An infiltration model based on flow variability in macropores: development, sensitivity analysis and applications
Introduction
Runoff generation at a site determines the flow pathways in catchments and thus the flow hydrograph for rainfall events. Only a realistic simulation and prediction of flow pathways leads to a correct description of the internal hydrological behavior in a catchment (Beven, 2001). Although the prediction of the flow hydrograph with rainfall-runoff models, which do not account for the relevant runoff processes, may be acceptable, we get ‘the right results for the wrong reason’ (Klemes, 1986). A correct description of the internal hydrological behavior is especially important for the simulation of solute transport processes in the catchment (e.g. nitrate or phosphate leaching) and for the assessment of land-use or climate change scenarios. The influence of macropore flow on hillslope runoff generation is well established (Bronstert, 1999, Faeh et al., 1997, McDonnell, 1990, Návar et al., 1995, Smettem et al., 1991, Weiler et al., 1998, Weiler and McDonnell, 2004), particularly for extreme rainfall events with either a very high intensity for a short duration or a lower intensity for a longer duration. (e.g. Faeh et al., 1997, Niehoff et al., 2002). Despite abundant experimental evidence of its significance, macropore flow, a subtype of preferential flow, is a process that is often not considered in rainfall-runoff models. Indirect evidence of macropore flow at the catchment scale can be obtained through model interpretation (Germann and Beven, 1981, Burch et al., 1989, Niehoff et al., 2002, Beckers and Alila, 2004).
Macropore flow causes a rapid downward movement of water in structural pore spaces such as worm channels, shrinkage cracks, and root holes subsequently bypassing portions of the soil profile. For example, earthworm activity, particularly the anecic earthworm species Lumbricus terrestris, causes macropores in soils in humid climate (Langmaack et al., 1999). Macropore flow influences the infiltration of rainfall and therefore the runoff generation and solute transport in natural soils where these structures are common (Larsson, 1999). The impact of macropores is governed to a large extent by water supply to macropores, water flow in macropores, and the water transfer from the macropores into the surrounding soil matrix (Beven and Germann, 1982, Faeh et al., 1997, Buttle and Leigh, 1997). The flow rate in earthworm channels can be very high compared to the rate in the soil matrix. Even for relatively small earthworm channels, the flow rate in macropores seems to be always higher than the rainfall intensity (Bouma et al., 1982, Wang et al., 1994, Shipitalo and Gibbs, 2000, Weiler, 2001). Thus, the flow capacity of the macropore system is usually not the limiting factor during the infiltration process.
Water transfer from macropores into the surrounding soil matrix has been referred to as lateral infiltration from the macropores (Beven and Clarke, 1986). In this study, the term interaction is used to describe this critical process of water flow in structured soils (LogsdonM et al., 1996, Faeh et al., 1997). Despite some experiments to measure interaction in single artificial or natural macropores in the laboratory (Smettem, 1986, Ghodrati et al., 1999) or in the field using dye tracers and soil water measurements (van Stiphout et al., 1987), a consistent description, parameterization and verification of the interaction process remains elusive.
Macropore flow initiation is the process of water supply to macropores. It is a function of initial water content, rainfall intensity, rainfall amount, hydraulic conductivity of the soil matrix, and surface contributing area (Léonard et al., 1999, Trojan and Linden, 1992). Water can flow into a macropore if its water entry pressure is exceeded, either at the soil surface or the nearly saturated subsurface (Ela et al., 1992, Li and Ghodrati, 1997, Weiler and Naef, 2003a). Weiler and Naef (2003b) found a distinct distribution of flow rates in macropores depending on where macropore flow is initiated.
The objective of this study was to develop, evaluate and test a model that combines well-known approaches of simulating initiation and interaction with the yet uncommon initiation-based flow rate distribution. This model was then used as a tool to evaluate the hypothesis that the initiation process is crucial to infiltration in macroporous soils and hence to runoff generations in general. This hypothesis was tested theoretically with a sensitivity analysis evaluating for a synthetic dataset the influence of the initiation process on interaction and on the hydrological response of the soil. The new conceptual framework of the model was tested practically by simulating results from sprinkling experiments at three field sites in Switzerland (Weiler and Naef, 2003a).
Section snippets
Model theory
The INfiltration-INitiation-INteraction Model (IN3M) allows for water flow within discrete macropores, as well as for water flow in the soil matrix based on the dual permeability concept. Analytical solutions of the Green-Ampt equation and a simple accounting scheme serve as the basis for the model. The use of analytical solutions removes any concern regarding the numerical instability observed in most preferential flow codes (Simunek et al., 2003) and results in code that can be more readily
Strategy
Since IN3M can simulate the flow rate distribution in macropores, the influence of initiation on interaction and thus on the vertical flow in the macropores and on the water content change in the soil matrix can be analyzed under the model assumptions. For this purpose the simulation of vertical water flow in the soil matrix was switched off and simulations were run for three different conditions of water flow distribution in the macropores at the upper boundary:
- (a)
same flow rate in each macropore
Study sites
At three field sites in Northern Switzerland (Heitersberg, Koblenz, and Niederweningen), we conducted sprinkling experiments with a Brilliant Blue solution at two irrigation rates, 60 mm h−1 (high) and 12 mm h−1 (low) with a total irrigation of ∼75 mm, and two different initial soil moisture conditions referred to as ‘dry’ (no input within 4 weeks) and ‘wet’ (∼75 mm rainfall at the previous day). The dye solution was applied using a sprinkling device consisting of a linearly moving spray bar with 15
Discussion
None of the models that describes flow and/or transport in soils containing macropores, like MACRO (Jarvis et al., 1991), QSOIL (Faeh et al., 1997), RZWQM (Ahuja et al., 1995), tipping bucket model (Emerman, 1995), SAMP (Ewen, 1996), LASOMS (Chen and Wagenet, 1992) and PREFLO (Workman and Skaggs, 1990), have yet implemented the variation of flow rate in macropores due to the initiation process. This study assessed the hypothesis that the flow rate distribution in macropores due to the
Conclusion
Evaluating and testing a model (IN3M) that combines well-known approaches of simulating initiation and interaction with the yet uncommon initiation-based flow rate distribution demonstrated that the initiation process is crucial to correctly simulate infiltration in macroporous soils. Without considering the initiation process, total water flux from the macropores into the soil matrix is overestimated and thus hydrological response is underestimated. Further work is necessary to explore the
Acknowledgements
This work was funded by the Swiss Federal Institute of Technology in Zürich within the project ‘Investigation of the water exchange mechanisms between preferential flow paths and the soil matrix’. I would like to thank Felix Naef for his support during this study and especially Thomy Keller for his help during the field experiments. Kerstin Stahl, Kevin McGuire, and Kellie Vache provided helpful comments on the manuscript.
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