Effect of naturally restored grassland on the ephemeral gully erosion in the loess hilly and gully region.

Ephemeral gully erosion is an important erosion type in hilly and gully regions of Loess Plateau. While previous studies mainly focused on ephemeral gullies in agricultural land, little is known about the effects of naturally restored grassland on ephemeral gully erosion. In this study, taking the bare ephemeral gullies as the baseline, we conducted in-situ flushing tests to explore runoff and sediment yield characteristics and erosion mechanism of grassland ephemeral gullies under the runoff conditions of 5, 10, 15, 20 and 25 L·min-1. Compared to the bare ephemeral gully, average flow velocity, stable runoff rate, Reynolds number and Froude number of grassland ephe-meral gullies was reduced by 25.4%-67.3%, 8.4%-26.6%, 54.9%-80.5%, 18.6%-65.1%, respectively, whereas resistance coefficient was increased by 0.09-7.18 folds. Compared to the bare ephemeral gully, the maximum sediment yield rate, stable sediment yield rate, average sediment yield rate of grassland ephemeral gullies was decreased by 55.1%-90.9%, 61.8%-95.4%, and 64.8%-92.4%, respectively. The sediment yield reduction benefit of the naturally restored grassland under the discharge flow rate of 5-25 L·min-1 could reach 65.9%-88.8%, which decreased with increasing discharge flow rate. Compared to the bare ephemeral gully, average stream power and average shear stress of grassland ephemeral gullies was reduced by 54.9%-80.5% and 12.4%-51.1%, respectively, whereas the critical stream power and critical shear stress was increased by 1.43 folds and 33.7%, respectively. The average sediment yield of grassland and bare ephemeral gullies was signifi-cantly linearly related to average stream power and shear stress. Naturally restored grassland significantly increased the erosion resistance and reduced runoff erosion potential of ephemeral gullies.