## 3 Results
### Validation  
Leave one site out cross validation was conducted to assess crop and grass flux prediction consistency on unseen data and also to identify influential flux towers (Table 5). Leave one site out cross validation is a pessimistic error estimation particularly for influential flux towers. The crop mean MAD was 1.10 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\), and ranged from 0.70 to 1.71 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\). The most influential flux tower was Lamont ARM Main which represents a low yield dryland cropping system; this tower was the southern-most crop site, and one of two winter wheat sites. Mandan was the second most influential flux tower, which may be related to the crop type with Mandan being one of only two alfalfa flux towers. Additional cropland flux towers located in poorly represented crops would improve model confidence and robustness. Flux-tower known weekly NEP was regressed on predicted weekly NEP when each respective flux tower location was withheld as a test in the cross validation by site analysis. An overall, highly significant (\(p<0.01\)) regression model had an coefficient of determinations (\(R^{2}\)) of 0.57, an RMSE of 1.62 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\), and a MAD of 1.14 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) (Table 2)--which was quite similar to the table means of the leave-one-out flux tower site (Table 5). Similar NEP mapping accuracies were reported by [13] but, at a global scale, it had a much coarser spatial resolution (0.5 degree) and a longer mapping time step. The grassland leave one site out cross validation (also shown in Table 5) analysis indicated slightly lower NEP error terms than the cropland model with a mean RMSE of 1.12 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) (ranging from 0.9 to 1.61 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\)); and a mean MAD of 0.83 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) (ranging from 0.62 to 1.09 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\)). Fort Peck was the most influential grassland flux tower because it captured an extreme drought year (2002). Rannells Ranch was the second most influential flux tower of the grassland model. An overall regression of withheld weekly grassland NEP had similar error magnitudes (Table 2) as the leave one site cross validation means (Table 5). The lower NEP error terms that were observed for grass relative to crops persisted in the weekly regression of withheld grass predictions with a RMSE of 1.14 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) and a MAD of 0.81 g C\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) (Table 2). These lower grass leave-one-out cross validation error terms may be related to the diverse crop types (the monoculture nature of crop stands resulted in abrupt vegetation differences) and the intensive management of crops. Grassland communities are typically more diverse mixtures of forbs and C\({}_{3}\) and C\({}_{4}\) grasses, often have gradual transitions in species compositions, and management impacts are not as intensive as croplands. Leave-one-out cross validation quantified the robustness of mapping models through time (Table 6). The cropland mean RMSE and the mean MAD from the leave-one-year-out analysis had similar magnitudes as the leave-one-site-out analysis. Interestingly, we see 2001 was the most influential year--the year the Lamont winter wheat flux tower recorded very low crop yields. Similarly, the leave-one-year-out cross validation had similar across-year mean RMSE and MAD as the leave-one-out site across site mean values. This demonstrates a consistency in the crop and