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Vadose Water Nimmo J R. (2009) Vadose Water. In: GeneAuthor's E. Likens, personal (Editor) copy Encyclopedia of Inland Waters. Volume 1, pp. 766-777 Oxford: Elsevier. Vadose Water J R Nimmo, U.S. Geological Survey, Menlo Park, CA, USA Published by Elsevier Inc. Introduction the porous medium in an oven until the weight is constant, and then calculate how much water was The term vadose is derived from the Latin vadosus, in the soil from the difference between the dry weight meaning ‘shallow.’ In this sense, however, it refers to and the initial wet weight. Other methods in wide- shallow depths beneath the land surface, not shallow spread use have advantages, such as being less disrup- portions of surface water bodies. The vadose zone is tive. Neutron scattering is commonly used for frequently called the unsaturated zone, and some- monitoring y as a function of depth in the field. This times the zone of aeration, as its pore space usually method is based on the high effectiveness of water, contains air as well as water. The vadose zone extends among the various components of the wet soil, in from the land surface to the water table (the lowest slowing neutrons. Commercially available equipment water table if there is more than one). has a neutron source and detector housed in a cylin- Prediction of the transport rates of water and other drical probe that can be lowered to various depths in substances within the vadose zone is critical to infil- a lined hole. Another way to monitor y in the lab or tration, runoff, erosion, plant growth, microbiota, field is by measurement of the dielectric constant of contaminant transport, aquifer recharge, and dis- the medium, usually by time-domain reflectometry charge to surface water. Vadose-zone flow is funda- (TDR). For most applications, TDR electrodes in mentally complicated by nonlinearity and hysteresis the form of metal rods are inserted into the soil. of unsaturated hydraulic properties, and extreme sen- Liquid water has a much higher dielectric constant sitivity to materials and hydraulic conditions. There than do other vadose zone constituents, so a measure- is much variety in its natural constituents: soils, rocks, ment of this property can indicate the amount of water, air, plants, animals, and microorganisms. Mod- water present within the volume sensed. A less com- ern hydrology must consider interactions not only mon method is to measure electrical conductivity, among these constituents themselves, but also with which increases with y. This principle can be ap- a wide variety of contaminants, including pesti- plied tomographically for observing two- or three- cides, fertilizers, irrigation wastewater, manure, sew- dimensional details of changing water distributions age, toxic chemicals, radioactive substances, bacteria, in the field. The reflective or absorptive behavior mine wastes, and organic liquids. of ground penetrating radar can also be used di- The porous medium of the vadose zone is typically rectly or tomographically to indicate water content soil (Figure 1), but may also be porous rock, or any distributions. other material that occurs near the earth’s surface. In general, the forces of molecular attraction are greater between solid and water than between solid and air. Water pressure and energy Matric pressure, usually Consequently, water behaves as the wetting phase, symbolized c, may be thought of as the pressure of and air as the nonwetting phase. Within the pores, the water in a pore of the medium relative to the water tends to cling to solid surfaces in films and in pressure of the air, in other words, the pressure differ- partially filled pores with curved air–water interfaces ence across an air–water interface. The liquid–solid (Figure2 ). attraction that curves the air–water interfaces also causes capillary rise (Figure 3). Water that has risen to higher levels in the tube, because of the attraction of the tube walls for water, is at a lower pressure than Fundamental Processes of Vadose Water the bulk water outside the tube. The narrower the tube, the stronger is the capillary effect. Similarly, in Unsaturated Hydrostatics an unsaturated porous medium, water is generally at Water content The most basic measure of the water lower pressure than the air, so c is negative, the y is volumetric water content, often symbolized , concave side of air–water interfaces is toward the air. defined as the volume of water per bulk volume of the The most direct measurement of c is by a tensiom- medium. eter. In firm contact with the porous medium, this y The most standard measurement of is the gravi- device allows for equilibration of pressure between metric method. The procedure is to dry a sample of the water in unsaturated pores and the water in a 766 Encyclopedia of Inland