## 4 Results  
### Assessment of the Derived Monthly Net Primary Production (NPP)  
A comparison of monthly NPP from eddy covariance flux sites and MODIS monthly NPP shows that a highly significant and positive correlation exists and a close 1:1 match of field-based and MODIS NPP is observed (Figure 5). Across the study area, the correlation coefficients between monthly NDVI values and monthly MODIS NPP values (both for 2000-2010) are universally positive, and statistically significant (Figure 3). The mean correlation coefficients of tree, bush, and grassland savanna is 0.71, 0.73, and 0.74, respectively, which indicates that a strong linear relationship for all cover types. As NDVI has strong linear relationship with NPP in the study area, bootstrapping regression can be used to derive estimated or predicted NPP from the NDVI data set [17]. To assess the performance of predictions, we calculated the per-pixel root mean square error (RMSE) based on the predicted NPP and original MODIS NPP from 2007 to 2010 (Figure 6). The RMSE for tree savanna, bush savanna, and grassland savanna is 44.15, 38.78 and 33.03, respectively. This indicates that the predictions of NPP of grassland savanna are more accurate than that of bush and tree savanna, but that all performed well and are more  
Figure 4: The histogram of (**a**) intercepts and (**b**) slopes derived from the bootstrapping regression model for one example pixel (Lat: \(-15.44^{\circ}\)S; Lon: \(23.25^{\circ}\)E). than acceptable for use here. The spatial pattern of mean annual predicted NPP (calculated across water years, from September to August) is consistent with that of mean annual MODIS NPP (Figure 7a,b) for the overlapping 4 years. The predicted NPP value of different land cover classes is also close to the original NPP value. Figure 5: Comparison between monthly net primary production (NPP) from eddy covariance flux towers and Moderate-resolution Imaging Spectroradiometer (MODIS) monthly NPP. The 1:1 line is shown, along with the fitted line of linear regression. Figure 6: Spatial pattern of root mean square error between predicted net primary production (NPP) and original Moderate-resolution Imaging Spectroradiometer (MODIS) NPP for 2007–2010. The model's seasonal NPP results were evaluated using MODIS NPP from 2007 to 2010. We calculated the mean monthly NPP by land cover classes in the entire study area (Figure 8) and comparisons were made (Figure 9). Model estimates closely follow the seasonality and magnitude of MODIS NPP for each land cover class. It looks that NPP is consistently overestimated in comparison to MODIS NPP during the green-up phase. A period of overestimate suggests the ratio of the increasing rate of NPP to that of NDVI (slope) at this period is lower than the fitted slope. For one thing, the greenness of vegetation which can be better indicated by the NDVI values rises rapidly during the green-up phase; for another, the NPP might not increase as rapid as the greenness during the green-up phase as NPP is significantly controlled by leaf-intercepted photosynthetically active radiation. A highly significant correlation coefficient of 0.78 is observed for all savanna types combined. The performance of the bootstrapping regression model for estimating monthly NPP is slightly different in terms of savanna types as suggested by comparison between model-derived NPP and MODIS NPP. There is a highly significant linear correlation for tree savanna, bush savanna, and grassland savanna with correlation coefficients of 0.77, 0.74, and 0.80, respectively. A close match with 1:1 line for tree savanna, bush savanna, and grassland savanna is observed with slope coefficients of 0.88, 0.76, and 0.81, respectively (Figure 9). To validate annual aggregated NPP, we randomly selected pixels across the study region and extracted annual aggregated NPP of model estimates and MODIS for the 4 overlapping years.