This paper reviews the importance of large continuous openings (macropores) on water flow in soils. The presence of macropores may lead to spatial concentrations of water flow through unsaturated soil that will not be described well by a Darcy approach to flow through porous media. This has important implications for the rapid movement of solutes and pollutants through soils. Difficulties in defining what constitutes a macropore and the limitations of current nomenclature are reviewed. The influence of macropores on infiltration and subsurface storm flow is discussed on the basis of both experimental evidence and theoretical studies. The limitations of models that treat macropores and matrix porosity as separate flow domains is stressed. Little-understood areas are discussed as promising lines for future research. In particular, there is a need for a coherent theory of flow through structured soils that would make the macropore domain concept redundant.

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Macropores and water flow in soils

This article is concerned with the predominantly vertical movement of water through soils that to some degree have a network of large channels (macropores) and the consequences of this movement for the convective transport of solutes and suspended matter. Beven and Germann (1982) have reviewed the experimental evidence indicating that infiltration and redistribution of water in soils containing macropores are not adequately described by theories that treat the soil as a homogeneous medium conforming to Darcian principles of water flow. Such theories, developed for the mixing of solutions in hallow tubes (Taylor, 1953) and porous rock strata (Brigham et al., 1961), have been applied to miscible displacement experiments with columns of sand, resins, glass beads, or finely sieved and repacked soil. The underlying assumptions are that an unreactive solute moves through the medium at the same velocity as the water and all the analysis and interpretation of these experiments have been reviewed several times (Gardner, 1965; Biggar and Nielsen, 1967; Biggar and Nielsen, 1980; Wagenet, 1983) and will not be repeated here.

The Influence of Macropores on the Transport of Dissolved and Suspended Matter Through Soil

[1] Dyes are important tracers to investigate subsurface water movement. For more than a century, dye tracers have provided clues about the hydrological cycle as well as flow and transport processes in the subsurface. Groundwater contamination often originates in the vadose zone. Agrochemicals applied to the soil surface, toxic compounds accidentally spilled by human activities, and contaminants released from waste repositories leach through the vadose zone and can ultimately pollute groundwater resources. Dyes are an important tool to assess flow pathways of such contaminants. This review compiles information on dyes used as hydrological tracers, with particular emphasis on vadose zone hydrology. We summarize briefly different human-applied tracers, including nondye tracers. We then provide a historical sketch of the use of dyes as tracers and describe newer developments in visualization and quantification of tracer experiments. Relevant chemical properties of dyes used as tracers are discussed and illustrated with dye intermediates and selected dye tracers. The types of dyes used as tracers in subsurface hydrology are summarized, and recommendations are made regarding the use of dye tracers. The review concludes with a toxicological assessment of dyes used as hydrological tracers. Many different dyes have been proposed as tracers for water movement in the subsurface. All of these compounds, however, are to some degree retarded by the subsurface medium. Nevertheless, dyes are useful tracers to visualize flow pathways.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001RG000109

Dyes as tracers for vadose zone hydrology

A case study on infiltration into dry clay soil i. morphological observations

Publisher Summary This chapter focuses on the role of the soil structure (the soil–pore system) through which both water and roots move. The principle reason for concentrating on soil structure is that, of the soil's intrinsic physical properties, it is the one most easily, frequently, and widely altered, particularly by cultivation. The transmission porosity is responsible for rapid water movement and provides pre-existing channels for root growth. Disruption or lack of, transmission pores is common in cultivated soil. This leads to impeded water flow and root growth within the profile. Structural instability is frequently encountered at the surface of tilled soils. It gives rise to crusting and translocation of clay into the subsoil. Crusting can reduce the infiltration rate by up to two orders of magnitude. Organic matter accumulation at the soil surface, by conservation tillage practices, provides the most cost-effective remedy. Crop roots have been studied less than crop canopies, but it appears that considerable inter-and intra-specific variation exists in such properties as maximum rooting depth, lateral branch production, xylem vessel diameter, and root hair length. Mechanical impedance and transient waterlogging are structurally related causes of restriction to root development in arable soils. There is evidence that loosening of zones of high strength improves crop–water relations.

The influence of soil structure on water movement, crop root growth, and water uptake

Hydraulic conductivities of Houston Black clay were measured in two field basins, in 73-cm and 21-cm diameter undisturbed cores, and in small cores prepared from sieved soil. Conductivities averaged about 2.5 cm/day for the field basins, about 0.3 cm/day for the undisturbed cores, and 0.1 cm/day for the disturbed soil cores. After steady-state flow through the larger, undisturbed cores was obtained, water tagged with fluorescein was used to displace water in the cores until tagged water first appeared in the effluent. Examination of cross sections of the cores at different depths showed distinctive isolated areas where tagged water was present in the soil pores. The results imply that most water contained within structural units was inactive in the flow process compared to water flow around the units. The core walls apparently cut off some large pores associated with the soil structure, causing the decreased conductivities. Therefore, only field-determined hydraulic conductivities are appropriate for use when predicting water flow in natural profiles of a swelling clay soil.

D. E. Kissel

Papers

Water Movement in Undisturbed Swelling Clay Soil

Chloride Movement in Undisturbed Swelling Clay Soil1

Nitrate and Chloride Leaching in a Swelling Clay Soil

Losses of Nitrogen in Surface Runoff in the Blackland Prairie of Texas

Occurrence of 2,4,5-T and Picloram in Subsurface Water in the Blacklands of Texas