Application of an index of sediment connectivity in a lowland area

Sediment connectivity at the landscape scale has gained interest in the last few decades. Distributed approaches, such as topographic indices, are widely used to evaluate this connectivity. However, most of the research efforts are concentrated in mountainous areas while little work has been done in lowland areas where evidence of high connectivity has been reported. The objectives of this study are as follows: (i) to integrate landscape infiltration/runoff properties in the assessment of connectivity to account for lowland processes and (ii) to apply this approach to a large territory with both mountainous and lowland areas. The topographic index of connectivity (IC) of Borselli et al. (2008) was computed for the Loire-Brittany River Basin (> 10(5) km(2)). A distributed parameter (IDPR) that reflects landscape infiltration and saturation properties due to underlying geological formation characteristics is introduced. We integrated this parameter in a revised index (IC (revised) ) as an indicator of landscape hydrologic connectivity. Results at the pixel scale are aggregated at the watershed scale. Two maps of connectivity are produced, considering the initial IC and the revised form (IC (revised) ). As expected, the IC gives the highest connectivity in the steepest areas and does not reflect the existing connectivity in lowland areas. On the contrary, the IC (revised) computed in this study profoundly modifies the sediment connectivity values. These changes are evenly distributed over the entire territory and affected 51.5 % of the watersheds. As a result, we obtained a better correlation between calculated connectivity and the observed drainage density (which reflects the actual connections between hillslopes and rivers) in areas where slopes are gentle (< 7 %). Topographic indices do not reflect the real sediment connectivity in lowland areas, but their adaptation by considering runoff processes of such areas is possible. The IC (revised) presents an interesting perspective to define other highly connected areas at the country scale, as 17 % of the French territory is characterized by very gentle slopes with high runoff capacity.