# Estimation of Terrestrial Water Storage Changes at Small Basin Scales Based on Multi-Source Data
## 6 Conclusions  
In this study, we took the GRB as the study area and presented a new method for calculating TWSC at small-basin scales. First, we divided the GRB into 172 sub-basins based on the slope runoff simulation algorithm. Then, we simulated the ET and runoff of each sub-basin with the PML_V2 and SWAT. Finally, through the water balance equation, the TWSC of each sub-basin was obtained. We evaluated its effectiveness by comparing the estimated TWSC against the CSRM, in situ groundwater level data, and SMSC. We also analyzed the temporal and spatial variations in precipitation, ET, runoff, and TWSC in the GRB from 2002 to 2018. From the estimated results and spatio-temporal analyses, we can draw the following conclusions. The results of this research demonstrate that compared with the in situ data, the simulated runoff is reliable, and the proposed method is capable of obtaining the TWSC at small scales. The TWSC calculated by our method agrees well with the CSRM, in situ groundwater level data, and SMSC. Compared with the CSRM and reconstructed natural monthly TWSC, the RMSE is 37.1 mm/month and 21.9 mm/month, respectively. By comparing with in situ groundwater level data, and SMSC, it shows higher CC. These results indicate that the proposed method makes sense and is beneficial. The spatial and temporal variations in precipitation and runoff in the GRB are similar, with the minimum value in 2011 and the maximum value in 2016, and the high-value area is mainly concentrated in the downstream area. The annual ET changes gently, with the minimum value in 2007 and the maximum value in 2004. The TWSC fluctuates greatly in the study period, with the minimum value in 2003 and the maximum value in 2015, and the spatial distribution is uniform. From this study, the accuracy of the evaluation method still needs to be further improved because of the lack of accurate and comparable TWSC observations. We used the GRACE product, the groundwater monitoring level observations, and soil moisture storage to evaluate the method's accuracy. However, this evaluation method is only qualitative and not quantitative. Nevertheless, we could conclude that in small basins, our method provides reliable data that are suitable for the study of small-basin TWSC. In future work, the quality of the TWSC could be improved by using more accurate runoff and ET simulations. Conceptualization, Y.Z., M.W., Q.L. and X.L. ; methodology, Y.Z., M.W., Q.L. and S.Z. ; data analysis, Y.Z. and Q.L. ; validation, Q.L. ; writing--original draft preparation, Q.L., Y.Z. ; writing--review and editing, Y.Z. ; project administration, Y.Z., M.W. and X.L. ; funding acquisition, Y.Z., M.W. and X.L. All authors have read and agreed to the published version of the manuscript. **Funding:** This research is funded by the Funds of China Geological Survey (No. DD 20190329), National Natural Science Foundation of China (No. 42004073, No. 41974019, and No. 41874091); Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. 26420190050-CUGL190805); and Open Fund of State Key Laboratory of Remote Sensing Science (No.OPSLRSS20107). **Data Availability Statement:** The in situ data used in this study were mainly provided by the institution introduced in Section 2.2.1. The China meteorological forcing dataset (CMFD) is available at ([[http://data.tpdc.ac.cn/en/data/8028b944-daaa-451-8769-965612652c49/](http://data.tpdc.ac.cn/en/data/8028b944-daaa-451-8769-965612652c49/)]([http://data.tpdc.ac.cn/en/data/8028b944-daaa-451-8769-965612652c49/](http://data.tpdc.ac.cn/en/data/8028b944-daaa-451-8769-965612652c49/)); accessed on 13 August 2021). CMA Land Data Assimilation System Version 2.0 (CLDAS-V2.0) is available at ([[http://data.cma.cn/data/detail/dataCode/NAFF_CLDAS2_0_NRT/](http://data.cma.cn/data/detail/dataCode/NAFF_CLDAS2_0_NRT/)]([http://data.cma.cn/data/detail/dataCode/NAFF_CLDAS2_0_NRT/](http://data.cma.cn/data/detail/dataCode/NAFF_CLDAS2_0_NRT/)); accessed on 12 August 2021).