- Previous sedimentation studies
The relevance of the alpine area and the Barasona reservoir with its management problems have generated an interest in gaining a greater knowledge of the role of headwater catchments as suppliers of water to lowlands together with the need to assess erosion rates. Different authors have carried out studies in the area to assess the specific sediment yield contribution to the reservoir. Fargas et al. (1997) proposed and validated a methodology using basic terrain information in a GIS environment and concluded that approximately 25% of the area shows a high risk of sediment emission and identified the central part of the catchment (Internal Depressions and Intermediate Depression) as the main contributing area. Martinez-Casasnovas and Poch (1998) measured the badland erosion rates in the central part of the catchment and estimated the sediment supply to the drainage system that was finally transported to the Barasona reservoir. Similar trends have been reported using fallout caesium-137 to assess erosion rates and patterns in the central Ebro Basin (Navas et al. 1997). Using bathymetric techniques Avedaño-Salas et al. (1997) estimated the sediment yield of the Barasona catchment to be 437500 t year-1. Valero-Garcés et al. (1999) and Navas et al. (2004) reconstructed the depositional history of the Barasona reservoir based on a 137Cs-derived chronology and identified similar potential areas of high sediment yield risk in the catchment by mineral characteristics. Mamede et al. (2006) examined the sedimentation processes in the Barasona reservoir. Alatorre et al. (2010) applied the WATEM/SEDEM model to the Barasona reservoir catchment and found good agreement between the model predictions of sediment yield and observed data. In this same context, López-Tarazón et al. (2009 and 2012) studied suspended sediment transport, in-channel sediment storage and temporal dynamics in the Isábena River by in-channel field measurements, means of direct sampling, and turbidity recording across the Isábena catchment.
- The SWAT model sediment modeling component
The sediment from sheet erosion for each HRU is calculated using the Modified Universal Soil Loss Equation (MUSLE; Williams 1975).
sed=(11.8×Qsurf×qpeak×areahru)0.56×KUSLE×CUSLE×PUSLE×LSUSLE×CFRG
where sed is the sediment yield on a given day (metric tons), Qsurf is the surface runoff volume (mm H2O/ha), qpeak is the peak runoff rate (m3/s), areahru is the area of the HRU (ha), KUSLE is the USLE soil erodibility factor (0.013 metric ton m2 hr/(m3-metric ton cm)), CUSLE is the USLE cover and management factor, PUSLE is the USLE support practice factor, LSUSLE is the USLE topographic factor and CFRG is the coarse fragment factor (Neitsch et al. 2010).
MUSLE is a modified version of the Universal Soil Loss Equation (USLE) developed by Wischmeier and Smith (1978) which predicts average annual gross erosion as a function of rainfall energy. In MUSLE, the rainfall energy factor is replaced with a runoff factor. This improves the sediment yield prediction, eliminates the need for delivery ratios and allows the equation to be applied to individual storm events. Sediment yield prediction is improved because runoff is a function of antecedent moisture condition as well as rainfall energy.
Some soils erode more easily than others even when all other factors are the same. This difference is termed soil erodibility and is caused by the soil properties. Direct measurement of the erodibility factor is time consuming and costly. Williams (1995) proposed an equation to calculate the soil erodibility factor when the silt and very fine sand content account for more than 70 % of the soil particle size distribution.
KUSLE =fcsand∙fcl-si∙forgc∙fhisand
where fcsand is a factor that gives low soil erodibility factors for soils with high coarse-sand contents and high values for soils with little sand, fcl-si is a factor that gives low soil erodibility factors for soils with high clay to silt ratios, forgc is a factor that reduces soil erodibility for soils with high organic carbon content, and fhisandis a factor that reduces soil erodibility for soils with extremely high sand contents.
The USLE cover and management factor, CUSLE, is defined as the ratio of soil loss from land cropped under specified conditions to the corresponding loss from clean-tilled, continuous fallow (Neitsch et al. 2010). The plant canopy affects erosion by reducing the effective rainfall energy of intercepted raindrops. Because plant cover varies during the growth cycle of the plant, SWAT updates CUSLE daily. The USLE support practice factor, PUSLE, is defined as the ratio of soil loss with a specific support practice to the corresponding loss with up-and-down slope culture. The USLE topographic factor, LSUSLE, is the expected ratio of soil loss per unit area from a field slope to that from a 22.1-m length of uniform 9 percent slope under otherwise identical conditions. The coarse fragment factor, CFRG, is the factor related to the percent rock in the first soil layer.
