Analyzing gully evolution in a rolling landscape over seven decades in SW Europe. Lessons for the future

The study presented in this communication evaluates the evolution of gully erosion over seven decades in the western Campiña of the Guadalquivir River Basin in southern Spain. Gully erosion, a major driver of soil degradation, seems to be intensified in this region in last years. The research utilizes photointerpretation techniques to track the development of gullies from 1956 to 2022, correlating this with rainfall data, land use changes, and the hydrological responses of watersheds. Nine gully systems were selected, and their areas were digitized from historical orthophotos, with volumetric erosion estimates made for select periods using available digital elevation models (DEMs).

The results show that gully development accelerated in response to a combination of factors, particularly the shift from herbaceous crops to olive groves. As olive cultivation expanded and ground cover was reduced, soil became more susceptible to erosion. This was especially evident after high-intensity rainfall events, such as those between 2009 and 2011, which caused significant gully growth. At the end of the study period, approximately 6% of the first-order catchments analyzed were occupied by gullies.

The average erosion rates recorded in this study (47 tons per hectare per year) were consistent with those reported in other Mediterranean regions, and showed a pronounced upward trend, with peak erosion rates reaching 282 tons per hectare per year during the 2008-2010 period. This increase is attributed to more frequent extreme rainfall events, as well as changes in land use. Furthermore, a comparison of gully morphology with global data suggests that the gullies in this region are shallower for a given width, possibly due to the low stability of the soils to lateral collapse.

A key finding is the role of human interventions in gully systems, such as partial filling, contour reshaping, and land leveling, which frequently modified gully erosion rates. These activities result in the underestimation of the volume eroded by up to 28%, with some particular cases showing deviations as high as 393%. This highlights the need for adjusting the temporal scale of monitoring gully erosion to capture relevant interventions, or alternatively inform gully erosion rates with detailed information on the land management between study periods.  

The study concludes by recommending that the design of hydraulic structures for gully control be reevaluated in light of the increasing intensity and frequency of rainfall events with high return periods. Additionally, sustainable land use practices should be implemented from the outset to mitigate gully formation, for example, at the time of establishing a new crop or when making a change in land use. Finally, an empirical expression between contributing area and peak flow for different return periods is stablished to facilitate the implementation of control measures in a region.

Acknowledgements:

This work has been possible thanks to the contribution of the Postdoctoral fellowship (POSTDOC_21_00342) from the Andalusian Plan for Research, Development and Innovation (PAIDI 2020) and the project TUdi (GA 101000224) from European Union's Horizon 2020 research program.