# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 4 Results
### Drought Process  
We used the 254-month continuous time series of TWSC in Figure 6 to generate the GRACE-DSI series from January 2003 to February 2024 for monitoring the 2023-2024 extreme drought in the ARB. To verify the reliability of GRACE-DSI, we compared it with three traditional drought indices (SPEI, SPI, and SCPDSI) as well as the CMI (Figure 7). Since the SPEI dataset is only updated up to 2022, the data comparison was limited to the period from 2003 to 2022. In Figure 7, GRACE-DSI has similar variation characteristics to and strong correlations with the four drought indices (all correlation coefficients are greater than 0.52). The correlation with SPI-6 is the highest (0.69), the correlation with SPEI-3 is the lowest (0.52), the correlation coefficient with SCPMSI is 0.61, and the correlation coefficient with CMI is 0.56. These results indicate a significant positive correlation between GRACE-DSI and the four drought indices. However, the four drought indices are mainly calculated based on meteorological data and cannot reflect terrestrial water storage anomalies during  
Figure 5: Time series of TWSC results from GRACE and Swarm solutions in the ARB during 2014 and 2022 (**a**), uncertainties in TWSC results (**b**), and scatterplots (**c**). Figure 6: Time series of TWSC results from satellite gravity (GRACE and Swarm solutions) and GLDAS model in the ARB during January 2003 and February 2024 (**a**) and scatterplots (**b**). droughts. In contrast, GRACE-DSI was constructed based on TWSC, giving it a natural advantage in this regard. In summary, GRACE-DSI is capable of implementing drought monitoring in the ARB. We used the difference between PRE and ET as the natural net recharge (P-ET) to assess the impact of natural factors on the 2023-2024 extreme drought in the ARB (Figure 8). P-ET shows significant seasonal variations, and there is a similar changing trend between GRACE-DSI and CMI. According to GRACE-DSI, the 2023-2024 extreme drought began in September 2023, after which GRACE-DSI continued to decline, with the peak occurring in November 2023. By February 2024, the drought was in the recovery stage. P-ET started to show a significant downward trend from March 2023 (19 cm) and continued until August 2023 (8 cm), after which it began to rise gradually. GRACE-DSI, however, showed a significant decline in June. By comparing the time points of the occurrence of the decline and the peak, we found that there was a 3-month lag effect in the response of GRACE-DSI to P-ET. CMI showed negative values from January 2022 and a downward trend until November 2023 (consistent with GRACE-DSI). This indicates that this drought started as a meteorological drought (CMI) and then propagated to form a hydrological drought (GRACE-DSI). Based on the comparison between CMI and GRACE-DSI, it can be seen that the duration of the meteorological drought was significantly longer than that of the hydrological drought. This may be related to human activities. Drought prevention and mitigation measures such as reservoir water storage and regional water resource scheduling have interfered with the propagation between meteorological droughts and hydrological droughts, resulting in significant differences in drought characteristics between meteorological droughts and hydrological droughts [56]. Figure 7: Time series of the monthly GRACE-DSI, CMI and three kinds of traditional drought indices in the ARB during 2003 and 2022 (**a**,**c**,**e**,**g**) and their scatterplots (**b**,**d**,**f**,**h**). From Figure 8, a total of six drought events occurred in the ARB during January 2003 to February 2024, which are in 2005, 2010-2011, 2015-2016, 2017-2018, 2020 and 2023-2024, respectively. We counted the characteristics of these six droughts (Table 4). Comparing the six droughts, although the duration of the 2023 extreme drought (6 months) is not long, its peak and average GRACE-DSI (\(-1.29\) and \(-0.94\)) are second only to the 2015 extreme drought, and its drought severity (\(-5.62\)) was not high and ranked only fourth.