Evaluating the flow and sediment effects of gully land consolidation on the Loess Plateau, China

Gully land consolidation (GLC) is a slope-gully reclamation program that was implemented after "Grain for Green" project on the Loess Plateau, China. Through the GLC project, the Chinese government planned to increase the area of cropland by 337.80 km(2) from 2013 to 2017. To quantify the impact of GLC on runoff and sediment transport, the coupled SWAT (Soil and Water Assessment Tool) model, which was integrated with a process-based terrace algorithm, was employed in this research. The results showed that the improved SWAT model was proven to be a powerful tool for detecting the effect of GLC on flow and sediment discharge. The effect of GLC on flow and sediment loading was concentrated in the flooding period, its runoff interception efficiency was 169,350 m(3) /(km(2).yr), and sediment trap efficiency was 3979 t/(km(2).yr), showing that it had good flow and sediment reduction benefits. However, the GLC project drastically altered the water and sediment flux equilibrium, and excessive expansion was likely to lose parts of riparian-ecosystem functions of the downstream river. In view of this, we integrated the improved SWAT (embedded in the terrace module), double-mass curve and Pettitt test to identify the recommended ratio of GLC area to the total area in the Yanhe watershed. The results demonstrated that if the reclaimed gully area accounted for 0.8% (61 km(2)) of the total area (7608 km(2)), the runoff abruptly changed. Therefore, the reclaimed gully area should remain within 0.8% of the total area in the Yanhe watershed, if so, GLC not only alleviates the contradiction between agricultural development and ecological restoration, but also gives full play to its functions of reducing flood and restricting sediment transport.