## 4 Results
### Hydrological Drought Assessment Based on Simulated Runoff  
In order to assess the hydrological drought for recent decades in China, the time series of runoff for all the stations from 1996 to 2016 were generated using Equation (7) based on microwave \(T_{b}\) data. The SRI-3 for each station was then calculated to analyze hydrological drought in the selected 9 major basins across Mainland China.  
The Mann-Kendall (M-K) test [61; 62] was used to analyze the SRI-3 as shown in Figure 7, presenting the trend of change and significance. According to the analysis, 40 of the 79 stations indicate an increasing trend in SRI-3, with 30 stations showing a significant increase. Conversely, 39 stations present decreasing trends, 18 out of which experienced significant decreases. As shown in Table 2, among the 9 basins, the SRI-3 values of the PR, SWR and HR show primarily increasing trends as these basins were dominated by drought intensification from 1996 to 2016. The SRI-3 of the YZR, SER, Hai, SLR and YR, on the other hand, primarily indicates decreasing trends, showing a slight drought relief in this period. The three stations with the highest rising SRI-3 trends are the Qilinzui station on the Zeng River, a tributary of the Pearl River, the Yunhe station on the Dayun River, a tributary of the Huai River, and the Xiantao station on Han River, a tributary of the Yangtze River.  
Figure 4: (a) Statistics of each category and (b) comparisons between basins for \(R\) and \(NS\) of the 79 stations.  
Figure 5: Spatial distribution of \(R\) and \(NS\) for the 79 stations.  
Figure 6: Observed vs. simulated monthly runoff at typical stations during the calibration and validation period: (**a**) Yambajan Station of Duilong River; (**b**) Gengzhang Station of Niyang River; (**c**) Xidaqiao Station of Aksu River; (**d**) Yangçun Sation of Yarlung Tsanpo River; (**e**) Pingle Station of Gui River; (**f**) Zhimeda Station of Yangtze River.  
Statistics on the frequency of drought events occurring at the 79 stations during 1996-2016 (shown in Figure 8) indicate that, apart from the permanent dry regions (mainly in the IR), the areas with the most frequent occurrence of drought are mainly located in the upper reaches of the PR, the middle and upper reaches of the YZR, the middle reaches of the Yarlung Tsangpo River (in the SWR) and the Hai. Basically, these areas belong to Yunnan Province, Sichuan-Chongqing regions, Lhasa City and Beijing-Tianjin-Hebei region. Furthermore, the largest drought events in 1996-2016 were also identified and their Dd and Ds were calculated, as shown in Figure 9. It can be found that in terms of spatial distribution, Dd and Ds are larger in the upper reaches of the YR, the Tibetan region and some areas in the middle reaches of the YZR, while the drought in the SLR is more severe despite its short duration. In terms of time of occurrence, half of these strongest drought events occurred in the periods 1996-1999 and 2009-2014, which was also proved by the analysis from the China Meteorological Agency (CMA) ([[http://www.cma.gov.cn/en2014/](http://www.cma.gov.cn/en2014/)]([http://www.cma.gov.cn/en2014/](http://www.cma.gov.cn/en2014/)), accessed on 31 August 2021).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Basin** & **Increasing Trend** & **Percentage (\%)** & **Decreasing Trend** & **Percentage (\%)** \\ \hline
**YZR** & 12/25 * & 48.0 & 13/25 & 52.0 \\
**YR** & 3/7 & 37.5 & 5/7 & 62.5 \\
**PR** & 3/4 & 75.0 & 1/4 & 25.0 \\
**SWR** & 9/16 & 56.3 & 7/16 & 43.8 \\
**SER** & 1/3 & 33.3 & 2/3 & 66.7 \\
**HR** & 4/7 & 57.1 & 3/7 & 42.9 \\
**Hai** & 2/5 & 40.0 & 3/5 & 60.0 \\
**SLR** & 2/7 & 28.6 & 5/7 & 71.4 \\
**IR** & 2/4 & 40.0 & 2/4 & 40.0 \\ \hline \hline \end{tabular} Note: * The number of stations with the SRI-3 of the corresponding trend/the total number of stations in the corresponding basin.  
\end{table}
Table 2: Statistics of SRI-3 trends for the selected 79 stations.  
Figure 7: Standardized runoff index on a 3-month scale (SRI-3) trends for 79 stations from 1996–2016.