## 3 Result
### Sensitivity to Lake Hydrology in Tibet  
We considered the long-period gravity trends caused by a systematic water mass increase in the lakes over the entire Tibetan plateau. In order to test the sensitivity of the MOCAST+ mission, we considered two models; one model includes only the 14 lakes for which we have a DAHITI time-series at our disposal. The other model includes the effect of all the lakes of the plateau; as detailed previously, the effect was estimated by extrapolating an average lake level rate derived from the DAHITI observations to all the remaining lakes. In the following Figure 13a,b, we report the two models considered. The colour scale is the same for both plots, so that the potential important contributions--in terms of the gravity signal--given by all the unmonitored lakes appear clearly.  
For the known lakes, we report the localized spectra for Qinghai Lake and for a cluster of lakes in the Central Tibet. For this central area, two window caps were considered, one centred on the largest anomaly and the other, of greater size, which encloses the entire central Tibetan region.  
Considering the yearly solutions, all the yearly gravity signals were below the noise level of both MOCAST+ mission and GRACE. Therefore, we rescaled the curves considering the expected errors of a two-year solution and the trend signal accumulated after two years  
Figure 12: Gravity effects for a selection of South American Reservoirs/Lakes calculated at 250 km and their respective spectra: (**a**) Long-period trend gravity signals; (**b**) Yearly seasonal amplitude signals. The dashed outlines mark the various spherical caps considered. (**c**) Spectra of long-period yearly gravity rates for 3 South American lakes/reservoirs (black lines), 1-year error curves of MOCAST+ (blue) and GRACE (red). Rates expressed as the gravity change accumulated in one year. Spectra of fields calculated at 250 km altitude. (**d**) Spectra of seasonal amplitude for 3 Lakes/reservoirs in South America (black lines). Monthly error curves of MOCAST+ (blue) and GRACE (red). Spectra of fields calculated at 250 km altitude.  
(Figure 13c). The anomalies of the Qinghai and the central Tibet plateau (the one with smaller localization cap) after two years are clearly below the sensitivity of GRACE but would be detectable by the MOCAST+ configuration. We remark that in both cases the mass changes of the single lakes are very small and generally less than 1 Gt/yr. However, the spectral analysis proves that a long-period monitoring of these subsurface storage units is feasible only with mission concepts with noise characteristics at least as adept as MOCAST+. The spectral analysis of the cumulative effect of all lakes in the Tibetan plateau is given in the plot of Figure 13d. We see that when considering this window with a radius of \(8^{\circ}\), only a MOCAST+ mission would be able to detect the yearly long-period trend. To capture a picture at this spatial resolution GRACE requires at least 2 years of observations. GRACE can see only a low-pass filtered anomaly, with a maximal d/o 15 as evident from Figure 13e, where the signal curve is higher than the GRACE noise only for a very wide cap window of an \(18^{\circ}\)radius.