Abstract
Abstract. The importance of soil water flow paths to the transport of nutrients and contaminants has long been recognized. However, effective means of detecting concentrated subsurface flow paths in a large landscape are still lacking. The flow direction and accumulation algorithm based on single-direction flow algorithm (D8) in GIS hydrologic modeling is a cost-effective way to simulate potential concentrated flow paths over a large area once relevant data are collected. This study tested the D8 algorithm for simulating concentrated lateral flow paths at three interfaces in soil profiles in a 19.5-ha agricultural landscape in central Pennsylvania, USA. These interfaces were (1) the interface between surface plowed layers of Ap1 and Ap2 horizons, (2) the interface with subsoil water-restricting clay layer where clay content increased to over 40%, and (3) the soil-bedrock interface. The simulated flow paths were validated through soil hydrologic monitoring, geophysical surveys, and observable soil morphological features. The results confirmed that concentrated subsurface lateral flow occurred at the interfaces with the clay layer and the underlying bedrock. At these two interfaces, the soils on the simulated flow paths were closer to saturation and showed more temporally unstable moisture dynamics than those off the simulated flow paths. Apparent electrical conductivity in the soil on the simulated flow paths was elevated and temporally unstable as compared to those outside the simulated paths. The soil cores collected from the simulated flow paths showed significantly higher Mn content at these interfaces than those away from the simulated paths. These results suggest that (1) the D8 algorithm is useful in simulating possible concentrated subsurface lateral flow paths if used with appropriate threshold value of contributing area and sufficiently detailed digital elevation model (DEM); (2) repeated electromagnetic surveys can reflect the temporal change of soil water storage and thus is a useful indicator of possible subsurface flow path over a large area; and (3) observable Mn distribution in soil profiles can be used as a simple indicator of water flow paths in soils and over the landscape; however, it does require sufficient soil sampling (by excavation or augering) to possibly infer landscape-scale subsurface flow paths. In areas where subsurface interface topography varies similarly with surface topography, surface DEM can be used to simulate potential subsurface lateral flow path reasonably so the cost associated with obtaining depth to subsurface water-restricting layer can be minimized.
Highlights
Contribution of concentrated subsurface lateral flow in soils to rapid transport of nutrients and chemicals has been well recognized (e.g., Tsukamoto and Ohta, 1988; Elliot et al, 1998)
The spatial patterns of potential lateral flow paths at the three interfaces simulated with different thresholds of contributing area (i.e., 100, 500, and 1000 m2) are illustrated in Fig. 2a–c for the soil-bedrock interface, where the patterns using the thresholds of 1000 and 500 m2 were close to each other but quite different from that using the threshold of 100 m2
Through validation by soil hydrologic monitoring, electromagnetic induction (EMI) surveys, and soil morphological observations, it was apparent that concentrated subsurface lateral flow occurred at the interfaces with the clay layer and the underlying bedrock in the agricultural landscape studied, but not at the interface between the surface plowed layers of Ap1 and Ap2 horizons
Summary
Contribution of concentrated subsurface lateral flow in soils to rapid transport of nutrients and chemicals has been well recognized (e.g., Tsukamoto and Ohta, 1988; Elliot et al, 1998). Generating three-dimensional (3-D) scheme of subsurface flow paths in a landscape can help nutrient management and pollution control. Most studies on concentrated subsurface lateral flow reported in the literature have been conducted in forested catchments (e.g., Kitahara et al, 1994; Sidle et al, 2001; Lin et al, 2006), with much fewer studies conducted in agricultural landscapes. The soil-bedrock interface has been recognized in a number of recent studies as an important concentrated subsurface. Fiori et al (2007) reported that the principal mechanism for the stream flow generation was subsurface flow along the soil-bedrock interface Freer et al (1997) reported a positive correlation between total flow volume and the contributing area calculated from a digital elevation model (DEM) of the soil-bedrock interface (instead of the soil surface). Noguchi et al (1999) demonstrated through dye tracing that bedrock topography was important in contributing to preferential flow in a forested hillslope. Buttle and McDonald (2002) found that water flow at bedrock surface occurred in a thin saturated layer. Haga et al (2005) demonstrated that saturated subsurface flow above the soil-bedrock interface was dominant subsurface runoff. Fiori et al (2007) reported that the principal mechanism for the stream flow generation was subsurface flow along the soil-bedrock interface
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