Erosion and sediment delivery are estimated as a function of peak runoff rate and volume (e.g. runoff energy factor) and physical factors such as soil erodibility, slope steepness and length, cover factor, and supporting practice factor, which corresponds to flow volume within the channel on a given day. The computed loads are then routed through the channel network based on a simplified version of Bagnold´s method (1977), in which sediment deposition or sediment erosion is determined based on the unique sediment transport capacity of the individual routing reach and by the upstream continuum of sediment from other subbasins and channel reaches (Neitsch et al. 2010).
References:
Alatorre LC, Beguería S, García-Ruiz JM (2010) Regional scale modeling of hillslope sediment delivery: a case study in the Barasona Reservoir watershed (Spain) using WATEM/SEDEM. J Hydrol 391:109–123
Avendaño-Salas C, Sanz-Montero E, Cobo-Rayán R, Gómez-Montaña JL (1997) Sediment yield at Spanish reservoirs and its relationship with the drainage basin area. In: Proceedings of the 19th Symposium of Large Dams. ICOLD (International Committee on Large Dams), Florence, Italy, pp 863–874
Bagnold RA (1977) Bedload transport in natural rivers. Water Resour Res 13:303–312
Fargas D, Martínez-Casasnovas JA, Poch RM (1997) Identification of critical sediment source areas at regional level. Phys Chem Earth 22:355–359
López-Tarazón JA, Batalla RJ, Vericat D, Francke T (2009) Suspended sediment transport in a highly erodible catchment: the river Isábena (Central Pyrenees). Geomorphology 109:210–221
López-Tarazón JA, Batalla RJ, Vericat D, Francke T (2012) The sediment budget of a highly dynamicmesoscale catchment: the River Isábena. Geomorphology 138:15–28
Mamede GL, Bronstert A, Francke T, Müller EN, De Araujo JC, Batalla RJ, Güntner A (2006) 1D process-based modelling of reservoir sedimentation: a case study for the Barasona Reservoir in Spain. In: Rui MLF, Alves ECTL, Leal JGAB, Cardoso AH (eds) Proceedings of the International Conference on Fluvial Hydraulics - River Flow 2006, Taylor and Francis, London, UK, Vol. 2, pp 1585-1594
Martínez-Casasnovas JA, Poch RM(1998) Estado de conservación de los suelos de la cuenca del embalse Joaquín Costa. Limnetica 14:83–91
Navas A, García-Ruiz JM, Machín J, Lasanta T, Walling D, Quine T, Valero B (1997) Soil erosion on dry farming land in two changing environments of the central Ebro Valley, Spain. In: Walling DE, Probst JL (eds) Human impact on erosion and sedimentation. IAHS Publ 245, IAHS Press, Wallingford, UK, pp 13–20
Navas A, Valero-Garcés BL, Machín J (2004) An approach to integrated assessment of reservoir siltation: the Joaquín Costa reservoir as a case study. Hydrol Earth Syst Sci 8:1193–1199
Neitsch SL, Arnold JG, Kiniry JR, Srinivasan R,Williams JR (2010) Soil and Water Assessment Tool input/output file documentation: Version 2009 USDA. Soil and Water Research Laboratory/ Blackland Research Center, Texas, USA
Valero-Garcés BL, Navas A, Machín J, Walling D (1999) Sediment sources and siltation in mountain reservoirs: a case study from the Central Spanish Pyrenees. Geomorphology 28:23–41
Williams JR (1975) Sediment-yield prediction with universal equation using runoff energy factor. In Present and prospective technology for predicting sediment yield and sources: Proceedings of the sediment yield workshop. USDA sediment Lab, Oxford, Mississippi, USA, pp244-252
Wischmeier WH, Smith DD (1978) Predicting rainfall losses: A guide to conservation planning. In: US Gov Print Office (ed) USDA Agricultural Handbook No 537, Hyattsville, Maryland, USA