Hydrological connectivity via gully formation in tropical watersheds limits the effectiveness of riparian buffers in protecting streams

We tested the hypotheses that increased hydrological connectivity is associated with gully formation due to sugarcane agriculture, and, that this in turn, compromises the effectiveness of stream riparian buffers at protecting streams. Based largely on results from temperate regions, national-level programs to restore riparian buffers have been promoted in tropical countries like Brazil. The assumption is that the buffers would mitigate the impacts of agriculture on stream hydrology and water quality. We show however that the combination of intensive agriculture and heavy seasonal rainfall (which is characteristic of many tropical regions) causes gully formation thereby expanding the drainage network and increasing the hydrological connectivity between streams and their uplands. The result is that excess runoff and associated materials from croplands and from the gullies themselves can enter stream channels directly without interaction with a riparian buffer. Focusing on 11 first-order streams in Brazil, we quantified the relationship between a suite of landscape metrics, the hydrologic response to storm events (i.e., stream flashiness) and changes in suspended sediment concentrations. We included novel metrics describing the hydrological connectivity via gully. We demonstrate that streams more hydrologically connected to their uplands via gullies were flashier and had greater suspended sediment concentrations. Based on the quantitative relationships we found, we propose alternatives to current management practices.