3 W ater Release Characteristic John Townend University of Aberdeen, Aberdeen, Scotland Malcolm J. Reeve Land Research Associates, Derby, England Andre´e Carter Agricultural Development Advisory Service, Rosemaund, Preston Wynne, Hereford, England I. INTRODUCTION The water release characteristic is the relationship between water content (usually volumetric water content) and matric potential (or matric suction) in a drying soil. The water release characteristic is one of the most important measurements for characterizing soil physical properties, since it can (1) indicate the ability of the soil to store water that will be available to growing plants, (2) indicate the aeration status of a drained soil, and (3) be interpreted in nonswelling soils as a measure of pore size distribution. There are a range of methods used for measurement of the water release characteristics of soils. This chapter describes the physical properties that deter- mine the release characteristic, outlines the most common methods used to mea- sure it and their suitability for a range of analytical environments, and briefly illustrates the ways in which the results can be presented and applied. Copyright © 2000 Marcel Dekker, Inc. II. THE SOIL WATER RELEASE CHARACTERISTIC A. Energy of Soil Water Soil water that is in equilibrium with free water is by definition at zero matric potential. Water is removed from soil by gravity, evaporation, and uptake by plant roots. As the soil dries, water is held within pores by capillary attraction between the water and the soil particles. The energy required to remove further water at any stage is called the matric potential of the soil (more negative values indicate more energy is required to remove further water). The term matric suction is also used. This represents the same quantity but is given as a positive value (e.g., a matric potential of Ϫ1 kPa is the same as a matric suction of 1 kPa). The units used to express the energy of soil water are diverse, and Table 1 provides a conversion for some of those more commonly used. The kilopascal is the most commonly applied SI unit. Schofield (1935) proposed the pF scale, which is the logarithm of the soil water suction expressed in cm of water. The scale is analo- gous to the pH scale and is designed to avoid the use of very large numbers, but it has not been universally adopted. As the soil dries the largest pores empty readily of water. More energy is required to remove water from small pores, so progressive drying results in de- creasing (more negative) values of matric potential. Not only is water removed from soil pores, but the films of water held around soil particles are reduced in thickness. Therefore there is a relationship between the water content of a soil and its matric potential. Laboratory or field measurements of these two parameters can be made and the relationship plotted as a curve, called the soil moisture character- istic by Childs (1940). Soil water retention characteristic, soil moisture charac- teristic curve, pF curve, and soil water release characteristic have also been used as synonymous terms. B. Hysteresis The term ‘‘water release characteristic’’ implies a measurement made by desorp- tion (drying) from saturation or a low suction. However, this curve is different 96 Townend et al. Table 1 Conversion Factors for Energy of Soil Water Ϫ1 Ϫ1 kPa ϭϪ1Jkg ϭϪ0.01 bar ϭϪ10 hPa ϭϪ10.2cmHOat20Њ C ϭϪ0.75 cm Hg 2 pF ϭ log (Ϫcm H O at 20Њ C) (e.g., Ϫ10.2 cm ϭ pF 1.01) 10 2 Copyright © 2000 Marcel Dekker, Inc. from the sorption (wetting) curve, obtained by gradually rewetting a dry sample. Both curves are continuous, but they are not identical and form a hysteresis loop (Fig. 1). Partial drying followed by rewetting, or partial wetting followed by drying, can result in intermediate curves known as scanning curves, which lie within the hysteresis loop. The phenomenon of hysteresis (Haines, 1930) has been frequently documented, more recently by Poulovassilis (1974) and Shcher- bakov (1985). The main reasons for hysteresis, described in detail by Hillel (1971), are 1. Pore irregularity . Pores are generally irregularly shaped voids inter- connected by smaller passages. This results in the ‘‘inkbottle’’ effect, illustrated in Fig. 2. 2. Contact angle. The angle of contact between water and the solid walls of pores tends to be greater for an advancing meniscus than for a receding one. A given water content will tend therefore to exhibit greater suction in desorption than in sorption. 3. Entrapped air. This can decrease the water content of newly wetted soil. Water Release Characteristic 97 Fig. 1 The hysteresis loop. Scanning curves occur when a partially dried soil is rewetted or a wetting soil is redried. Copyright © 2000 Marcel Dekker, Inc. 4. Swelling and shrinking. Volume changes cause changes of soil fabric, structure, and pore size distribution, with the result that interparticle contacts dif- fer on wetting and drying. Poulovassilis (1974) added that the rate of wetting or drying may also affect hysteresis. For accurate work a knowledge of the wetting and drying history of a soil is therefore essential to interpret results. However, for most practical applications the drying curve only is measured and the effect of hysteresis ignored. Although an understanding of