Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
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Uniform and Preferential Flow Mechanisms in the Vadose Zone
Uniform and Preferential Flow Mechanisms in the Vadose Zone
Pages 149-188
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From page 149... ...
Since preferential flow occurs at a number of scales, scale is used as the primary classification criterion. Three distinctive scales are recognized on the basis of three different conceptual and physical models for water flow in the vadose zone: pore scale, Darcian scale, and areal scale.
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From page 150... ...
Uniformilow 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.
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From page 151... ...
This changes the conceptual model from one based on a fluid continuum at the pore scale to one based on the concept of a representative volume at a larger spatial scale. At the Darcian scale, water movement through a one-dimensional, unsaturated, vertical soil column is mathematically expressed by Darcy-Buckingham's equation: q = -K(h)
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(1991~. At the areal scale, application of the Darcy-Buckingham equation is no longer practical since it would require long and expensive field campaigns to characterize and quantify the spatial variability of the vadose zone at the Darcian scale.
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Flow in a fracture requires a measurement of its width; unsaturated flow through a soil profile requires measurements or indirect determination of the six soil parameters Ks, Os, Or' n, X, and a; evaluation of a regional water balance in the vadose zone requires long-term monitoring of soil water contents, meteorological variables, and groundwater levels. This leads to the observation that the timeframe of a study often will increase if the spatial scale of its conceptual model becomes larger.
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From page 154... ...
154 an Cal o an ·_4 Cq Cq ·_4 C)
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From page 155... ...
For these reasons we have selected a practical classification criterion based upon the three conceptual models discussed in the previous section which lead to three typical spatial scales often encountered in vadose zone studies. Thepore scale deals with water flow processes described by Hagen-Poiseuille's equations, the Darcian scale with processes considered to take place within fictitious representative volumes and described by Darcy's equation, and the areal scale with processes affected by major landscape elements such as local depressions, faults, and discontinuous layers in the vadose zone.
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From page 158... ...
For example, the infiltration rate of a soil depends not only on the time since infiltration started but also on antecedent water content (Philip, 1969~. When the infiltration rate, iota, decreases with time and with increasing antecedent soil water content, the opportunity for overland flow, outs, and macropore flow, q(0, t)
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159 o ~0 $o N o O— O , oo a a)
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From page 160... ...
Macropores and water flow in soils. Water Resources Research 18: 1311-1325.
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From page 161... ...
The most straightforward application is found under stable flow conditions characterized by horizontal wetting fronts parallel to the soil surface. For example, Figure 5-5 shows a stable wetting front in a loam soil along the Rio Salado near Socorro observed in August, 1995, a few days after heavy rainfall.
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From page 162... ...
water flow through the vadose zone have focused on agricultural soils, while our knowledge of stony soils remained limited. However, stony environments are widespread in river bed and mountain-front hydrogeological provinces that are characterized by recharge infiltration through stony layers (Issar and Passchier, 1990~.
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, and Raats (1984~. The conditions under which unstable wetting fronts form in the field are not yet fully understood, because systematic investigations of the phenomena in field soils have been rare.
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demonstrate that the irregular and incomplete wetting pattern in water-repellent soils can be predicted and explained by unstable wetting front theory. Figure 5-6 demonstrates the extreme variability in volumetric water content for the top layer of a water-repellent soil.
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From page 165... ...
Water Resources Research 29: 2183-2193. Copyright by American Geophysical Union.
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From page 166... ...
predict sharp, stable wetting fronts during infiltration in dry, homogeneous wettable soils. These theories were mostly verified in laboratory columns with diameters of a few centimeters, but also in field soils after periods with precipitation (Hendrickx and Yao 1996; Figure 5-5 of this chapter)
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Under steady-state water flow, distinct flow patterns developed, with regions of high flux and regions of low flux. The flow patterns observed depended on the magnitude of the steady water flux, and consequently on the water content of the porous medium.
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(W.H. Gardner, unpublished photographs, Department of Crop and Soil Sciences, Washington State University.)
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Karstic Vadose Zone Gunn (1983) investigated mechanisms by which flow is concentrated and transmitted to the underlying aquifer for a karst area in the Waitomo district of Wet Disniacement Fauit FIGURE 5-10 Displacement faults in sand near Socorro, New Mexico.
