# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 4 Results  
### Continuous TWSC Time Series Construction  
Although five TWSC results had the same peaks, troughs, and change trends (Figure 2), they had different uncertainties (Figure 3). This indicated that different GRACE/GFO solutions were discrepant. Therefore, it was necessary to fuse the five TWSC results to reduce the uncertainties. Figures 2 and 3 show that the fused TWSC result completely overlapped with the other five TWSC results and its uncertainty is much smaller than any single solution. This suggests that the fused TWSC results substantially reduce the uncertainty of TWSC results while maintaining their consistency. We also plotted the spatial distribution maps of the uncertainties of the five GRACE/GFO TWSC and the fused TWSC result (Figure 4). From Figure 4a-e, the uncertainties in most regions are less than 5 cm, and only in the central and eastern regions are the uncertainties relatively high. The fused TWSC results also exhibit similar spatial distribution characteristics, but the uncertainty values are all less than 2 cm in the most affected region, and they are less than 4.5 cm in the central and eastern regions (Figure 4f). Combining Figures 3 and 4, after the data fusion processing, the uncertainty of TWSC was significantly reduced. Hence, we used the fused TWSC results to detect and characterize the 2023 extreme drought in the ARB. Since GRACE and GFO are data of exactly the same nature, we denote the fused TWSC results GRACE TWSC in the following.  
From Figure 3, the GRACE TWSC has a data gap from July 2017 to May 2018, which creates a discontinuity in the TWSC time series. In our study, Swarm TWSC results were used to fill this gap. We compared Swarm TWSC results with GRACE ones in the ARB during 2014 and 2022 (Figure 5). It is clear that the two time series have the same peaks, troughs and trends (Figure 5a). However, the amplitude of the GRACE result is significantly  
Figure 2: Time series of TWSC from five single GRACE/GFO solutions and fused results in the ARB during January 2003 and February 2024.  
larger than that of the Swarm result, which is attributed to the larger uncertainty of the Swarm result (Figure 5b). The correlation coefficient (CC) and NSE between GRACE and Swarm results were 0.95 and 0.86, respectively (Figure 5c). This demonstrated that the Swarm TWSC result had similar performance to the GRACE one and has the potential to detect TWSC in the ARB. Therefore, it was feasible to fill the gap between the GRACE and GFO missions using Swarm solutions.  
We constructed a continuous 21-year TWSC series based on GRACE/GFO and Swarm solutions in the ARB (Figure 6). To verify the reliability of this series, we compared it with the Global Land Data Assimilation System (GLDAS) TWSC for the same time period. We found that these two series have peaks or troughs at the same time points and similar trends. This indicated that the two have strong consistency, which was also confirmed by the correlation coefficient results (0.88). However, the positions of the peaks and fluctuations in the two time series do not overlap, showing obvious differences. These differences are caused by multiple factors. Firstly, the GRACE results include TWSC caused by both natural factors and human activities, while the GLDAS results only consider natural factors. Secondly, neither the GRACE results nor the GLDAS results directly obtain TWSC: instead,  
Figure 4: Spatial distribution of uncertainties of TWSC from five single GRACE/GFO solutions and fused results in the ARB during January 2003 and February 2024.  
Figure 3: Uncertainties of TWSC from five single GRACE/GFO solutions and fused results in the ARB.  
they are estimated through calculation formulas or mathematical models, which involve errors in the formulas or models. Finally, the quality of input variables in the GLDAS model also has a certain impact on the final results [60]. Therefore, this 21-year continuous TWSC series has high reliability and applicability in the ARB.