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.