Waters (2009), vol. 1, pp. 766-777 Nimmo J R. (2009) Vadose Water. In: GeneAuthor's E. Likens, personal (Editor) copy Encyclopedia of Inland Waters. Volume 1, pp. 766-777 Oxford: Elsevier. Hydrology _ Vadose Water 767 µ 100 m Figure 1 Particles and pores of a silt loam soil, scanning electron micrograph of a thin section. Reproduced from Lebron I, Schaap MG and Suarez DL (1999) Saturated hydraulic conductivity prediction from microscopic pore geometry measurements and neural network analysis. Water Resources Research 35: 3149–3158, with permission from the American Geophysical Union (http://www.agu.org/pubs/copyright.html ). r Water Air Mineral Capillary h tube Figure 2 Pore space of a hypothetical unsaturated medium, illustrating the arrangement of solid, liquid, and gaseous phases. Few intergrain contacts appear in these figures because such Water contacts are essentially points in three-dimensional space, and mostly do not lie in the two-dimensional plane of the image. small chamber where a gauge or transducer reads Figure 3 Capillary rise in a tube. the pressure (Figure 4). Its use is limited to relatively wet soils. nylon fabric, or filter paper. This medium is placed Other methods are available for relatively dry media in contact with the medium to be measured so that and for easier application when less accuracy is accept- c becomes equal in both. Then the water content of able. Some of these are based on the humidity of the air the intermediary medium is measured by other means in soil pores. A low (strongly negative) c increases the (usually electrical conductivity, thermal diffusivity, or pore water’s effectiveness for absorbing water mole- mass) and translated into a matric pressure using the cules out of the vapor in the soil air, resulting in a lower known properties. relative humidity. The effect is slight, however; a 0toÀ15 atm range in c corresponds to a 100 to 99% range in relative humidity. Another class of Water retention Analogously to capillary rise, the methods uses an intermediary porous medium of smaller pores of a medium hold water more strongly known retention properties, typically gypsum blocks, than the larger pores do. To extract water from a Encyclopedia of Inland Waters (2009), vol. 1, pp. 766-777 Nimmo J R. (2009) Vadose Water. In: GeneAuthor's E. Likens, personal (Editor) copy Encyclopedia of Inland Waters. Volume 1, pp. 766-777 Oxford: Elsevier. 768 Hydrology _ Vadose Water Porous membrane 0.45 Water-filled tube Plano silt loam core sample Pressure-measuring device 0.40 Soil Water content (vol/vol) Very fine water-filled 0.35 pores − − 40 20 0 (a) Matric pressure (kPa) 0.30 Figure 4 A tensiometer establishes a continuous water phase 0.25 Water retention for sandy soil from the soil pores to a chamber for pressure is measured to indicate matric pressure. Pressure plate 0.20 Water-activity meter small pore requires application of a more highly neg- ative matric pressure. The volume of water held in the 0.15 soil at different matric pressures therefore depends on the pore-size distribution, and is a characteristic of a 0.10 particular porous material. This property, known as soil–water retention, is expressible as a set of y vs. c Volumetric water content 0.05 curves for a given medium. When c is close to zero, air–water interfaces are broadly curved, nearly all pores are filled, and y is high. If c is much less than 0.0 − 6 − 4 − 2 − 0 zero, the interfaces are more tightly curved, they can 10 10 10 10 (b) Matric pressure (kPa) no longer span the largest pores, and the pores have less water in them. Thus, greater y goes with greater Figure 5 Water retention relations. (a) Hysteretic water c retention of a silt loam soil, with arrows indicating the direction of (less strongly negative) . change on each curve. Reproduced from Nimmo JR and Miller EE Normally, during the process of wetting a porous (1986) The temperature dependence of isothermal moisture vs. medium, some air is trapped as the pores surrounding potential characteristic of soils. Soil Science Society of America it become water-filled. In soil brought to c ¼ 0, it is Journal 50: 1105–1113, with permission from the Soil Science common for trapped air to occupy one-tenth or more Society of America. (b) Drying retention curve for a sandy soil from of the total pore space. Trapped air will eventually the Amargosa Desert Research Site. The points are dissolve and diffuse away, but vadose-zone moisture measurements by two different methods and the smooth curve is a fit of the model of Rossi and Nimmo (1994). Reproduced from conditions commonly change rapidly enough that Andraski BJ (1996) Properties and variability of soil and trench the pore space always contains some amount of fill at an arid waste-burial site. Soil Science Society of America air.
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