- 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
137
Cs-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 coarsesand 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 𝑓ℎ𝑖𝑠𝑎𝑛𝑑 is 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:
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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
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López-Tarazón JA, Batalla RJ, Vericat D, Francke T (2009) Suspended sediment
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