# Impacts of Compound Hot-Dry Events on Vegetation Productivity over Northern East Asia
## 3 Results  
### CHD Events and the Eco-Hydrological Processes  
Using the definition methodology mentioned in Section 1, we identified all CHD events in each grid cell in northern East Asia from June to August between 2003 and 2019. Figure 2a depicts the time series of the total area with CHD events, facilitating the identification of periods characterized by regionally extensive and long-lasting CHD events. The most prolonged and regionally extensive CHD events occurred during the summer seasons of 2017 and 2015, with durations of 17 days each. Subsequently, we selected the four most regionally severe CHD events based on the spatial extent for detailed case studies, namely Case2017, Case2015, Case2010, and Case2007. Further information regarding each event's occurrence dates and durations is in Table 2. Figure 2b,c display the spatial distribution of temperature and precipitation anomalies for the four CHD events. All four events show Tmax anomalies above +3 \({}^{\circ}\)C and SPI anomalies below \(-\)0.5, covering over 34% (1.88 \(\times\) 10\({}^{6}\) km\({}^{2}\)) of the study area. Case2017 represents the most extensive coverage among these CHD events, with a land area exceeding 61% (3.36 \(\times\) 10\({}^{6}\) km\({}^{2}\)) affected by drought and heat. The spatial pattern of Case2015 is similar to that of Case2017, albeit with a slightly lower heatwave intensity and higher drought severity in specific areas compared to Case2017. The impact area of Case2010 is more inclined towards the southeast than the first two events, predominantly encompassing forested areas with a small amount of grassland. In the fourth event, Case2007, the affected area is primarily concentrated at the junction of China, Russia, and Mongolia, located in the TCZ, which is also encompassed by Case2017 and Case2015 (45\({}^{\circ}\)-55\({}^{\circ}\) N, 105\({}^{\circ}\)-125\({}^{\circ}\) E).  
CHD events strongly impact eco-hydrological processes, with our primary focus on the abnormal changes in GPP, ET, and SM. Apart from Case2010, the other three CHD events exhibited noticeable negative anomalies in GPP (Figure 2d). In Case2017 and Case2015, the distribution of GPP anomalies showed remarkable similarity, with 44% and 57% of the vegetation area demonstrating negative GPP anomalies, particularly in the grasslands within the TCZ, where the decline in GPP was most severe. In Case2007, 87% of the vegetation area experienced negative GPP anomalies, and this event witnessed the most significant GPP decrease, with an average reduction of -0.99 gC/m\({}^{2}\)/day. Conversely, Case2010 saw a positive impact on GPP due to the combined effects of drought and heatwaves, with 74% of the vegetation area displaying positive GPP anomalies. These findings indicate that different CHD events have varying effects on vegetation productivity, and various types of ecosystems also exhibit distinct responses to these events. During the CHD event, the abnormal performance of ET was closely related to vegetation types. Figure 2e shows that negative ET anomalies were mainly observed in grassland ecosystems.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Case ID** & **Start and End Dates** & **Duration (Days)** \\ \hline Case2017 & 22 June 2017–8 July 2017 & 17 \\ Case2015 & 4 July 2015–20 July 2015 & 17 \\ Case2010 & 22 June 2010–29 June 2010 & 8 \\ Case2007 & 23 July 2007–30 July 2007 & 8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Dates and durations of the four selected cases of the regional compound hot–dry events over northern East Asia during summer.  
In contrast, positive ET anomalies were primarily found in forest ecosystems (except for a few boreal evergreen needleleaf trees in the TCZ). Furthermore, the regions with the most significant decline in ET were consistent with those with the largest GPP decrease (see Figure 2d,e). From the spatial distribution of SM anomalies (Figure 2f), it can be observed that the SM in the study area was generally low, except for a small localized area in Case2010.