP_{calc}}\) (g C m\({}^{-2}\) d\({}^{-1}\)) as dependent variable, R\({}^{2}\): coefficient of determination, GPP: gross primary production, fPAR: fraction of absorbed photosynthetic active radiation.