Spatial discretization of large watersheds and its influence on the estimation of hillslope sediment yield

Authors: GONZALEZ, VIRGINIA - Departamento De Ingeniería Hidráulica Y Ambiental, Pontificia Universidad Cato´lica De Chile; CARKOVIC, ATHENA - Departamento De Ingeniería Hidráulica Y Ambiental, Pontificia Universidad Cato´lica De Chile; LOBO, GABRIEL - Departamento De Ingeniería Hidráulica Y Ambiental, Pontificia Universidad Cato´lica De Chile; Flanagan, Dennis; BONILLA, CARLOS - Departamento De Ingeniería Hidráulica Y Ambiental, Pontificia Universidad Cato´lica De Chile

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2015
Publication Date: 6/1/2015
Citation: Gonzalez, V.I., Carkovic, A.B., Lobo, G.P., Flanagan, D.C., Bonilla, C.A. 2015. Spatial discretization of large watersheds and its influence on the estimation of hillslope sediment yield. Hydrological Processes. DOI:10.1002/hyp.10559.

Interpretive Summary: Large rainfall events can result in substantial runoff and soil erosion occurring, and when eroded sediments reach off-site water bodies (streams, rivers, lakes, etc.) they can cause a number of economic and environmental problems. It is expensive, time-consuming, and often impractical to measure runoff and sediment losses in watersheds. Computer simulation models have been developed to determine the channel networks, watershed boundaries, and contributing hillslope regions using digital elevation information. These topographic delineation programs have recently been linked with watershed erosion prediction models, to allow rapid simulation of runoff and erosion from hillslope regions within the watershed and flow and sediment discharge from the entire catchment. This paper reports on the results of testing a geospatial erosion model named GeoWEPP (Geo-spatial interface for the Water Erosion Prediction Project) and the TOPAZ (Topographic PArametriZation) delineation program with different types of digital elevation information for two large watersheds in Chile. We found that the type of elevation data and the watershed delineation parameters with TOPAZ had little effect on the predicted runoff losses from these two watersheds. However, soil erosion amounts and the location of high soil loss rates within the watersheds were affected by the elevation data and input settings for TOPAZ. This study impacts scientists, conservation agency personnel, environmental protection agency staff, and others involved in estimating runoff and sediment losses from watersheds. Guidance and recommendations provided here, as well as a technique to determine the best unbiased watershed delineation, should be helpful to others applying GeoWEPP or similar natural resource models.

Technical Abstract: The combined use of water erosion models and geographic information systems (GIS) has facilitated soil loss estimation at the watershed scale. Tools such as the Geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the Topographic PArametriZation (TOPAZ) model is used as an automated procedure to extract hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates. In large watersheds, the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of the watershed discretization on the soil loss estimates using GeoWEPP. Two large watersheds were selected and discretized by varying the TOPAZ parameters and the DEM. The drainage networks built with TOPAZ were compared to topographic maps using a quantitative method developed for this purpose. The results showed that the discretization affected soil loss estimates and their spatial distribution but not total runoff predictions. Using 10% as an acceptable variation in the soil loss, the results showed that, depending on the DEM source and resolution, this value is reached with up to a 60% variation in the number of hillslopes and channels. Consequently, when comparing GeoWEPP estimates among different watersheds, we recommend the use of the same DEM resolution and a quantitative method to remove subjectivity when determining the discretization.