Adapting SWAT hillslope erosion model to predict sediment concentrations and yields in large Basins

Abstract

The Soil and Water Assessment Tool (SWAT) is used worldwide for water quality assessment and planning. This paper aimed to assess and adapt SWAT hillslope sediment yield model (Modified Universal Soil Loss Equation, MUSLE) for applications in large basins, i.e. when spatial data is coarse and model units are large; and to develop a robust sediment calibration method for large regions. The Upper Danube Basin (132,000km(2)) was used as case study representative of large European Basins. The MUSLE was modified to reduce sensitivity of sediment yields to the Hydrologic Response Unit (HRU) size, and to identify appropriate algorithms for estimating hillslope length (L) and slope-length factor (LS). HRUs gross erosion was broadly calibrated against plot data and soil erosion map estimates. Next, mean annual SWAT suspended sediment concentrations (SSC, mg/L) were calibrated and validated against SSC data at 55 gauging stations (622 station-years). SWAT annual specific sediment yields in subbasin reaches (RSSY, t/km(2)/year) were compared to yields measured at 33 gauging stations (87station-years). The best SWAT configuration combined a MUSLE equation modified by the introduction of a threshold area of 0.01km(2) where L and LS were estimated with flow accumulation algorithms. For this configuration, the SSC residual interquartile was less than +/-15mg/L both for the calibration (1995-2004) and the validation (2005-2009) periods. The mean SSC percent bias for 1995-2009 was 24%. RSSY residual interquartile was within +/-10t/km(2)/year, with a mean RSSY percent bias of 12%. Residuals showed no bias with respect to drainage area, slope, or spatial distribution. The use of multiple data types at multiple sites enabled robust simulation of sediment concentrations and yields of the region. The MUSLE modifications are recommended for use in large basins. Based on SWAT simulations, we present a sediment budget for the Upper Danube Basin.

The Hillslope Length Impact on SWAT Streamflow Prediction in Large Basins

“…Choosing a constant hillslope length of 50 m (L3) however was a good second alternative to be considered when DEM resolution is of 25 m or coarser, especially if the DEM accuracy is low. Furthermore, given the importance of hillslope length on sediment outputs reported in literature (Chaplot, 2014;Zhang et al, 2014) the impact of hillslope length on sediment predictions was explored in Vigiak et al (2015), in a study that confirmed the suitability of L2 method for prediction of sediment yields.…”

Identifying erosion hotspots in Lake Tana Basin from a multisite Soil and Water Assessment Tool validation: Opportunity for land managers

“…The SWAT model assumes that HRUs would represent hillslopes, where sheet and rill erosion are the dominant sediment sources and hillslope deposition is the main sink. In reality, HRUs may extend above the sheet and rill erosion or hillslope deposition (Vigiak, Malagó, Bouraoui, Vanmaercke, & Poesen, 2015), and SY may exceed gross erosion when other sources may contribute, such as gully erosion or landslides (de Vente & Poesen, 2005;Vigiak et al, 2015). In this case, the soil erodibility factor (USLE_K) in the SWAT model may increase to reflect the effect of gully erosion and landslides on SY (Awulachew, Smahktin, Molden, & Peden, 2012).…”

Hydrosedimentological modeling with SWAT using multi-site calibration in nested basins with reservoirs

“…These studies aimed mainly at simulating streamflow (9 studies), secondarily streamflow, sediments and nutrients (3) and finally only streamflow and sediments (1), using 2 to 13 (mean 5) sub-basins nested in meso to macroscale basins situated in North America (8), Asia (2), South America (1), Africa (1) and Oceania (1). A few studies also evaluated other approaches to calibration LIEW, 2010;VIGIAK et al, 2015) and analysis of uncertainties LIAN, 2013;ZHANG et al, 2015b), associated with the multi-site calibration of nested basins.…”

“…Only 4 of the studies on multi-site calibration described above have reservoirs within their drainage area, and only Vigiak et al (2015) calibrated their sedimentological parameters without showing details on the hydrological calibration. Considering only simple calibration, many studies with SWAT were performed in basins with the presence of reservoirs, some evaluating the hydrological effect GARBRECHT;ARNOLD, 2003;LINO et al, 2009;WAGNER et al, 2011;ZHANG et al, 2012), sedimentological effect (MISHRA; FROEBRICH; and water quality effect (ZHANG et al, 2011) of these structures.…”