## 6 Summary and Conclusions  
To explore the utility of microwave remote sensing data for runoff modelling and thus drought assessment in Mainland China, we combined the \(M/C\) signal method with the SRI algorithm based on the MEASUREs high-resolution passive microwave remote sensing \(T_{b}\) observation dataset for 79 typical stations in the 9 major basins across Mainland China. Specifically, SRI-3 was calculated based on the simulated data to assess spatially and temporal patterns of droughs over 1996-2016. The main conclusions of this study can be summarized as follows:  
(1) 92.4% and 59.5% of the selected 79 stations got the \(R\) and \(NS\) greater than 0.5 when comparing the simulated and observed runoff. Among the 9 major basins, the \(M/C\) signal method shows the best performance in the IR and the SWR. Meanwhile, the simulated results at the stations such as the Gengzhang, Yangcun, Zhimenda and Pingle, which are located on the Qinghai-Tibet Plateau and Yunnan-Guizhou Plateau, are also quite outstanding. Comparing the simulated and observed monthly runoff processes, it is obvious that the \(M/C\) signal method is far more effective for the dry season than the flood season as the flood peaks of some stations were underestimated. (2) The monthly runoff data obtained by the \(M/C\) signal method for 1996-2016 was applied for calculating the SRI-3. 30 out of the 79 stations indicate significant increasing trends in SRI-3 and 18 show significant decreasing trends. By analyzing the frequency of drought events and the most severe drought events at each station during this 21-year period, the duration and severity of droughs in the non-permanent dry areas of the Hai, the middle reaches of the YZR and the Southwest of China were found to be more frequent and severe than that in other regions. (3) After analyzing 7 topographic and hydro-meteorological influencing factors, elevation, climate type and mean discharge were found to be the dominated factors. Stations over small rivers in plateau areas with sparse vegetation tend to have better simulated results. To be more specific, areas with elevation higher than 1000 m, river mean discharge between 100 and 1500 m\({}^{3}\)/s and a polar climate are defined to be more suitable for applying passive microwave remote sensing data to runoff modelling. Meanwhile, these regions are usually prone to drought, proving that passive microwave remote sensing can be a feasible way for hydrological drought assessment. This study proposed an effective way for hydrological drought assessment based on microwave remote sensing in Mainland China. It can be further applied to studies on all forms of drought together with other multi-source satellite data. This study can provide guidance for other practical hydrological applications, especially in regions of limited ground data. The following are available online at [[https://www.mdpi.com/article/10.3390/w13172429/s1](https://www.mdpi.com/article/10.3390/w13172429/s1)]([https://www.mdpi.com/article/10.3390/w13172429/s1](https://www.mdpi.com/article/10.3390/w13172429/s1)). Figure S1. Box plots of normalized centered root-mean-square error (NCRMSE) and mean relative error (MRE) for the 79 stations; Figure S2. NCRMSE vs. \(R\) and MRE vs. \(R\) for the 79 stations; Figure S3. Comparisons of the standardized runoff index on a 3-month scale (SRI-3) trends, drought event frequency, distribution of drought duration (Dd) and drought severity (Ds) for the 79 stations from 1996-2008 based on simulated and observed runoff, respectively; Table S1. Multiple regression analysis of the influencing factors. Conceptualization, Z.Z. and X.Q. ; methodology, Z.Z. and Z.Y. ; formal analysis, X.Q. and Z.Z. ; resources, J.H. and J.X. ; data curation, J.H. and Y.W. ; validation, J.X. ; supervision, Z.Z. and Z.Y. ; writing--original draft preparation, X.Q. ; writing--review and editing, Z.Z. ; funding acquisition, Z.Z. and Y.W. All authors have read and agreed to the published version of the manuscript.