hysteresis is central to any explanation of soil water release characteristics, the overriding influence on the shape of the water release curve is soil composition. C. Effect of Soil Properties The amount of water retained at low suctions (0 –100 kPa) is strongly dependent on the capillary effect and hence, in nonshrinking soils, on pore size distribution. Sandy soils contain large pores, and most of the water is released at low suctions, whereas clay soils release small amounts of water at low suctions and retain a large proportion of their water even at high suctions, where retention is attribut- able to adsorption (Fig. 3). Clay mineralogy is also important, smectitic clays with high cation-exchange capacity and specific surface area having greater adsorption than kaolinitic clays (Lambooy, 1984). Organic matter increases the amount of 98 Townend et al. Fig. 2 The ‘‘inkbottle’’ effect. The pore does not fill until the suction is quite low due to its large diameter (a). Once full, this pore does not reempty until a high suction is applied because of the small diameter of the pore neck (b). Copyright © 2000 Marcel Dekker, Inc. water retained, especially at low suctions, but at higher suctions soils rich in or- ganic materials release water rapidly. The presence of free iron oxides and calcium carbonate has also been shown to affect the release characteristic (Stakman and Bishay, 1976; Williams et al., 1983), though the effect of free iron is difficult to separate from the effect of the high clay contents and good structural conditions with which it is often associated (Prebble and Stirk, 1959). Water Release Characteristic 99 Fig. 3 Water release characteristics for subsoils of different texture. (After Hall et al., 1977.) Copyright © 2000 Marcel Dekker, Inc. Soil structure and density have significant effects. For example, compaction decreases the total pore space of a soil (Archer and Smith, 1972), mainly by re- ducing the volume occupied by large pores, which retain water at low suctions (Fig. 4). Whereas the volume of fine pores remains largely unchanged, that occu- pied by pores of intermediate size is sometimes increased, and this can increase the amount of water retained between specific matric suctions of agronomic im- portance (Archer and Smith, 1972). D. Suction and Pore Size In a simple situation of a rigid soil containing uniform cylindrical pores, the ap- plied suction is related to the size of the largest water-filled pores by the equation 4s d ϭ (1) rgh where d is the diameter of pores, s is the surface tension, r is the density of water, h is the soil water suction, and g is the acceleration due to gravity. At 20Њ C Eq. 1 gives d ϭ 306/h, where h is in kilopascals and d is in micrometers. Pores larger than diameter d will be drained by a suction h. 100 Townend et al. Fig. 4 The effect of compaction on the water release characteristic of an aggregated soil. Copyright © 2000 Marcel Dekker, Inc. The volume of water released by an increase in matric suction from h 1 to h 2 therefore equals the volume of pores having an effective diameter between d 1 and d 2 , where d and h are related by Eq. 1. This simple relationship will operate only in nonshrinking soils and where the pore space consists of broadly circular pores with few ‘‘blind ends’’ or random restrictions (necks). Real soils can contain pla- nar voids, pores with blind ends, and/or restrictions. If a void of 200 mm diameter has a neck exit of only 30 mm, water in the void will be released only when the suction exceeds 10 kPa. Thus the water release characteristic is at best only a general indicator of the effective pore size distribution. The size distribution of pores in a soil can be used as a means of quantifying soil structure (Hall et al., 1977) or to give a general indication of saturated hydrau- lic conductivity, the value of which is largely determined by the volume of larger pores. Aeration is also largely a function of larger pores. Whereas larger pores may be defined as macropores and related to the water released at an arbitrary low suction, other pore sizes may be termed meso- or micropores (Beven, 1981), the latter being related to the water release characteristic at higher suctions. Con- versely, the water release characteristic of soil can also be used to estimate the distribution of the size of the pores that make up its pore space. In clay soils, however, this is complicated by the fact that shrinkage results in pores reducing in size as water is withdrawn. III. MEASUREMENT METHODS There are three distinct ways to obtain a release characteristic. The usual proce- dure is to equilibrate samples at a chosen range of potentials and then determine their moisture contents. Suction tables, pressure plates, and vacuum desiccators are examples of this approach. In the second procedure, samples are allowed to dry out progressively and their potential and moisture content are both directly measured. A third option is to produce a theoretical model of the water release characteristic, based on other parameters measured from the soil such as the par- ticle size distribution, or fractal dimensions obtained from image analysis of resin- impregnated samples of the soil. A. Methods for Equilibrating Soils at Known Matric Potentials 1. Main Laboratory Methods for Potentials