## 3 Results and Discussion
### Acquired Results at the Country and Water District Levels
The time series of the auxiliary variable used in the present work to downscale GRACE TWSA, i.e., GPM IMERG, is shown in Figure 7. The corresponding downscaled TWSA shown in Figure 6, can be found in the Supplementary material accompanying the present work (GREECE_TWSA.tiff). The spatial pattern of GRACE-FO original data in Figure 5 indicates that the north and eastern parts of the country are mostly affected by negative TWSA, especially during late summer (August) and autumn months. Those parts correspond to  
Figure 4: Scatterplots of the downscaled GRACE TWSA (m) results against the original GRACE TWSA (m) from July 2018 to March 2021 (the **left**) plot corresponds to the downscaled results before the residual correction, and the (**right**) plot presents the downscaled results after the residual correction). areas where most agricultural activities are found; the water demand is high and receives much less precipitation compared to western Greece. This specific spatial pattern is also reflected in the downscaled results shown in Figure 6; however, the impact of low precipitation is also evident from May to December 2020, where most of the country demonstrates negative TWSA in Figure 6, which agrees with the low precipitation patterns evidenced in Figure 7.  
Results of the downscaling methodology are presented as Water District means for the whole country. The WD means for the coarse resolution GRACE TWSAs were evaluated after resampling the coarse resolution dataset to 0.1\({}^{\circ}\) by a bilinear interpolation and then extracting the regional mean TWSA from the new resampled GRACE TWSAs. Thus, Figure 8 presents the GRACE-FO TWSA before and after downscaling along with GPM precipitation, averaged over each one of the 14 Water Districts of Greece. The associated uncertainty related to each Water District can be found in the Supplementary Material (Table S1) acquired from the gridded fields of leakage and GRACE measurement errors [44], where a mean error of 15.9 mm at the country scale is estimated. GR13 and GR14 Water District refer to Crete and Aegean Islands, respectively, and their results in terms of GRACE TWSA error are reported only slightly higher compared to those of the continental part of the country. However, since the land areas occupied by those two regions are below the typical GRACE footprint and are surrounded by sea, the downscaling outcome should be evaluated very cautiously, as it might have been impacted by sea mass changes. Comparing the graphs of Figure 8, it is evident that at the regional scale, there is consistency between the original and downscaled TWSA, i.e., the downscaled result preserves the major GRACE patterns, whereas the time lag of 2 months slightly shifted the downscaled TWSA. The Water Districts in the eastern and northern  
Figure 5: Original GRACE-FO TWSA time series as an ensemble mean of the three analysis centers (CSR, GFZ, JPL) from June 2018 to March 2021 (months correspond to GRACE-FO months found in Supplementary Material of the present work). Units are in mm with reference to the 2004–2009 mean. parts of the country (GR6 to GR12) received much lower precipitation and demonstrated mostly negative TWSA with higher temporal variability. On the contrary, the western Water Districts with much higher precipitation demonstrated mostly positive TWSA and much lower temporal variability compared to those in northeastern Greece. Regarding the time lags used in the downscaling of TWSA, Figure 8 reveals that at the Water District level, the downscaled product offers a more consistent time variation of TWSA with that of precipitation. In some Water Districts, there might be additional time lags that could offer additional information to the downscaling process. This is because the downscaling process has been conducted at the country level, thus suppressing local differences in the correlation of precipitation lags with TWSA. Thus, in the next section, a demonstration of the downscaling methodology is presented for two individual Water Districts.