Effects of agricultural drainage systems on sediment connectivity in a small Mediterranean lowland catchment

Abstract

Traditional drainage systems combining man-made channels and subsurface tile drains have been used since Roman times to control water excess, favouring adequate soil water regime for agriculture purposes. However, mechanization of agriculture, abandonment or land use changes lead to a progressive deterioration of these drains in the last decades. The effects of these structures on hydrological and sediment dynamics have been previously analyzed in a small Mediterranean lowland catchment (Can Revull, Mallorca; Spain) by establishing an integrated sediment budget with a multi-technique approach. Moreover, the recent advances in morphometric techniques enable the completion of this analysis by the accurate identification of active areas (i.e. sources, pathway links, and sinks) and improve the understanding of (de-)coupling mechanisms of water and sediment linkages. In this study, an index of connectivity (IC) derived from a LiDAR-based high resolution DTM (> 1 pt m-2; RMSE < 0.2 m) was used to evaluate the spatial patterns of connectivity of the catchment under two different scenarios: (1) the current scenario, including an accurate representation of the channels (CS - Channelled Scenario), and (2) a hypothetical scenario in which artificial channels were removed (US - Unchannelled Scenario). Design and configuration of the drainage system in Can Revull generated changes, favoring lateral decoupling between different compartments, with hillslopes-floodplain and floodplain-channels relationships showing a general decrease of IC values, and high longitudinal connectivity along the artificial channel network. Field observations corroborated these results: structures enabled rapid drainage of the water excess also promoting low surface runoff within the field crops, proving to be an effective management practice for erosion control in agricultural Mediterranean lowland catchments. By contrast, US demonstrated that the abandonment of agricultural practices and the subsequent destruction of the drainage system could lead to its lower efficiency increasing soil losses by erosion.