2.4 Subsurface Water Movement 15
2.4.4 Lateral preferential flow 27
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modeling (Beven, 1982; Germann et al., 1986; McDonnell, 1990; Faeh et al., 1997;
Bronstert, 1999; Beckers and Alila, 2004). Considering the wide spatial and temporal variability of soil macropores and their functional behavior (Weiler and McDonnell, 2007), there are a number of areas to be addressed for better conceptualization of lateral preferential flow at the hillslope scale. Therefore, in the recent trend of studies on macropore dominated lateral flow generation in hillslopes, due importance has been given on extensive field investigation for better understanding of the processes and subsequent development of physical based models (Bronstert and Plate, 1997;
Faeh et al., 1997; Beven, 2000b; Weiler and McDonnell, 2007; Anderson et al., 2008).
Some preferential flow models used at hillslope or catchment scale do not actually include lateral preferential flow. The rapid subsurface stormflow is described mainly with vertical preferential flow connected to shallow lateral groundwater flow on the bedrock surface (Zehe et al., 2001; Christiansen et al., 2004; Beckers and Alila, 2004). Sidle et al. (2000) reported that subsurface flow contributes more to storm runoff than overland flow in steep forested catchments. However, the flow domain in a forested hillslope soil can be subdivided in two different components viz.
matrix flow and preferential flow through well connected macropore network (Uchida et al., 2002). The lateral flow velocity through the soil macropores is expected to be very high as compared to that of the surrounding soil matrix. However, in the process of rapid subsurface stormflow generation soil macropores are supposed to contribute the major portion of subsurface runoff occurring within first few hours of high intensity storm events (Sloan and Moore, 1984). To incorporate this behavior some researchers introduced a dual-porosity concept (Gerke and van Genuchten, 1993; Ray et al., 1997; Larsson and Jarvis, 1999) or considered different flow velocities in soil
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matrix and macropores (Beckers and Alila, 2004). However, some of the models suggested the adoption of an effective lateral hydraulic conductivity, which can be several times higher than the actual saturated hydraulic conductivity of the soil matrix, to account for the higher subsurface flow rates (Sloan et al., 1983; Fan and Bras, 1998; Troch et al., 2002; Rezzoug et al., 2005). Such approximations might give a reasonable correlation with the total subsurface flow hydrograph, but the bifurcation of runoff hydrograph into soil matrix and macropore flow is not addressed thoroughly. The actual contribution of macropores in total subsurface stormflow can be one of the important aspects that need to be evaluated. Beven and Germann (1982) in their famous review paper suggested that the size of macropores and their numbers do not necessarily indicate the generation of active preferential flow; but it is the connectivity of the mocropores that imparts the hydraulic effectiveness. Different number of macropores can be effective under different hydro-geologic conditions.
The process of water supply to macropores is a function of antecedent moisture condition, rainfall intensity, rainfall amount, hydraulic conductivity of soil matrix, and surface contributing area (Trojan and Linden, 1992; and Léonard et al., 1999).
Bronstert and Plate (1997) proposed a comprehensive hillslope model (Hillflow 3D) to simulate the hydrological processes like interception, evapotranspiration, infiltration into soil matrix and macropores, lateral and vertical water flow in soil matrix and preferential flow paths, surface runoff, and channel discharge. Jones and Connelly (2002) presented a semi-distributed physically based pipe flow model to simulate ephemeral and permanent flow in soil pipes. The water flow in pipes was calculated as a function of wetted perimeter of soil pipe, length of pipe, and velocity at which water enters the pipe. The complexity of a branching network was also taken into account. Herbst and Diekkruger (2003) developed a
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simplified finite element approach to model the spatial variability of soil moisture in a micro-scale catchment. The surface runoff and macropore flow processes were conceptualized in the model. The model performed reasonably well to predict the measured hydrograph. But, the subsurface flow response was underestimated by the model. Brooks (2003) reported that the bulk lateral hydraulic conductivity of a soil greatly depends on depth of saturation. For macroporous soils the use of a double exponential relationship between saturated hydraulic conductivity and depth was suggested. This approach was similar to the dual porosity approach, where the lateral saturated hydraulic conductivity is raised exponentially when the topsoil becomes saturated. The model could account for rapid lateral flow under saturated soil conditions, but the process of rapid subsurface flow in unsaturated conditions were not addressed. In one of the recent studies Ticehurst et al. (2003) used a physically based model to conduct a sensitivity analysis of subsurface lateral flow in south-east Australia. Uchida et al. (2005) reported that lateral preferential flow is highly threshold dependent and a certain amount of rainfall threshold is required to activate lateral preferential flow. Therefore, it seems to be more realistic to develop a model which can compute the contribution of lateral subsurface flow through soil matrix and macropores separately and also can indicate the number of macropores which are hydraulically effective under different hydrologic conditions. Weiler and McDonnell (2007) presented a new approach to formalize the qualitative explanation of complex preferential flow into a numerical model structure. Field observations were used to evaluate the model for its ability to capture flow and hydrograph composition. Model outputs were generated with different pipe network geometries for its calibration and validation. It was suggested that preferential flow can be parameterized within a process-based model structure. Anderson et al. (2009) conducted tracer experiments
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in a preferential flow dominated hillslope to measure the subsurface flow velocities. It was reported that the subsurface flow velocities were more closely related to the 1-h rainfall intensity than to the antecedent moisture conditions. Very little water table response of the plot indicated that preferential flow operated independently from soil matrix and the majority of flow was carried through the preferential pathways during the storms. It was also found that the subsurface flow velocities were more for shorter length of slopes. It was concluded that the preferential flow network of the hillslope was an important factor to control the subsurface flow velocity. Anderson et al.
(2010) characterized the subsurface flow processes in a watershed by monitoring water table dynamics using piezometers. The study was focused to characterize water table-runoff relationship, to identify the existence of preferential flow, and to test the feasibility of identifying areas within the watershed that are dominated by lateral preferential flow. Tang et al. (2011) studied lateral subsurface flow generation to quantify its contribution in nutrient loading in streams. Hillslope hydrology and stream hydrology were simultaneously monitored and the subsurface flow was separated from observed storms by chemical mixing model. It was found that lateral subsurface flow mainly delivered nitrates to the stream. Therefore, it was suggested to put more attention on lateral subsurface flow generation processes of hillslopes to control non-point source surface water pollution.