# Estimation of Terrestrial Water Storage Changes at Small Basin Scales Based on Multi-Source Data
## 5 Discussion
### Impact of ET and Runoff  
In the process of ET simulation in small basins, we replaced the forcing data used in the PML_V2 model with the CMFD dataset, but the ET calculated by the PML_V2 model was globally calibrated. There was no region-specific calibration parameter and its accuracy may need further improvement in specific regions at small scales. [PERSON] et al. [44] showed that when using the PML_V2 model to simulate ET, due to the forcing data of Collection 6 MCD12 has spectral confusion between evergreen broadleaf forests (EBF) and evergreen needleleaf forests (ENF). It is difficult to distinguish several classes using MODIS coarse resolution data, which may lead to some potential uncertainty in ET estimation. [PERSON] et al. [24] also showed that due to the MODIS algorithm not considering the effects of cloud cover and leaf shadow effects, MODIS-ET is more accurate in the dry season than in the flood season. The impact of ET by using satellite products also needs to be considered in future research.  
We found that the CC between TWSC and SMSC in the sub-basins of the downstream region is generally larger, and the regions with smaller CC are mainly concentrated in the upstream region (Figure 12). It can be seen from Figure 2(a),c that the downstream region is also the area with large precipitation and runoff. We speculate that this phenomenon may be related to the accuracy of the runoff simulation. [PERSON] et al. [67] found that the sensitivity of SWAT model parameters and the accuracy of runoff simulation are significantly different between the periods of high and low precipitation. To address this issue, [PERSON] et al. [68] applied runoff data from periods of low precipitation to calibrate the SWAT model in a Canadian basin dominated by ice melt recharge and improved the simulation accuracy of the model. In the GRB, there are large spatial differences inprecipitation between upstream and downstream regions. However, when we simulated runoff using the SWAT model, the model calibration was carried out for the whole basin due to limited in situ runoff data, which may lead to a higher accuracy of the runoff simulation in the downstream region compared to the upstream region. With more runoff data, we can determine the model parameters from upstream and downstream regions, and further improve the accuracy of the upstream runoff simulation.  
Besides, when the SWAT model was applied to simulate the runoff process at a small basin, due to the lack of data about the groundwater process and aquifer system, especially the natural and artificial water storage systems closely related to hydrological conditions, such as dams and reservoirs, wetlands and ponds, the precision is constrained. [PERSON] et al. [69] also proved that when using the hydrological model to simulate runoff, the shape of a flow duration curve is mainly affected by reservoirs, land-use types, and upstream abstractions of water. The GRB is widely distributed, and several artificial water conservancy facilities such as reservoirs and dams play an important role in the hydrological cycle. If these data are coupled into the model, the accuracy of runoff simulation will be further improved.