of 0 to Ϫ1500 kPa Diverse methodologies for the determination of water release characteristics have evolved since Buckingham (1907) introduced the concept of using energy rela- tions to characterize soil water phenomena. The most important techniques of measuring water release characteristics in the laboratory and the ranges of suction for which each method can be used are shown in Table 2. Water Release Characteristic 101 Copyright © 2000 Marcel Dekker, Inc. a. Vacuum or Suction Methods for Measurement at High Potentials (Ͻ 100 kPa suction) The basis of these methods is that soil is placed in hydraulic contact with a me- dium whose pores are so small that they remain in a saturated state up to the highest suction to be measured. The suction can be applied by using either a hang- ing water column or a pump and suction regulator. The soil in contact with the medium loses or gains water depending on whether the applied suction is greater or less than the initial value of soil water suction. Because it is more common to carry out such measurements on the desorption segment of the hysteresis curve, we are usually concerned with the loss of water. Attainment of equilibrium with the applied suction can be determined by regularly weighing the soil sample or by measuring the outflow of water until either the weight loss or outflow ceases or becomes minimal. The main restriction to such methods is the bubbling pres- sure of the medium used. The bubbling pressure (which is negative) is the suc- tion applied to the medium that empties the largest pores, thus allowing air to 102 Townend et al. Table 2 Methods of Determining Soil Water Release Characteristics in the Laboratory Method Approximate range (kPa, suction) Type of potential measured Early reference to method Bu¨chner funnel 0 –20 Matric Haines, 1930 Porous suction plate 0–70 Matric Loveday, 1974 Sand suction table 0 –10 Matric Stakman et al., 1969 Sand–kaolin suction table 10 –50 Matric Stakman et al., 1969 Porous pressure plate (including Tempe cell) 0 –1500 Matric Richards, 1948 Reginato and van Bavel, 1962 Pressure membrane 10 –10,000 Matric Richards, 1941 Richards, 1949 Centrifuge 10 –3000 Matric Russell and Richards, 1938 Osmosis 30 –2500 Matric and osmotic Zur, 1966 Pritchard, 1969 Consolidation 1–1000 Matric Croney et al., 1952 Vapor pressure (vacuum desiccator) 3000 –1,000,000 Matric and osmotic Croney et al., 1952 Sorption balance 3000 –1,000,000 Matric and osmotic Wadsworth, 1944 Filter paper 0–10,000 Matric McQueen and Miller, 1968 Copyright © 2000 Marcel Dekker, Inc. pass through the pores and causing a breakdown in the applied suction. Various experimental arrangements to apply the suction are discussed in the following sections. Bu¨chner Funnel. In the simplest application of the suction principle, a Bu¨chner funnel and a filter paper support the soil. The apparatus, introduced by Bouyoucos (1929) and later adapted by Haines (1930) to demonstrate hysteresis effects, is still occasionally referred to as the Haines apparatus, even in installa- tions where the funnel is fitted out with a porous ceramic plate (Russell, 1941; Burke et al., 1986; Danielson and Sutherland, 1986). One type of installation is illustrated in Fig. 5. One end of a flexible PVC tube is connected to the base of a funnel and the other end to an open burette. The tubing should be flexible but resistant to collapse, which can result in measure- ment errors. The tubing and funnel are filled with deaerated water and the burette adjusted until the water is level with the ceramic plate or filter paper. Air bubbles trapped within the funnel can be expelled upward by tapping the funnel while applying a gentle air pressure through the end of the burette. If a porous ceramic plate is used, as in Fig. 5, deaerated water will need to be drawn through the plate by applying a vacuum to the open end of the burette while the funnel is inverted in the water. Once the system is air-free, a prewetted soil sample (normally a soil core) is placed in contact with the filter paper or ceramic plate. The water level is maintained level with the base of the sample until it is saturated, whereupon the volume in the burette is recorded. A suction, h cm of water, can then be applied by adjusting the burette so that the water level in it is h cm below the midpoint of the sample. Water that flows out of the sample in response to the applied suction can be measured by the increase in volume of the water in the burette after the water level has stopped rising. No detectable change in burette water level within 6 hours is suggested as a satisfactory definition of equilibrium (Vomocil, 1965), but a shorter period with- out change might be acceptable. Small evaporative losses through the open end of the burette can be suppressed by adding a few drops of liquid paraffin to the water in it. Evaporative losses from the sample can be minimized by covering the open top of the funnel or creating a closed system as in Fig. 5. If the final level in the burette is hЈ, then the final suction applied is hЈ, rather than h. However, by altering the level of the free water surface to h at each inspection, the desired suction can be maintained. By repeating the exercise at successively increasing suctions, a soil moisture characteristic curve can be plotted by calculating back from the final moisture content of the soil sample (determined gravimetrically) using the vol- umes of water extracted between successive applied suctions. Using a filter paper, the maximum suction that can be applied is only 50 – 70 cm of water before air entry occurs around the sides of the paper; but using a porous ceramic plate, the maximum suction attainable is much higher, depending Water Release Characteristic 103 Copyright © 2000 Marcel Dekker, Inc. 104 Townend et al. Fig. 5 Bu¨chner funnel or Haines apparatus tension method. Copyright © 2000 Marcel Dekker, Inc. [...]