## 3 Result
### Sensitivity of MOCAST+ to Hydrology and Glaciers in the Andes
#### 3.1.2 Sensitivity to Subsurface Hydrology in South America  
We considered long-period trends and seasonal signals for three major basins in South America, namely, the Amazon, Parana, and Orinoco. In Figure 10a-c the yearly long-period gravity rates in these three basins and the outline of the caps considered for the spectral analysis are reported. For the Amazon, given its large dimensions and large signals, we considered two windows centred on the two local minima in the gravity anomalies. A comparison with the yearly error curves of MOCAST+ and GRACE is offered in Figure 10d. The impact of MOCAST+ is again evident in terms of an increase in spatial resolution; in particular, for the Amazon basin, the two \"blobs\" are better depicted. The largest minimum (bounded by white dots in Figure 10a) is theoretically observable by both the GRACE and MOCAST+ missions; while the other local minimum, located towards the Andes orogen, would be detectable after 1 year of observations only by a MOCAST+ mission concept. Figure 10: (**a–c**) Long-period gravity rates in the three basins considered and the outline of the caps used for the localized spectral analysis. Calculations at 250 km altitude. (**d**) Spectra of long-period yearly rates in 3 South American hydrologic basins considered (black lines). Rates expressed as the gravity change accumulated in one year. Error curves of MOCAST+ (blue) and GRACE (red). Spectra calculated at 250 km altitude. In addition, a smaller basin, such as the Parana basin, generates long-period signals just visible by the MOCAST+ mission and clearly not at all detectable by GRACE. The last basin considered, the Orinoco, is the smallest basin and also has the smallest long-term mass variations. It also shows a more complex pattern of anomalies in the spatial domain (Figure 10a). The signal degree variance for the whole basin, calculated within a cap of radius \(10^{\circ}\), is lower than the MOCAST+ error by about one order of magnitude. Even considering a more localized window centred on the largest anomaly, the signal remains below the MOCAST+ sensitivity. Regarding the subsurface hydrology, we further tested the improvement of the MOCAST+ mission for detecting the seasonal signals. In order to verify a detectability criterion, the monthly error curves of various missions were compared to a 1-month signal during the seasonal peak. The three snapshots of the gravity field for the Amazon, Parana, and Orinoco are shown in Figure 11a-c, and were taken at the months of February, January, and July, respectively. These months correspond to the maximum seasonal peak. We see that the Amazon has the largest signal both in terms of amplitude and of areal extension; the Parana has a seasonal signal mostly localized nearby the Amazon,  
Figure 11: (**a–c**) Snapshot corresponding to the month when the maximum gravity of the seasonal amplitude was observed in the three basins considered and the outline of the caps used for the localized spectral analysis. Fields calculated at 250 km altitude. (**d**) Spectra of seasonal amplitude for 3 South American hydrologic basins (black lines). Monthly error curves of MOCAST+ (blue) and GRACE (red). Spectra calculated at 250 km altitude. which is also in phase with the Amazon. The Orinoco on the other side has an important gravity signal, as large as the Parana, but more localized. It is also not in phase with the seasonal signals of the other two basins. Considering the different areal extensions of the anomalies, we opted for caps with different radii for the computation of the localized spectra, in particular, the radii of \(15^{\circ}\), \(10^{\circ}\), and \(7^{\circ}\), for the Amazon, Parana, and Orinoco basins, respectively. The contribution of MOCAST+ is again its ability to recover the anomalies at a higher spatial resolution. The improvement is, as expected, more pronounced for the smaller basins, such as the Orinoco and Parana, compared to the Amazon.