Effects of gully control measures on sediment yield and connectivity in wooded rangelands

Gully rehabilitation is often applied as part of catchment management strategies aimed at reducing downstream sediment yields. However, the influences of gully control measures on the runoff and sediment transport processes in agroforestry systems have been seldom studied. In this paper, a thorough analysis of these processes was carried out in a valley-bottom gully located in a dehesa from SW Spain. The gully was monitored before and after implementation of different runoff and sediment control measures that included: gabion check-dams, brushwood check-dams and livestock exclusion through fencing. The aims of this work are: (1) to analyze the effect of the gully control measures on the hydrological dynamics and sediment load, and (2) to evaluate their effect on sediment connectivity at the catchment and channel scales. Changes in topography and connectivity were estimated using sequential Digital Elevation Models (DEMs) with a resolution of 0.02 m generated by Structure - from - Motion photogrammetry from aerial images acquired by an Unmanned Aerial Vehicle. Discharge and suspended sediment were monitored at the outlet. Results indicate that flood discharge was not influenced by the control measures, but suspended sediment concentration was reduced by 65%. The integration of the difference of connectivity index with topographic change maps highlighted the impact of the gully control measures on changes in sediment connectivity. A strong relationship between geomorphic dynamics and the spatial pattern of hydrological and sediment pathways were observed in the gully. The connectivity index (IC) increased in eroded areas, while deposition sites showed a decrease in the IC. Connectivity also decreased in the bank headcuts located within an area isolated from livestock. The implementation of runoff and sediment control measures in the channel was successful in stabilizing the expansion of the channel network and had beneficial effects in the short-term but further monitoring would be necessary to understand long-term effects.