Ecological Restoration Declined the Rainstorm—Flood Relationship for past 40 years on Chinese Loess Plateau

The Loess Plateau has been experiencing large-scale ecological construction over the past  40 years. The watershed of Zhou River (1334 km² ) in the core land of the Loess Plateau was chosen as a case study to understand the impact of land use and cover change on the relationship between rainstorm and flood. The land use of the watershed in 1985, 2000 and 2015 was extracted from corresponding Landsat images and the series of event data from 1980 to 2019 of rainfall and runoff were collected from seven rain measuring stations and one gauging station (774 km²) distributed in the watershed. he different magnitude of rainstorm-flood was classified to investigate its responses of ecological restoration by mathematical statistics method. The results showed that the land use conversions were mainly from cropland to forest and grassland, and from grassland to woodland in the time. The proportion of forest and grassland increased from 60.5% in 1985 to 72.1% in 2000 and reached as 85.3% in 2015. While in the meantime, the rainstorm frequency and magnitude of quantity had no obvious trend, however, the annual maximum flood peak discharge in the watershed showed a significantly downward trend, and a change point occurred in 2003. Also the flood magnitude and frequency decreased significantly. Under the same peak discharge, the flooding lag time and runoff coefficient showed an obviously increasing trend compared to that before 2003. And after 2003, most of the floods turned to be once every two years (peak discharge of 169 m³/s) after that year. For floods greater than 10 year-return period, the runoff coefficient is mainly affected by forest land increasing, and the contribution rate is 0~78%. For floods less than 10 year-return period, the runoff coefficient is mainly affected both by the increase of forest land and grassland area, and their contribution rates are 0~57% and 11%~65%, respectively. Outcomes from this study are helpful to understand the shifts of the hydrological process with ecological restoration and assist the sustainable catchment management and land use planning on the Loess Plateau.