## 4 Results and Discussion
### Drought Condition Monitoring with Time-Series Biomass Maps  
We further used all the 68 biomass field samples and the corresponding synthetic NDVI pixels to generate synthetic NDVI-derived SVM-AGB model. Specifically, we estimated the AGB for the entire grassland area of Xilinhot, Inner Mongolia, for the growing season during 2005-2013. Figure 6 illustrates the spatial distribution of biomass in 2007 (drought year) and 2011 (non-drought year) for each 8-d intervals during the growing season. Figure 7 shows the fluctuations of the regional mean biomass in the study areas during 2005-2013. The biomass variation of the regional mean in 2011 was almost identical with the 10-yr mean (Figure 7), and therefore, biomass maps in 2011 can be used to represent the general vegetation growth conditions in Xilinhot. Generally, biomass for May-September exhibited large spatial heterogeneity, which is of great significance for detecting grassland conditions. The AGB generally showed a decreasing trend from the southeast to the northwest. In 2011, in the first week of May (day of year (DOY): 121), the grassland biomass was generally below 20 g/m\({}^{2}\) and the regional mean was 16.9 g/m\({}^{2}\). During July, nearly half of the grassland biomass values reached 40-50 g/m\({}^{2}\). Grassland biomass reached its peak values during the end of July and the start of August. In the last week of July (DOY: 209), the highest value reached to 190 g/m\({}^{2}\) in the southeast region, while in the northwest region, the biomass was as low as 35 g/m\({}^{2}\). Then, biomass started to decrease during the third week of August. In the first week of September (DOY: 233), the regional mean AGB was 34.0 g/m\({}^{2}\), and it decreased to 26.2 g/m\({}^{2}\) during the last week of September. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**AGB Model**} & \multirow{2}{*}{**Regression Equation**} & \multicolumn{3}{c}{**Training Set**} & \multicolumn{3}{c}{**Testing Set**} \\ \cline{3-8}  & & \(\mathbf{R^{2}}\) & \
\begin{tabular}{c} **RMSE** \\ **(g/m\({}^{2}\))** \\ \end{tabular} & \(RMSE_{r}\) & \(\mathbf{R^{2}}\) &
\begin{tabular}{c} _RMSE_ \\ **(g/m\({}^{2}\))** \\ \end{tabular} & \(RMSE_{r}\) \\ \hline Linear regression model & \(\text{y}=2010\)\({}^{*}\)\(x-1043\) & 0.66 & 34.13 & 46.3\% & 0.64 & 34.19 & 42.1\% \\ \hline Power function model & \(\text{y}=2.55\)\({}^{*}\)\(10^{5}\)\({}^{*}\)\(x^{14.15}\) & 0.68 & 33.21 & 44.8\% & 0.69 & 31.23 & 40.9\% \\ \hline Exponential model & \(\text{y}=6.771\)\({}^{*}\)\(10^{-5}\)\({}^{*}\)\(e^{(24.71}\)\({}^{*}\)\(x)\) & 0.68 & 33.36 & 44.9\% & 0.69 & 31.24 & 41.1\% \\ \hline
**SVM-AGB** & \(\mathbf{/}\) & \(\mathbf{0.73}\) & \(\mathbf{30.61}\) & \(\mathbf{43.0\%}\) & \(\mathbf{0.72}\) & \(\mathbf{22.89}\) & \(\mathbf{37.1\%}\) \\ \hline \hline \end{tabular} Note: x represents MODIS NDVI; y represents AGB.  
\end{table} 
Table 6: Comparison of the accuracy of different AGB estimation models based on MODIS NDVI. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**AGB Model**} & \multirow{2}{*}{**Regression Equation**} & \multicolumn{3}{c}{**Training Set**} & \multicolumn{3}{c}{**Testing Set**} \\ \cline{3-8}  & & \(\mathbf{R^{2}}\) & \ \begin{tabular}{c} _RMSE_ \\ **(g/m\({}^{2}\))** \\ \end{tabular}  & \(RMSE_{r}\) & \(\mathbf{R^{2}}\) & \begin{tabular}{c} _RMSE_ \\ **(g/m\({}^{2}\))** \\ \end{tabular}  & \(RMSE_{r}\) \\ \hline Linear regression model & \(\text{y}=2178\)\({}^{*}\)\(x-1140\) & 0.71 & 31.40 & 42.6\% & 0.79 & 26.48 & 34.6\% \\ \hline Power function model & \(\text{y}=1.044\)\({}^{*}\)\(10^{5}\)\({}^{*}\)\(x^{12.59}\) & 0.68 & 33.62 & 44.6\% & 0.84 & 28.03 & 38.0\% \\ \hline Exponential model & \(\text{y}=3.902\)\({}^{*}\)\(10^{-4}\)\({}^{*}\)\(e^{(24.61}\)\({}^{*}\)\(x)\) & 0.67 & 34.14 & 45.1\% &Figure 6: Spatial distribution of biomass in 2007 and 2011 for each 8-d intervals during the growing season. Returning to the drought analysis, comparisons of the biomass were made between 2007 (drought year) and 2011 (non-drought year).