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The success of recent land management efforts to reduce soil erosion in northern France

Frankl, Amaury, Prêtre, Vincent, Nyssen, Jan, Salvador, Pierre-Gil
Geomorphology 2018 v.303 pp. 84-93
aerial photography, cropland, erosion control, gully erosion, land management, landscapes, rain, ravines, runoff, sediment deposition, sediments, temporal variation, vegetation, watersheds, winter, France, Western European region
Soil erosion is an important problem in open-field agricultural landscapes. With almost no permanent vegetation in small headwater catchments, and with few physical obstacles to reduce runoff velocities, runoff concentration along linear landscape elements (plot boundaries) or thalwegs frequently causes ephemeral gullies to form — the latter reflecting the poor hydrogeomorphic condition of the land- and soilscape. To address this problem, and to remediate negative on- and off-site effects, land management efforts have multiplied over the past decades in many regions. This includes, amongst other measures, the implementation of vegetation barriers called ‘fascines’. In the loess-dominated Aa River basin of northern France, where cropland accounts for 67% of the cover, we investigated the effect of fascines on ephemeral gully erosion dynamics, together with rainfall characteristics and cropland management. This was accomplished through a spatially explicit study of 269 sites prone to ephemeral gullying using a diachronic analysis of historical aerial photographs. Between 1947 and 2012, ephemeral gully densities at the scale of the Aa River basin (643km²) varied between 0.39 and 1.31mha⁻¹ (long-term average of 0.68mha⁻¹ (with local maxima up to 9.35mha⁻¹). Densities are, however, much higher when only considering the most erosion-vulnerable municipalities (long-term average of 2.23–4.30mha⁻¹); those values should be used when comparing results from this study to other reports of ephemeral gully erosion. Fascines were introduced in 2001 and were present in ~30% of the gully erosion sites by 2012. Although the presence of fascines has an effect on gully length reduction, spatial and temporal variations in gully length were mainly driven by cumulative precipitation. Measurement of sediment deposition at 29 fascines in 2016 showed that only 47% of the fascines functioned as sediment sinks. They stored on average 1.7Mg of sediment per winter half-year, corresponding to 0.009Mgha⁻¹. The results suggest that fascines positively impact the landscape's resilience and reduce ephemeral gully erosion rates. The use of vegetation barriers such as fascines are increasingly implemented for erosion control in western Europe, but pose problems for the management of open-field landscapes.