Conceptual Models of Flow and Transport in the Fractured Vadose Zone

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CONCEPTUAL MODELS OF FLOW AND TRANSPORT IN THE FRACTURED VADOSE ZONE

INTRODUCTION

It has long been recognized that water flow in soils can either be uniform (Green and Ampt, 1911) or non-uniform (Lawes et al., 1881). Uniform flow leads to stable wetting fronts that are parallel to the soil surface; non-uniform flow results in irregular wetting. As a direct consequence of these irregular flow patterns, water moves faster and with increased quantity at certain locations in the vadose zone than at others. This non-uniform movement of water and dissolved solutes is commonly denoted preferential flow.

The term preferential flow neither distinguishes between the causes of the non-uniform flow pattern nor differentiates between the types of patterns. As such the term preferential flow comprises all phenomena where water and solutes move along certain pathways, while bypassing a fraction of the porous matrix. The reasons for the non-uniform flow patterns are manifold, and several identified mechanisms have coined an own term: Macropore flow is preferential water movement along root channels, earthworm burrows, fissures, or cracks. It occurs predominantly in fine-textured soils or media with a pronounced structure. Water bypasses the denser and less-permeable soil matrix by using the pathway of least resistance through macropores. Unstable flow is often observed in coarse-textured materials, and may be induced by textural layering, water repellency, air entrapment, or continuous non-ponding infiltration. As in the case of macropore flow, a considerable portion of the porous matrix is bypassed by the infiltrating water. Funnel flow refers to the lateral redirection and funneling of water caused by textural boundaries. Water again moves along the pathway of least resistance and can be redirected through a series of less permeable layers embedded in the soil profile. Each of these types of preferential flow is caused by different physical mechanisms. Often, several mechanisms act simultaneously, which leads to a broad variety of flow patterns.

The objectives of this paper are to describe the mechanisms and processes that lead to uniform and preferential flow in the vadose zone, and to elucidate the differences in the types of flow patterns observed. The different types of preferential flow are discussed in terms of three different conceptual and physical models for water flow which are frequently used in vadose zone hydrology and lead to the recognition of three spatial scales. We present illustrative examples and provide the basis for conceptual models to describe preferential flow phenomena.

CONCEPTUAL AND PHYSICAL MODELS

The physical principles that govern flow and capillary processes in the vadose zone are well understood and many excellent text books are available that deal with this topic at both introductory (Campbell, 1985; Hanks and Ashcroft, 1986; Hillel, 1998; Jury et al., 1991; Koorevaar et al., 1983; Marshall and Holmes, 1979) and advanced levels (Bear, 1972; Childs, 1969; Corey, 1990; Dullien,

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