## 5 Results
### The Causes of Drought Events  
We compared the time series of GRACE/Swarm-DSI, PPT anomaly and ET anomaly in the ARB during 2003 and 2020 (Figure 11). The PPT and ET anomalies were calculated by subtracting the mean values of the corresponding months from the original time series [29]. Figure 11 shows that PPT anomaly has better consistency with GRACE/Swarm-DSI than ET anomaly. When the PPT anomaly is positive (which means more PPT than normal), GRACE/Swarm-DSI is generally positive, that is, there is no drought. When PPT anomaly is negative (which means less PPT than normal), GRACE/Swarm-DSI is also generally negative, that is, there may be drought (Figure 11a). It explains that GRACE/Swarm-DSI and PPT anomaly have a close connection. However, there is a certain delay in the response of PPT anomaly and GRACE/Swarm-DSI. From Figure 11b, the correlation between GRACE/Swarm-DSI and ET anomaly is not significant. We calculated the correlation coefficients between GRACE/Swarm-DSI and PPT anomaly (0.41), ET anomaly (0.20), respectively, and the above results have passed the 99% confidence level. The results of correlation coefficient verify the conclusion of Figure 11. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**No. of Events**} & \multirow{2}{*}{**Time Span**} & \multirow{2}{*}{**Duration**} & \multicolumn{3}{c}{**GRACE/Swarm-DSI**} & \multicolumn{3}{c}{**Drought Area Percentage**} & \multirow{2}{*}{**Previous Studies**} \\ \cline{3-3} \cline{5-10}  & & & **Peak** & & **Average** & & **Drought** & **Peak** & **Average** & **Cumulative** & **Validation** \\  & & **Magnitude** & **Magnitude** & **Severity** & **Magnitude** & **Magnitude** & **Magnitude** & **Magnitude** & **(Y/N)** \\ \hline \hline \multirow{9}{*}{\(11\)} & 200301–200306 & 6 & \(-0.81\) & \(-0.65\) & \(-3.92\) & 66.79\% & 56.77\% & 340.61\% & Y [19] \\  & 200312–200408 & 9 & \(-0.8\) & \(-0.43\) & \(-3.86\) & 76.99\% & 51.22\% & 460.94\% & Y [19] \\  & 200412–200512 & 13 & \(-0.98\) & \(-0.63\) & \(-8.24\) & 78.67\% & 60.28\% & 783.70\% & Y [15] \\  & 200703–200705 & 3 & \(-0.39\) & \(-0.29\) & \(-0.88\) & 46.02\% & 43.31\% & 129.93\% & Y [19] \\  & 200708–200710 & 3 & \(-0.38\) & \(-0.31\) & \(-0.93\) & 55.89\% & 46.05\% & 138.15\% & Y [19] \\  & 200911–201101 & 15 & \(-0.86\) & \(-0.4\) & \(-6.03\) & 64.72\% & 49.35\% & 740.27\% & Y [16,56] \\  & 201209–201301 & 5 & \(-0.29\) & \(-0.22\) & \(-1.08\) & 33.67\% & 28.74\% & 143.72\% & Y [19] \\  & 201511–201612 & 14 & \(-0.95\) & \(-0.69\) & \(-9.63\) & 66.56\% & 54.93\% & 769.06\% & Y [27] \\  & 201707–201709 & 3 & \(-1.16\) & \(-1.05\) & \(-3.16\) & 98.90\% & 89.91\% & 269.74\% & Y [57] \\  & 201801–201805 & 5 & \(-0.64\) & \(-0.39\) & \(-1.93\) & 70.08\% & 42.64\% & 213.18\% & N \\  & 202003–202012 & 10 & \(-0.86\) & \(-0.39\) & \(-3.95\) & 65.05\% & 41.16\% & 411.62\% & Y [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary table of drought events. Figure 10: The time-series of GRACE/Swarm-DSI in ARB during 2003–2020. The red and orange bars represent the drought severity and drought area percentage for each month during drought event, respectively. To further study the relation between GRACE/Swarm-DSI and PPT anomaly, ET anomaly, we analyzed the performance of PPT and ET anomaly during the drought events. We found that all drought events experienced less PPT than usual, but only four drought events experienced more ET than usual, which occurred from December 2003 to August 2004, from November 2009 to January 2011, from September 2012 to January 2013 and from July 2017 to September 2017. The above results suggest that PPT is an important factor affecting the occurrence of drought events in the ARB, while ET has little effect. We calculated the maximum correlation coefficients and the delay months between GRACE/Swarm-DSI and PPT anomaly, ET anomaly (Figure 12).