Scale effects on the estimation of erosion thresholds through a distributed and physically-based hydrological model
Highlights
► Erosion processes particularly severe in arid environments. ► Physically-based and distributed hydrological modelling to evaluate the risk areas. ► Scale effect that affects the location of threshold points. ► Errors associated with the drainage calculation method and DEM resolution.
Introduction
Many researchers have studied slope incision resulting from runoff concentration to estimate the volume of erosion and the location of areas at risk. From the first incision in the soil, erosion processes are diverse and develop on different spatial and temporal scales. Depending on various factors such as soil type, vegetation cover, and climate, incision in a certain area can evolve very quickly from rills only a few centimetres wide to permanent gullies and channels. Particularly in semiarid environments, processes such as overland flow, subsurface flow, and small landsliding lead to the formation of large permanent gullies (Vandaele et al., 1996, Bull and Kirkby, 1997), and are responsible for significant soil loss (e.g., Poesen and Hooke, 1997, Poesen et al., 1997, Vandekerckhove et al., 1998, Gallart et al., 2002). In addition, in steep areas with low vegetation cover, these processes can be rapid, resulting in a significant, and in many cases irreversible, alteration of the landscape, incurring very high restoration and correction costs. The prediction of areas susceptible to these processes is important for better land-use planning and management.
The relationship between the local slope of a gully or a channel head, S, and the drainage area, A, has often been used for the identification of erosion thresholds (Montgomery and Dietrich, 1988, Montgomery and Dietrich, 1994, Willgoose et al., 1991, Tarboton et al., 1992, Prosser and Abernethy, 1996, Schoorl et al., 2000, Vandekerckhove et al., 2000, Hancock and Evans, 2006a, Svoray and Markovitch, 2009), mainly due to its potential for describing surface morphology and ability to integrate the major geological, climatic and vegetation-cover features of a basin (Hancock, 2005). This relation can be represented by the power function (Vandaele et al., 1996)where S is the critical slope (m m− 1), A is the drainage area (ha), α is a coefficient and b is an exponent. This topographic threshold has also been used to identify potential areas of rill and gully development (Moore et al., 1988, Rutherfurd et al., 1997). However, in some cases, weak S–A relationships limit the accuracy of this methodology.
The physical characteristics of soil affect infiltration, subsurface flow and the S–A relation (Montgomery and Dietrich, 1994, Prosser and Slade, 1994, Prosser et al., 1995, Vandaele et al., 1996, Vandekerckhove et al., 2000). In this way, several studies have identified the threshold of erosion using hydrological models of surface runoff based on spatially lumped soil properties such as unsaturated conductivity, matric potential, soil thickness and resistance to surface flow (Dietrich et al., 1992, Prosser and Abernethy, 1996, Schoorl et al., 2000, Massip, 2001). These approaches, however, do not integrate the spatial distribution and temporal evolution of the processes involved in the infiltration–runoff relationship, relevant to the initiation of rills and gullies (Prosser et al., 1995). On the other hand, the resolution of digital elevation models (DEMs) and distributed soil characteristics directly affect the result of hydrological and geomorphological modeling (Moore et al., 1991, Fryer et al., 1994, Zhang and Montgomery, 1994, Quinn et al., 1995, Walker and Willgoose, 1999, Hancock, 2005, Hancock and Evans, 2006a).
This study analyses the influence of DEM resolution on the identification of potential areas of slope incision based on erosion thresholds from the S–A relation, distribution of soil characteristics and the associated processes of runoff, infiltration and subsurface flow during a precipitation event. The physically based, distributed hydrological model, WiMMed (Herrero et al., 2010), was used for a basin where its water balance has already been evaluated in previous studies (Aguilar, 2008, Millares, 2008, Egüen et al., 2010). Precipitation events with different temporal distributions and intensities were used to assess the simulation results.
Section snippets
Study site
The Barbacana sub-basin belongs to the Guadalfeo River Basin located in the southeast of the Iberian Peninsula (Fig. 1). It is a north-facing sub-basin of small dimensions in the Sierra de la Contraviesa, with a total area of 13.4 km2 and elevations between 589 and 1375 m. The basin relief is generally marked by relatively gentle hillside slopes which average 0.2 m m− 1, and steeper slopes which are related to the drainage system and average 0.7 m m− 1, with local maxima of up to 1.4 m m− 1.
Various soil
Methodology
The methodological framework used in this work is illustrated in Fig. 2. It is based, firstly, on the selection of various events from precipitation records to evaluate the importance of infiltration and subsurface flow in relation to erosion threshold. Precipitation time series were collected from the two stations near the study area (Contraviesa and Torvizcón) to evaluate the intensity, magnitude, duration and distribution of each event.
The S − A relation was analysed for points previously
Results and discussion
The comparative analysis of aerial images obtained between 1956 and 2007 at the sampling points reveals the limited advance of already developed channel heads. Changes in land use from Mediterranean scrub to almond and vineyard cultivation occurred with greater frequency prior to 1956 (Fig. 5). This fact explains the presence of a greater number of incision sites and the rapid development of channelled areas seen in the 1956 aerial reconnaissance. The channelled areas were later abandoned, and
Conclusions
This work highlights the importance of properly selecting the local rainfall events and DEM resolution when determining erosion thresholds, especially in hillslopes and other areas where diffusive processes prevail. The low correlation in the S–A relation obtained at the field points prevents the a priori determination of erosion thresholds from the relation S = αAb; therefore, a more detailed study was needed to estimate the risk of incision at the study area.
The results obtained from the
Acknowledgments
This research has been supported by the Water Agency of the Andalusian Regional Government (Junta de Andalucía) as part of the Guadalfeo Project, and the European Community's Seventh Framework Programme (FP7-REGIONS-2009-1 “Regions of Knowledge”) under the Novel Integrated Water Management Systems for Southern European Regions (NOVIWAM) project with grant agreement no. 245460.
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