## 4 Results and Discussion
### Drought Condition Monitoring with Time-Series Biomass Maps
In 2007, biomass generally exhibited temporal variations similar to the biomass variations in 2011. Importantly, during the end of June to the end of July (DOY: 177-209) (indicated by red rectangles), large areas of biomass in 2007 showed much lower biomass compared with the biomass in 2011 (Figure 6), especially in the southwestern region of Xilinhot. During the third and fourth week of July (DOY: 201 and 209) in 2007, large areas of biomass in the southwest were below 100 g/m\({}^{2}\), while in 2011, the biomass values reached up to 150 g/m\({}^{2}\). For the remainder of the growing season, no large differences were detected between the biomass maps in 2007 and 2011. Overall, the time series of biomass during the growing season of 2005-2013 (black line) generally showed similar temporal variations with the 9-yr mean (2005-2013) (dashed line) (Figure 7). However, biomass in 2007, 2009, and 2010 showed larger negative anomalies compared with the 9-yr means largely because of the droughts in this area. The year of 2007 was recorded as a drought year [58], and the biomass values during June and October 2007 were mostly below the multi-year mean (Figure 7). This was consistent with the findings of [PERSON] _et al._[59] and [PERSON] _et al._[60]. [PERSON] _et al._[61] used the Modified Grassland Index (MGI) to monitor grassland variations in Xilingol (2001-2010) and found that the MGI values in 2007 were much lower than MGI values in other years. In the study of [PERSON] _et al._[60], the annual and seasonal NDVI in 2007 was below that in other years (2000-2010) in Xilingol. The largest negative biomass anomaly during 2005-2013 was found during the second week of July in 2009 (51.7% less than the 9-yr mean), followed by the second week of August in 2009 (50.0% lower than the 9-yr mean) and the first week of August in 2009 (47.0% lower than the 9-year mean). [PERSON] _et al._[47] also found that during 2006-2012, the biomass in 2009 was the lowest and it was 31.5% lower than the 7-yr mean in Xilingol League (Xilinhot is located in the center of Xilingol League). This was mainly attributed to severe drought and insect outbreaks in this region in 2009 [62]. Biomass in 2010 showed large fluctuations. According to the national grassland report in 2010 issued by Farmer's daily [59], Xilingol League suffered from severe snow disasters during the end of 2009 and the beginning of 2010. For grasslands in Inner Mongolia, up to 32 million ha of grassland were affected [59], which could be a possible reason for the biomass fluctuations in 2010. Conversely, biomass in 2012 was much larger than that in other years. In the third week of July, the mean biomass of the study area even reached to 133.86 g/m\({}^{2}\), which was 88.9% higher than the 9-yr mean. [PERSON] _et al._[47] stated that 2012 was a prime harvest year for all of Xilingol, and the biomass value was 40% higher than the annual average (2005-2013) and twice the value in 2009. Figure 7: The 8-d intervals of the regional mean biomass estimated based on synthetic NDVI and SVM-AGB model during the growing season (May to September) of 2005–2013 in Xilinhot. Dash line represents the regional mean biomass of 9-yr mean (2005–2013). To further explore the capability of the synthetic NDVI for detecting anomalous vegetation activity, we took the biomass during the drought year (2007) as an example and then compared the data to the 9-yr mean (2005-2013) through using \(SAI_{AGB}\). In 2007, large vegetation anomalies were mainly found during June to the start of August (Figure 8, DOY: 153-217), which can be attributed to the rainfall shortages. Large water deficiencies were detected during the start of March to the end of June (DOY: 64-176) (Figure 8). Other studies have found that precipitation has obvious lag effects on vegetation in the study region, and the lag response time is typically 48 to 56 d [61,63]. Although precipitation in the last week of July was even more than twice the 9-yr mean, it did not relieve the stress on vegetation during this month.