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From page 170... ...
subcutaneous flow, defined as water flowing laterally through the upper, weathered layer of limestone. Three vertical flow mechanisms were recognized for water transmission through the vadose zone: (1)
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From page 171... ...
The first responded directly to rainfall with a mean travel time of less than a week, whereas the second had a mean travel time of 8 weeks. The travel times for vadose seepage varied between 0 and 19 weeks, indicating that part of the water flows rapidly through small fractures as macropore flow while the remainder flows slowly through the porous material as capillary flow.
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From page 172... ...
After Journal of Hydrology, 94, Johnston, C D., Preferred water flow and localized recharge in a variable regolith, pp.
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His method is based on actual evapotranspiration predictions using a numerical soil water balance model, while the runoff component is predicted by a runoff model (Boers et al., 1986~. The microcatchment in Figure 5-13 illustrates how the components of rainwater harvesting interact.
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From page 174... ...
Table 5-2 presents the annual water balance components at S adore (Niger) for one Neem tree in a basin of 8 m2 with a soil profile comprising 3 m fine sand above 2 m laterite gravel.
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Rainfall Only Rainfall and Runoff Average Year Runoff area 20 m2 Runoff area 40 m2 P 545 545 545 R 0 232 465 Tact 409 633 755 D 0 5 113 Dry Year Runoff area 20 m2 Runoff area 40 m2 P 258 258 258 R 0 78 155 Tact 138 205 277 D 0 0 O Wet Year Runoff area 20 m2 Runoff area 40 m2 P 673 673 673 R 0 285 571 Tact 481 720 849 D 0 38 185 Pipes Through Calcic Horizons The La Mesa surface in southwestern New Mexico has developed on Rio Grande deposits mainly consisting of sands and gravels. The surface was abandoned by the Rio Grande River during the middle Pleistocene and since that time a calcic soil with an indurated calcic horizon has formed.
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From page 176... ...
76 CONCEPTUAL MODELS OF FLOWAND TRANSPORT FIGURE 5-14 A pipe through an indurated calcic horizon on the La Mesa surface in New Mexico (Rodr~guez-Mar~n and Hendrickx, personal communication, 1999~.
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From page 177... ...
At larger spatial scales covering entire fields or landscape elements, flow phenomena are indirectly assessed by measuring soil moisture and soil structure through remote sensing, ground-penetrating radar, neutron-probe moisture measurements, and other geophysical techniques that allow large-scale mapping of pedological and geological features. Deep coring tests analyzed for chloride distributions often provide insight into flow mechanisms.
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From page 178... ...
Nevertheless, the case studies presented in this review have a number of general characteristics in common that can serve as a guideline for the development of conceptual models. All three spatial scales of preferential flow discussed have in common that an initially spatially uniform fluxis disturbed and water flowis confined to a smaller cross-sectional area of the vadose zone.
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From page 179... ...
. 10 100 FIGURE 5-16 Application of stability criteria for determination of preferential flow caused by unstable wetting fronts.
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~ Unstable | Darcian BY ~ H ~ Dan tan FIGURE 5-17 Flow diagram for evaluation of vadose conditions that will lead to preferential flow.
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From page 181... ...
An example of how precipitation amount, precipitation intensity, and antecedent soil water content affect the occurrence of unstable flow in a wettable sand soil is given in Figure 5-16. In Figure 5-17 we present a flow diagram for the evaluation of vadose zone conditions that will cause preferential flow at different scales as a result of a lateral;fiow trigger.
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From page 182... ...
2001. Stability analysis of the unsaturated water flow equation, 1.
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From page 183... ...
Unstable wetting fronts in water repellent field soils. Journal of Environmental Quality 22: 109-118.
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D., 1987. Preferred water flow and localized recharge in a variable regolith.
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From page 185... ...
C., 1973. Unstable wetting fronts in uniform and nonuniform soils, Soil Sci.
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Experimental studies of wetting front instability induced by gradual change of pressure gradient and by heterogeneous porous media.
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Stability of wetting fronts in homogeneous soils under low infiltration rates. Soil Science Society of America Journal 60: 20-28.
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