2.4 Subsurface Water Movement 15
2.4.3 Subsurface stormflow 20
Subsurface stormflow is the quick interflow through the upper soil layer mainly through the preferential pathways of soil. The lateral movement of water through non-capillary pores and concentrated flow in soil pipes and fissures (Gregory and Walling, 1973) has gained the attention of many researchers. Hursh and Brater (1941) suggested that in small forested catchments subsurface stormflow along soil layers together with groundwater movement near stream channels could account for a major portion of the hydrograph peak. However, overland flow seemed of much less importance in this regard. Roessel (1950) and Fletcher (1952) observed that
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subsurface stormflow through large non-capillary pores accounted for the major part of storm runoff from undisturbed forested watersheds. Subsurface stormflow can be significant under both saturated (Kirkby and Chorley, 1967; Kirkby, 1969; Kirkby, 1978; Calver et al., 1972) and unsaturated (Whipkey, 1969; Jones, 1971) conditions.
Subsurface stormflow response from hillslopes are influenced by several factors like antecedent moisture condition, rainfall intensity, rainfall depth, topography, and the physical characteristics of preferential flow network (Tsuboyama et al., 1994; Sidle et al., 1995 and 2000; Uchida et al., 2005; Tromp-van Meerveld and McDonnell, 2006a). van Schaik et al. (2008) studied the influence of preferential flow on hillslope hydrology in semi-arid region. It was found that the hillslope hydrological system consists of soil matrix and macropore domains, which can interact depending on water contents. It was also reported that based on storm characteristics the contribution of macropore flow in total subsurface runoff ranged from 13 to 80 percent.
Direct field measurements of subsurface stormflow by intercepting the flow digging a trench or pit, have been reported by many researchers (Tsukamota, 1961;
Hewlett and Hibbert, 1963; Whipkey, 1965; Dunne and Black, 1970; Knapp, 1973).
However, excavation of such pits may distort the flow net upstream to the face of the pit and thus the flow hydrograph may get affected (Knapp, 1973). Under extreme magnitudes of lateral preferential flows, the stability of the face of the pit may also be a matter of concern. In this regard, piezometers and soil profile moisture monitoring systems may be a better alternative. Hydrometric studies using dye tracing and subsurface flow measurements (Mosley, 1979 and 1982) concluded that rapid flow of new water through soil macropores was capable of accounting for storm period streamflow. However, some of the researchers have emphasized on the fact that the
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rapid discharge of water through the soil macropores is mainly the ‘old water’ which is stored in the soil from the antecedent storm events (Abdul and Gillham, 1984;
Stauffer and Dracos, 1986; McDonnell, 1990). Montgomery et al. (1997) demonstrated the importance of flow through near-surface bedrock on runoff generation and pore pressure development in shallow colluvial soil on a steep, unchanneled zero-order catchment under natural rainfall and sprinkling experiments.
Freer et al. (1997) highlighted the importance of understanding the dominant downslope hydraulic gradients at the hillslope scale, which may not necessarily be represented by surface topography. This is important for the evaluation of spatial model predictions where the underlying subsurface topography is significantly different from that of the surface and, therefore, would significantly alter the spatial pattern of any topographic indices derived from digital terrain models. It was emphasized that if the surface and subsurface hydraulic gradients and dominant flow paths are correlated then it is not necessary to document the subsurface topography.
Negishi et al. (2004) studied stormflow generation in a tropical headwater zero-order basin characterized by steep slopes and shallow soils in peninsular Malaysia. Kienzler and Naef (2006) investigated the interdependency of spatially and temporally variable factors towards preferential subsurface stormflow generation in shallow soil layers.
Modeling subsurface flow in macroporous soils is a critical task as the flow geometry through soil macropores can change even within very short distances (Beven and Germann, 1982). Preferential flow models have been developed for a single pipe (Tsutsumi et al., 2005), for hillslopes (Faeh et al., 1997; Weiler and McDonnell, 2007), and for watersheds (Beckers and Alila, 2004). Both analytical and computational techniques have been adopted for modeling subsurface stormflow. For modeling high velocity of flow through soil macropores Mosley (1979) used an
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effective hydraulic conductivity calculated based on tracer velocity, effective porosity, and a hydraulic gradient equal to the bed slope. Sloan et al. (1983) suggested the use of an effective hydraulic conductivity based on a baseline hydraulic conductivity obtained from direct measurement of soil samples collected from hillslopes. The lower limit of effective hydraulic conductivity is the baseline hydraulic conductivity and higher order values as a multiple of this can be adopted to account for higher flow rates owing to the effect of soil macropores. Smith and Hebbert (1983) used mathematical simulation for interdependent surface and subsurface hydrologic processes. Beven (1982) provided analytical solutions for some simple cases of subsurface stormflow with simple kinematic theory for saturated and unsaturated flows. Solutions for rising, falling, and partial equilibrium hydrographs were given. Sloan and Moore (1984) evaluated five mathematical models for predicting subsurface flow. The models included one- and two-dimensional finite element models based on Richard’s equation, a kinematic wave model, and two simple storage discharge models based on kinematic wave and Boussinesq assumptions. They reported that the performance of simple storage-discharge models were comparable to that of complex finite element models. Germann and Beven (1985) approximated infiltration behavior into soils with sorbing macropores considering a two-domain flow. The matrix domain, where the water is subjected to capillarity, the infiltration was approximated by Philip’s sorptivity concept (Philip, 1957). In macropore domain water was assumed to move only under gravity and the flow was approached by kinematic wave theory. A sink function was introduced to account for water sorption by the soil matrix. The model was tested for infiltration into an undisturbed block of soil containing macropores. Germann et al. (1986) demonstrated the importance of saturated layers on the initiation of subsurface
