## 4 Results
### Monitoring and Assessing the 2012 Drought  
Figure 9 shows the SPI-3 values of CDs located in the Great Plains from May to October. A large-scale meteorological drought started in June, mainly in the southeastern and western areas of the Great Plains. In July and August, the drought area expanded and was centered in the middle of the Great Plains. In September, although the SPI-3 of some northern CDs remained low, the drought in the southern CDs began to diminish. By October, large-scale drought conditions were no longer experienced over the Great Plains. Figure 9 shows that the drought swept through the Great Plains in the summer of 2012, starting in June, reaching a peak in July and August, and fading away gradually from September to October. A more serious drought occurred in CDs at middle and low latitudes, and both the grassland (western area) and the cropland (eastern area) in the Great Plains received scant precipitation during the summer (see Figure 1 for land-cover information). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Site Name**} & \multicolumn{4}{c}{**Percentage Decline Compared to 2010**} \\ \cline{2-5}  & **SIF (July)** & **GPP (July)** & **SIF (August)** & **GPP (August)** \\ \hline
**KFS** & 29 & 34 & 43 & 41 \\
**Kon** & 25 & 23 & 16 & 10 \\
**Ne-3** & 26 & 57 & 29 & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Percentage decline in SIF and GPP in July and August of 2012 compared to 2010. The percentage decline (P) of SIF (GPP) was calculated by the formula: \(\mathrm{P}=\frac{\mathrm{SIF}_{2010}-\mathrm{SIF}_{2012}}{\mathrm{SIF}_{2010}}\). Figure 8: Seasonal patterns of Global Ozone Monitoring Instrument 2 (GOME-2) solar-induced chlorophyll fluorescence (SIF) and flux estimated gross primary production (GPP). Subfigures (**a,b**) show the pattern for all 12 months of the year, while subfigures (**c,d**) show the pattern for six months (from May to October), because flux observations start in May and end in October at Ne-1 and Ne-3. Based on the significant response of SIF to drought and the consistency between SIF and GPP, we employed a simple model using the difference of the SIF in 2012 and the SIF in a historically humid year (here we used 2010) to monitor the plants' responses to this drought, and to make an assessment of the drought-induced reduction of GPP. Figure 10 shows the percentage decline of SIF from May to October in 2012 compared to 2010. The spatial pattern of Figure 10 is similar to that of Figure 9, in that the reduction of SIF was concentrated in the months with the poorest precipitation (from June to September), and the most significant and widespread reduction in SIF occurred in July and August. According to the results given in Section 4.2 and previous studies that considered the relationship between GOME-2 SIF and GPP, the greater reduction of SIF compared to the corresponding historical period was considered to indicate more reduction in the GPP. The production of the Great Plains in 2012 was significantly reduced during the growing season. In May, the reduction occurred mainly in the south, over a very limited area. However, in June a large-scale reduction occurred over the whole western area of the Great Plains, whereas production in the east remained at normal levels. In July and August, a significant reduction occurred in most areas of the Great Plains. The western and middle areas experienced the most severe reduction, while the reduction was relatively smaller in the eastern area. From September to October, the large-scale reduction gradually diminished. Most of the reduction was concentrated in the middle and western areas in June, July, and August. As mentioned in Section 4.1, plants are most sensitive to water stress in the peak growing season. Thus, the impact of drought is most significant in this period. Overall, the SIF not only successfully tracked the evolution of the meteorological drought, but allowed us to assess the drought-induced reduction of GPP as well.