## 2 Materials and Methods  
### Study Site Summary  
Data from seven AmeriFlux Midwestern agricultural sites in Nebraska, Illinois, Iowa, and Minnesota were used in this study (Figure 1 and Table 1). All sites are characterized by a humid continental climate (Dfa Koeppen climate classification) with hot humid summers and cold winters (Figure 2). The Nebraska sites are located at the University of Nebraska Eastern Nebraska Research and Extension Center near Mead, NE. All three Nebraska sites (US-Ne1, US-Ne2, and US-Ne3) have been under no-till or minimum-till management aside from initial disking conducted in 2001. Conservation-plow tillage has been in effect at US-Ne1 (only) since 2005. Only US-Ne1 and US-Ne2 are irrigated. Water applied was in accordance with best practices to reduce water stress and administer fertilizer, herbicide, and pesticides. The Illinois site, US-Bo1, is located approximately 12 km south of Champaign, Illinois. The site is not irrigated and is under no-till management administered by NOAA. The two Iowa Brooks Field sites, US-Br1 and US-Br3, are located approximately 8 km southwest of Ames, Iowa. The sites are not irrigated and are under a tillage management system typical throughout the Upper Midwest Corn Belt. The Minnesota Rosemount site, US-Ro1, is located approximately 24 km south of Saint Paul, Minnesota. This field is not irrigated and is under chisel plow tillage in the fall following maize harvest and in the spring after soybean harvest in the fall. US-Ro1 is managed jointly by the University of Minnesota and USDA. Each site was equipped with an eddy covariance tower measuring key fluxes on a continuous basis. For these sensors, measurement heights were adjusted at each site (except US-Bo1) either once per season (US-Ne1, US-Ne2, US-Ne3) or at more frequent intervals (US-Ro1, US-Br1, US-Br3) based on crop height to keep the fetch area within the field during unstable, neutral, or moderately stable atmospheric conditions. Figure 1: Map indicating the locations of the seven agricultural AmeriFlux sites in the U.S. Midwest used in this study. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Approximate** & \multicolumn{2}{c}{**Latitude,**} & \multicolumn{2}{c}{**L**ttitude,**} & \multirow{2}{*}{**Management**} & \multirow{2}{*}{**Maize Crop**} & \multirow{2}{*}{**Soybean Crop**} & \multicolumn{2}{c}{**Mean Annual**} & \multicolumn{2}{c}{**Mean Annual**} & \multicolumn{2}{c}{**Area (ha)**} \\
**Field Site** & \multicolumn{2}{c}{**AmeiFlux Site**} & \multicolumn{2}{c}{**Longitude,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} \\
**Location** & \multicolumn{2}{c}{**Elevation**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} & \multicolumn{2}{c}{**L**oppoint,**} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Mead, NE \\ \end{tabular} } & US-Ne1 & 41,1650\({}^{\circ}\) N, & & & & & & & \\  & 96.470\({}^{\circ}\) W, 361 m & & & & & & & & \\  & & 96.470\({}^{\circ}\) W, 362 m & & & & & & & \\  & & 41.179\({}^{\circ}\) N, & & & & & & & \\  & & 96.463\({}^{\circ}\) W, 363 m & & & & & & & \\ \hline \multirow{4}{*}{\begin{tabular}{c} Brooks Field, \\ near Ames, IA \\ \end{tabular} } & US-Br1 & 41,9740\({}^{\circ}\) N, & & & & & & & & \\  & 93.696\({}^{\circ}\) W, 313 m & & & & & & & \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Bondville, IL \\ \end{tabular} } & US-Bo1 & 40,0062\({}^{\circ}\) N, & & & & & & & \\  & 88.2904\({}^{\circ}\) W, 219 m & & & & & & & \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Rosemount, MN \\ \end{tabular} } & US-Bo1 & 44,7143\({}^{\circ}\) N, & & & & & & & \\  & 93.698\({}^{\circ}\) W, 290 m & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 1: AmeriFlux study site locations, management status, crop rotation, field years, mean annual temperatures, precipitation, and field area information.