Abstract
Abstract. Current soil erosion and reservoir sedimentation modelling at the meso-scale is still faced with intrinsic problems with regard to open scaling questions, data demand, computational efficiency and deficient implementations of retention and re-mobilisation processes for the river and reservoir networks. To overcome some limitations of current modelling approaches, the semi-process-based, spatially semi-distributed modelling framework WASA-SED (Vers. 1) was developed for water and sediment transport in large dryland catchments. The WASA-SED model simulates the runoff and erosion processes at the hillslope scale, the transport and retention processes of suspended and bedload fluxes in the river reaches and the retention and remobilisation processes of sediments in reservoirs. The modelling tool enables the evaluation of management options both for sustainable land-use change scenarios to reduce erosion in the headwater catchments as well as adequate reservoir management options to lessen sedimentation in large reservoirs and reservoir networks. The model concept, its spatial discretisation scheme and the numerical components of the hillslope, river and reservoir processes are described and a model application for the meso-scale dryland catchment Isábena in the Spanish Pre-Pyrenees (445 km2) is presented to demonstrate the capabilities, strengths and limits of the model framework. The example application showed that the model was able to reproduce runoff and sediment transport dynamics of highly erodible headwater badlands, the transient storage of sediments in the dryland river system, the bed elevation changes of the 93 hm3 Barasona reservoir due to sedimentation as well as the life expectancy of the reservoir under different management options.
Highlights
In drylands, water availability often relies on the retention of river runoff in artificial lakes and reservoirs
The complexity of such models varies with the detail of spatial and temporal process representation, ranging from models representing hillslope processes for individual storm events or seasons, e.g. the WEPP model by Nearing et al (1989) or EROSION-2D (Schmidt, 1991), to models designed for the catchment scale, up to large catchment scale models that model water and sediment fluxes for entire basins and longer time periods such as SWRRB (Arnold et al, 1989), SWIM (Krysanova et al, 2000), LASCAM
Both types of models fail to enable the quantification of sediment transfer from erosion hotspots of erosion, i.e. small hillslope segments that contribute a vast amount to the total sediment export out of a catchment but at the same time cover only a rather small part of the total area, such as badland hillslopes or highly degraded slopes which are often found in dryland settings (e.g. Gallart et al, 2002)
Summary
Water availability often relies on the retention of river runoff in artificial lakes and reservoirs. Considering the potential impacts of changing climatic or physiographic boundary conditions on water availability and reservoir sedimentation, numerical modelling tools can help to explain and predict possible future changes to water and sediment dynamics of large river basins For this purpose, a wide range of erosion and sediment transport models has been developed for the micro- to macro-scale over the last decades. Grid-based models such as the LISEM model (Jetten, 2002) may incorporate a higher degree of spatial information, but are often limited in their applicability due to computing time (for small grid sizes) and lack of exhaustive spatial data, which makes their application at the meso-scale inappropriate Both types of models fail to enable the quantification of sediment transfer from erosion hotspots of erosion, i.e. small hillslope segments that contribute a vast amount to the total sediment export out of a catchment but at the same time cover only a rather small part of the total area, such as badland hillslopes or highly degraded slopes which are often found in dryland settings A model application is evaluated for the Isabena catchment (445 km2) in the Pre-Pyrenees, simulating and discussing model performance and its limitations for badland hotspot erosion, transient storage of sediment in the riverbed, bed elevation change in the reservoir and management options for different life expectancies of a large reservoir
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