# Space-Ground Remote Sensor Network for Monitoring Suspended Sediments in the Yellow River Basin
## 3 Results and Analysis
### Hyperspectral Monitoring of Water and Sediment in the Qingshui River
The blue lines represent the ground footprints of the image distribution, the yellow line indicates the spatial extent of Ningxia, and the red lines mark the roads in the base map. “2024-06-05” represents the date 5 June 2024, and the same applies to the other figures. through discrete on-site manual sampling (as shown in Figure 5). The discrepancy between some of the estimated SSC values and the actual values is primarily due to errors in the SSC retrieval model, measurement errors in the actual SSC, and the time discrepancy between the collection of spectral data and water samples. Based on the times of several high-concentration sediment peaks detected by the spectral station, such as on 26 August 2022, 9 September 2022, and 20 September 2022, these are consistent with the precipitation times during the same period in the region, indicating that local precipitation in the Qingshui River basin can lead to rapid responses and changes in sediment concentration. Satellite Remote Sensing for Water and Sediment Regulation Before the 2023 Flood Season of the Yellow River  
The majority of sediment in the middle and lower reaches of the Yellow River comes from the Loess Plateau. The accumulation of sediment poses a significant risk, and intermediate reservoirs such as Xiaolangdi face ongoing sediment deposition issues. From the implementation of water and sediment regulations in 2002 until all the water and sediment reaching the sea by 2023, approximately 3.29 billion tons of sediment have been transported to the sea. The average depth of the main channel downstream has decreased by 3.10 m, and the minimum flow capacity of the main channel has increased from 1800 m\({}^{3}\)/s before the flood of 2002 to around 5000 m\({}^{3}\)/s now, reducing the likelihood of mid to small floodplain inundation downstream. Over the 22 years of water and sediment regulation, Xiaolangdi reservoir has discharged a cumulative total of 2.705 billion tons of sediment. The water colors in the Yellow River varied sharply in different seasons and different phases of water and sediment regulation (as shown in Figure 6). In Figures 7 and 8, based on high-resolution satellite remote sensing images, the water and sediment changes in the Xiaolangdi reservoir and the Yellow River estuary during the water and sediment regulation period before the 2023 flood are displayed. This water and sediment regulation started on 21 June and concluded on 11 July at 8 a.m. through coordinated water release from reservoirs such as Wanjiazhai, Sanmencia, and Xiaolangdi to implement the Yellow River water and sediment regulation before the 2023 flood, with all the water and sediment reaching the sea by around July 16. The Xiaolangdi and Sanmencia reservoirs discharged 125 million tons and 40 million tons of sediment, respectively, with the volume of water reaching the sea and the volume of sediment reaching the sea being 4.259 billion cubic meters and 27 million tons, respectively. As shown in Figure 7a,b, during the clear water discharge phase (Figure 6a), the sediment concentration in the water released from Xiaolangdi reservoir was low. On 4 July at 22:00, sediment discharge began from Xiaolangdi reservoir (Figure 6c), and, by 10:30 a.m. on 7 July, the maximum sediment concentration reached 417 g/L. Figure 7d,e shows extremely high sediment levels in the downstream channel of Xiaolangdi on 8 and 9 July. Figure 5: Time series of suspended sediments concentrations at the Wangtuan hyperspectral station: 26 August 2022 to 26 September 2022. The red circles are the actual SSC measured by discrete in situ water sampling; The blue lines are the SSC retrieved from the hyperspectral reflectance in the continuous measurements. Figure 8: The changes in the water and sediment status of the Yellow River estuary and adjacent water bodies during the water and sediment regulation period of the Yellow River before the flood in 2023 (from 21 June 2023 to 11 July 2023).