The application of semi-quantitative methods and reservoir sedimentation rates for the prediction of basin sediment yield in Spain

Traditionally, the problems in predicting sediment yield at the basin scale are related to model's high data requirements, the focus on a limited amount of erosion and sediment transport processes and a general lack of systems knowledge. Here, we explore the performance of semi-quantitative approaches to predict mean annual area-specific sediment yield (SSY) at the scale of small to medium-sized basins. The Factorial Scoring Model (FSM) [Verstraeten, G., Poesen, J., de Vente, J., Koninckx, X., 2003. Sediment yield variability in Spain: a quantitative and semiqualitative analysis using reservoir sedimentation rates. Geomorphology 50 (4), 327-348], the PSIAC model [PSIAC, 1968. Pacific Southwest Inter-Agency Committee. Report of the water management subcommittee on factors affecting sediment yield in the Pacific southwest area and selection and evaluation of measures for reduction of erosion and sediment yield] and some variations of these models were applied to predict SSY for the 60 reservoirs, using the reservoir sedimentation rates as published by Avendano Salas et al. [Avendano Salas, C., Sanz Montero, E., Gomez Montana, J.L., 1997. Sediment yield at Spanish reservoirs and its relationship with the drainage basin area. Dix-neuvieme Congres des Grands Barrages. Commission Internationale De Grands Barrages, Florence, pp. 863-874]. The FSM explained 72% of the variability found in reservoir sedimentation rates, which is significantly better than estimates based on basin area alone, explaining only 22%. The FSM uses five factors (i.e. topography, lithology, vegetation cover, gullies, basin shape) to characterise a drainage basin in the vicinity (<5 km) of the reservoir and the main tributaries feeding the reservoir, by providing a score. An index is calculated by multiplying the scores per factor. Together with basin area, this index is used to predict SSY. The PSIAC model works in a comparable way, though some other factors are used (i.e. topography, climate, runoff, lithology, soil, vegetation, land use, upland erosion, channel erosion), basin area is not part of the relation and the index is calculated by summation of all scores. PSIAC explained 58% of the variation in SSY. Advantages of both models over physically based and other empirical models are that less data are required, gully erosion is included and the importance of connectivity in the landscape is recognised. This study thus demonstrates that semi-quantitative approaches can provide fairly accurate and reliable estimates of SSY at the scale of small to medium sized basins, with relatively low data requirements.