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stormflow in a sloping forest soil. The kinematic wave theory was applied in the model and the distribution of flow parameters (macropore conductance and sorbance), that were used to represent macropore flow processes within a given soil, were derived. The model successfully reproduced the hydrograph peaks for a number of experiments. Germann and Beven (1986) used a distribution function approach to model water flow in soil macropores based on kinematic wave theory. Fisher (1997) developed a one-dimensional finite difference model for saturated subsurface flow within a hillslope by solving Boussinesq equation. The model uses rainfall, elevation data, a hydraulic conductivity, and a storage coefficient to predict the saturated thickness in time and space. But the model was limited in its ability to reproduce historical piezometric responses. Fan and Bras (1998) presented analytical solutions to hillslope subsurface stormflow and saturation overland flow. Combining continuity equation with a kinematic form of Darcy’s law, a quasi-linear wave equation was obtained and it was solved applying the method of characteristics to develop a one- dimensional model. Troch et al. (2002) produced a more general analytical solution to the hillslope-storage kinematic wave equation for subsurface flow. Weiler and McDonnell (2004) studied in detail about the actual structures in the soil that promote and conduct a lateral water transfer. They used a new virtual experiment approach to identify the control of water flow and solute transport at the hillslope scale for two hillslopes in New Zealand and Japan. Their investigations revealed threshold behavior observed at these and other experimental hillslope sites around the world.
Some catchment scale models have also shown significant improvements after incorporating rapid subsurface flow component into them. Vertessy and Elsenbeer (1999) described a process-based distributed model (Topog_SBM) for storm flow generation consisting of a simple bucket model for soil water accounting,
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a one-dimensional kinematic wave overland flow scheme, and a contour-based element network for routing surface and subsurface flows. Scanlon et al. (2000) used a modified version of TOPMODEL to simulate the observed catchment dynamics.
The model considered macroporous subsurface stormflow zone as a hydrological pathway for rapid runoff generation. Further, a generalized topographic index theory was applied to the subsurface stormflow zone to account for logarithmic storm flow recessions, indicative of linearly decreasing transmissivity with depth. Walter et al.
(2002) extended the TOPMODEL theory to develop a model (STOPMODEL) for shallow-soil subsurface flow. The common TOPMODEL theory implicitly assumes a water table below the entire watershed and this does not conceptually apply to systems hydrologically controlled by shallow interflow of perched groundwater.
STOPMODEL provides an approach for extending TOPMODEL’s conceptualization to apply to shallow, interflow-driven watersheds by using soil moisture deficit instead of water table depth as the state variable. STOPMODEL equations are derived using a hydraulic conductivity function that changes exponentially with soil moisture content.
Zhang et al. (2006) introduced the macropore domain in Representative Elementary Watershed (REW) model for describing preferential flow of water in both vertical as well as lateral directions. The lateral macropores were assumed parallel to the soil surface and no exchange between macropores and matrix was considered. The mass balance equation of the model had been reformulated to account for the quick subsurface stormflow. It was clearly demonstrated that subsurface stormflow contributed considerably to stream flow in the study area.
More recently, Tromp-van Meerveld and Weiler (2008) stressed on the need of including bedrock leakage, variable soil depth, and preferential flow to model subsurface flow response from the hillslopes. Graham et al. (2010) applied a forensic
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approach by combining irrigation and excavation experiments in the Maimai hillslope to determine typology and morphology of lateral subsurface flowpaths. The controls of topography and bedrock permeability on these flowpaths were also studied. The experiments showed that downslope flow was concentrated at the soil bedrock interface, with flowpath locations controlled by small features in the bedrock topography. Lateral subsurface flow was characterized by high velocities, several orders of magnitude greater than predicted by Darcy’s Law, using measured hydraulic conductivities at the site. It was found that the bedrock was moderately permeable and the vertical percolation of water into the bedrock was a potentially large component of the hillslope water balance. The experimental findings were incorporated in a conceptual model of hydrological processes to address the threshold response of hillslopes to rainfall (Graham and McDonnell, 2010). Vogel et al. (2010a) reported that the formation and intensity of preferential flow depends on the contrast between the hydraulic properties of the two flow domains as well as the properties of their interface. Physical coupling and numerical coupling of the respective governing equations were targeted. The governing equations were solved using sequentially coupled and fully coupled approaches. It was found that fully coupled approach was a numerically more robust alternative. Vogel et al. (2010b) studied soil moisture dynamics at an experimental hillslope site using a one-dimensional dual-continuum model. The water present in soil matrix and the one flowing through the preferential pathways were treated as two separate but mutually communicating soil water continua. The 18O isotope was used as a natural tracer to study the role of preferential flow in the formation of shallow subsurface runoff. It was found that the dual- continuum model could explain the observed process of subsurface hillslope discharge.
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