## 2 Study Area and Datasets
### Sentinel-1 Data  
Sentinel-1 is a two-satellite constellation carrying C-band (\(\lambda\) = 5.6 cm) SAR instruments to ensure data continuity that began with the ERS and Envisat satellites [42]. Sentinel-1A was launched on 3 April 2014, operating in a near-polar, sun-synchronous orbit with a 12-day repeat cycle and 175 orbits per cycle. Then, Sentinel-1B, sharing the same orbit plane with a 180\({}^{\circ}\) orbital phasing difference, was launched on 25 April 2016. Currently, with both satellites operating, an ideally six days revisit time can be achieved in most of the regions [43]. Sentinel-1 has four exclusive acquisition modes: Strip Map (SM), Interferometric Wide swath (IW), Extra Wide swath (EW), and Wave (WV) modes. Over land, the Sentinel-1 SAR instruments, with the exception of emergency situations, operate in IWmode by default, which provides a wide coverage of 250 km with a spatial resolution of 5 \(\times\) 20 m in range and azimuth directions, respectively [43]. The Sentinel-1 mission provides open-access data to users with a very low latency. All data can be freely downloaded through the Copernicus Open Access Hub [44]. A total of 49 Sentinel-1A/1B SAR images were selected over the study area from April 2015 to December 2016. These images were acquired in IW mode at both VV and VH (vertical transmit and horizontal receive) polarizations. Moreover, all of the images are in ascending pass (track 40), with incidence angle ranging from 30.8\({}^{\circ}\) to 45.9\({}^{\circ}\). Level-1 Ground Range Detected (GRD) product, which consists of focused SAR data that has been detected, multi-looked (5 \(\times\) 1 looks), and projected to ground range using an Earth ellipsoid model, is used in this study. The spatial resolution of high-resolution GRD product is about 20 \(\times\) 22 m, and the corresponding pixel spacing is 10 \(\times\) 10 m in range and azimuth directions, respectively [43]. Although Sentinel-1 SAR sensors are assumed to acquire images day and night regardless of weather conditions, it is sometimes hard for C-band signals to penetrate through thick clouds or severe storms. Therefore, two images acquired on 17 June 2015 and 12 February 2016 were removed, both of which were partially affected by the bad weather. Table 1 lists the acquisition dates, the gaps between consecutive images (\(\Delta\)days) and the sensor types (A or B) of the Sentinel-1 images used. Preprocessing of the Sentinel-1A/1B images were performed with the open source SNAP software (version 4.0) [45], as provided by ESA, which included a collection of Sentinel-1 specific processing tools. The workflow was divided into several steps. First, precise orbit files were downloaded and were applied to the raw images to obtain accurate satellite position and velocity information. Then, the adjacent slices of the same days were put together using the S-1 Slice Assembly tool. Second, radiometric calibration was performed to convert digital pixel values into backscattering coefficients. Third, to reduce the inherent speckle noise of SAR images, both multi-looking processing (2 \(\times\) 2 looks) and speckle filtering (Refined Lee filter) [46] were conducted. Fourth, Range-Doppler terrain correction was done with the Shuttle Radar Topography Mission (SRTM) 3 arc-seconds data [47] to correct  
Figure 1: (**a**) Location of the study area and Poyang Lake basin. The lake connects to the Yangtze River at Hukou. The red triangles represent the stream flow stations of five main tributaries, and the yellow polygon shows the boundary of Poyang Lake ungauged zone; (**b**) The preprocessed VV polarized Sentinel-1A image acquired on 18 April 2015, overlaid with the mean surface soil moisture measurements (0–10 cm) of 15 ground monitoring sites. for geometric distortions, producing orthorectified images with 20 \(\times\) 20 m resolution in Universal Transverse Mercator (UTM) projection (Zone 50 N). Fifth, the time series images were co-registered to a master image (18 April 2015) in the Stack tool, outputting the minimum extent of all images.