Background Gully erosion is the primary sediment source in the loess hilly-gully region of China. It is essential to systematically examine the gully morphological characteristics under different flow discharge conditions, which hold important implications for the understanding of gully erosion mechanisms, the establishment and improvement of gully erosion prediction models, as well as the development of soil and water conservation practices and policies.

Methods In the current study, a total of 10 fallow experimental plots with a length of 8 m, a width of 0.7 m, and a slope gradient of 25º, were constructed on typical slopes in the loess hilly-gully region. A field-scouring experiment was carried out in situ, at five different discharge rates of 0.25, 0.50, 0.75, 1.00 and 1.50 L/s. When the gully developed in each plot achieved its equilibrium or stable status, the scouring experiment was terminated and the structure from motion (SfM) photogrammetry was applied to measure the soil surface, and to derive the gully DEM and digital orthophoto map (DOM). Ten morphological parameters of each gully, i.e., length, maximum and mean surface width, maximum and mean depth, perimeter, area, volume, width-depth ratio and perimeter-area ratio were extracted, and their quantitative relationships with the flow discharge rate were analyzed.

Results 1) The scouring period required for each gully to achieve its equilibrium or stable status ranged from 69 to 168 min. With the increase of the discharge rate, in addition, this scouring period decreased in general. 2) The overall shape of each gully varied with the discharge rate. When the rate was 0.50 L/s or lower, several branches developed on the upper portion of the experimental plot merged into one or two main gullies downslope. Whereas at the discharge rate equal to or exceeding 0.75 L/s, a connected stripe-like gully through the plot was resulted. 3) Applying different flow discharges, the gully mean surface width ranged from 0.16 to 0.29 m, and the gully area from 1.30 to 2.49 m², both of which increased with the discharge rate significantly. The corresponding relationships of the gully mean surface width and area with the discharge rate, moreover, were both best fitted with the power functions, and the power exponents were 0.406 and 0.416, respectively. In contrast, the gully perimeter-area ratio, i.e., falling between 8.42 and 26.22, decreased logarithmically with the discharge rate significantly.

Conclusions These finding can provide valuable reference for the gully erosion prediction and the comprehensive control of soil and water loss in the loess hilly-gully region, as well as in other similar regions.