... analysis for engineering purposes, there are well-documented standard methods (British Standards Institution, 1975) using equipment of standard design There have been attempts at some degree of standardization for methods of determining the water release characteristic, e.g., by Burke et al (1986) However, a variety of analytical methods are still in use worldwide and will continue to be used as long as individual... loam at 10 kPa (Carter and Thomasson, 1989) The air-entry value of fine sand precludes the use of sand suction tables at suctions above about 10 kPa Stakman et al (1969) extended the range of the sand suction table by first applying layers of a sand–kaolin mixture and then pure kaolin to the top of a sand suction table The required suction was maintained by a vacuum pump The kaolin–sand suction table has... sampling location within the field and within the soil profile, the number of replicates, and the time of sampling Loveday (1974) provided a comprehensive discussion on sampling technique and sampler design 1 Location If soil samples are to be taken to represent an area of land such as a field or soil mapping unit, they should be taken from several soil pits located at random within the area, to characterize... viscosity of soil water For these reasons, field methods are less commonly used than laboratory methods Spaans and Baker (1996) suggested that the dry end of the water release curve can effectively be derived from the soil freezing characteristic (the relationship between quantity and energy status of liquid water in frozen soil) , which can be measured in the field during freezing weather in soils that... (Haverkamp and Parlange, 1986; Tietje and Tapkenhinrichs, 1993; Viaene et al., 1994) There have been many independent attempts to compare pedotransfer functions with each other and/ or with measured data, often using a combination of the above methods (Haverkamp and Parlange, 1986; Vereecken et al 1989; Felton and Nieber, 1991; Tietje and Tapkenhinrichs, 1993; Danalatos et al., 1994; Viaene et al., 1994; Nandagiri... to the soil was described by Croney et al (1952) A saturated soil sample, laterally confined and sandwiched top and bottom between two porous disks, is loaded with successive weights on a consolidation frame (oedometer) (Head, 1982) The excess pore water pressure induced by each load is dissipated through the porous disks at a rate dependent on the hydraulic conductivity of the soil, and the soil compresses... equipment The water content of the soil sample can readily be determined by Copyright © 2000 Marcel Dekker, Inc Water Release Characteristic 117 Fig 10 Comparison of water release characteristics obtained by consolidation ( -) and by sand suction table-pressure membrane apparatus (—) for two sieved and rewetted subsoil clays oven drying after removal of the filter paper, and hence a water release characteristic... to the desired depth and is carefully removed to preserve a known volume of soil as it existed in situ Dagg and Hosegood (1962) devised a sampler incorporating several existing designs, which, with further slight modifications, is used on a routine basis in England and Wales (Hall et al., 1977) A tin-plated sleeve 7.5 cm diameter and 5.0 cm high is placed in a machined steel barrel and a cutting ring... report an air-entry value for it Such a medium might be usable up to 33 kPa and might result in fewer problems than the sand– kaolin combination Because sand or silt suction tables provide an excellent low-cost method of measuring the soil water characteristic for a large number of samples at high potentials, they have been adopted by many researchers (see, e.g., Hall et al., 1977; Stakman and Bishay,... range of tensiometers (0 to Ϫ80 kPa), and although use of electric resistance sensors (Campbell and Gee, 1986) or thermocouple psychrometers can extend this range, there can be calibration problems, and a long time is needed before a soil water characteristic curve can be obtained If the soil rewets between readings, hysteresis can be a problem, and fluctuations in soil temperature cause further complications . capillary effect and hence, in nonshrinking soils, on pore size distribution. Sandy soils contain large pores, and most of the water is released at low suctions, whereas clay soils release small. measured from the soil such as the par- ticle size distribution, or fractal dimensions obtained from image analysis of resin- impregnated samples of the soil. A. Methods for Equilibrating Soils at Known. construct than a sand suc- tion table. It also suffers from problems of entrapped air (Topp and Zebchuk, 1979) and capillary breakdown and thus requires more maintenance than a sand suction table.