Dynamic modelling for linear erosion initiation and development under climate and land-use changes in northern Laos

Linear erosion (LE) induced either by piping or overland flow is one of the most active factors in the evolution of soils. During single storm events LE may remove enormous amounts of soil material from the uplands to the bottomlands and has thus become a broad challenge for food supply, food security, and human health. Recent and rapid changes in land-use and climate patterns in the sloping lands of tropical areas may dramatically increase LE. Our main objective was to investigate to what extent one could use direct flow velocity estimations from dynamic models for predicting LE initiation and development at the event level. The second objective was to estimate the impact of expected land-use and climate changes on LE. The study was conducted in the 0.62 km² watershed of northern Laos presented inChaplot et al. (2005). Field observations of the formation and the development of LE features throughout 2001 were compared to flow velocity estimations from an existing surface water routing algorithm developed at Utrecht University ([De Roo, A.P.J., Wesseling, C.G. and Ritsema, C.J. 1996. LISEM: a single event physically based hydrologic and soil erosion model for drainage basins. I: theory, input and output. hydrological processes 10 (8): 1107–1117.]). In 2001, two main rainfall events were responsible for the formation or development of 14 linear features with a total length of 972 m and an erosion rate of 3.5 Mg ha^{− 1}. The water routing algorithm was calibrated using the water and the sediment hydrographs observed at the watershed outlet during the first rainfall event. Assuming realistic estimations of flow velocity in hillslopes, a threshold of 0.062 m s^{− 1} for linear erosion estimated over 10-m cells was defined. This threshold, validated using the remaining rainfall event, accurately predicted the length (mean error of estimate of less than 15%) and location of LE features. Using this simulation tool, an increase of the percentage of land under cultivation from 9% to 100% resulted in 600% increase in linear erosion. The tested scenarios of climate changes had less impact on linear erosion.