4.3.2 Annual Variation Characteristics in Sediment Discharge during the Flood Season
This section evaluates the changes in sediment discharge during the flood season in the Qinhe River Basin for different periods. Data recorded at the Runcheng Station from 1971 to 2000 were used, and the significance of changes and future trends were assessed using the M-K trend and Hurst index tests. The results from the M-K trend test showed a highly decreasing trend in sediment transportation in the Qinhe River Basin during the flood season, with a Z-value of -2.72 at the 95% confidence level. Furthermore, the discharge of sediments during the flood season was found to be weakly persistent, with an H-value of 0.69 from the Hurst index test.
Figure 10 illustrates that the average sediment discharge during the flood season decreased by more than 79% since the implementation of major water and soil conservation measures in the Qinhe River Basin in the 1970s. The baseline, change I, and change II periods recorded an average sediment discharge of 4.711 × 106, 2.659 × 106, and 0.943 × 106 tons, respectively. Over the same period, the land use in the basin significantly changed. Specifically, the area occupied by grassland decreased in change period I, while areas covered by forest and cultivated lands increased. In change period II, forest and cultivated lands also increased. These changes led to increased vegetation cover, which improved soil and water conservation in the basin. Vegetation stabilizes soil and sand, reduces erosion associated with runoff, and effectively prevents water and soil loss. Moreover, water and soil conservation measures such as the construction of terraced fields and others significantly weakened the ability of runoff to transport soil on slopes in the basin. The NDVI index for the basin increased from 0.56 in change period I to 0.59 in change period II, confirming the increased vegetation cover in the basin.
The construction of water conservancy projects, which began in the 1950s and 1970s, regulated and stored water in the basin, reducing the amount of water and sediment transported to the river channel during the operation of reservoirs. Sediments were predominantly deposited in areas containing reservoirs during the flood season, and this reduced water and soil loss, positively impacting the ecological environment of the basin.