In connection with the SSC method: When measuring unsaturated K, we routinely make measurements that also yield the water-retention relation (water content as a function of matric pressure, q(y)). At each step in the measurement, after steady flow has been established, we can measure the equilibrium matric pressure in addition to the water content of the sample. This gives a triplet of numbers-- K, y, and q-- such that after the set of measurements is complete, one has data for the retention relation, q(y), as well as K(q) (and, as frequently also is useful, K(y). Our preferred technique for matric pressure measurement after centrifugation is with a tensiometer.
By the SPOC method: For detailed retention measurements of small samples, we use submersible pressure outflow cells (SPOCs). We use this method, described by Constantz and Herkelrath (1983), in a modified form with computer automation. The apparatus is also useful for approximate measurement of soil hydraulic diffusivity and unsaturated K by the one-step outflow method.
At low water contents and extreme suctions: In the dry soils of arid regions, water movement in the form of liquid may be less important than water movement in the form of vapor. For experiments under such conditions, the techniques normally favored for controlling and measuring water content and pressure, which act mainly on water in the liquid phase, are ineffective or unworkable. In these experiments we use vapor-oriented techniques, such as the chilled mirror hygrometer to measure matric pressure (Gee and others, 1993).
By the large-core controlled-volume method: With traditional water-retention techniques, such as the pressure plate or cell, fixed gage pressures are applied to the soil-air-water system and water is allowed to drain out of the sample until equilibrium water contents are reached. A different approach to defining points on a retention curve involves extracting or adding water to a sample in fixed volume increments and allowing the matric pressure to equilibrate with time. In each step, a known volume of water is extracted (or added), leaving the sample with a known average water content. Water then redistributes within the sample until pressure is equalized. Each measured equilibrium matric pressure is paired with the average volumetric water content to define points on the retention curve. With the “null” method described by Su and Brooks (1980), Dumbleton and West (1968), Leonard and Low (1962), Croney and Coleman (1954), and Miller (1951), the external pressure in the system is adjusted until it matches that of the soil water to determine matric pressure. Our variation of the null method (Winfield and Nimmo, 2002)uses a pressure transducer to monitor equilibration at a point within the sample while still holding the water content fixed for each point on the retention curve. The method has been tested for the determination of both wetting and drying curves.
The basic components of the system are a large tensiometer, a burette, a vacuum-pressure regulator, a pressure transducer, a datalogger (or alternately, a voltmeter and computer data acquisition system), and the soil sample. The soil cores are of relatively large diameter and length (10 cm by 15 cm), as it is desirable to include typical soil structure within them. Measurements are conducted in the original core liners to minimize sample disturbance which may occur during recoring. Starting from saturation, the steps for obtaining individual points on a drying curve are: (1) remove the desired volume of water from the sample by applying a suction at the base of the sample (using the vacuum-pressure regulator, tensiometer, and burette), (2) close valves to the burette/vacuum system, (3) monitor equilibration of pressure within the sample (using pressure transducer, tensiometer, and datalogger). After the desired number of points is achieved the oven-dry weight of the sample is determined. From this and the sample bulk volume, the extracted volumes are converted into volumetric water contents. Recording a number of volume and equilibrium-pressure pairs for a particular sample defines points along the drying curve.
Controlled-volume methods have the advantage of being relatively rapid for large samples. With the availability of low cost, fast-responding solid-state transducers, the use of a large tensiometer is practical, and may save time over some of the older controlled-volume “null” methods where the water level in the burette must be monitored manually in order to determine the matric pressure. The method is limited, at best, to pressure in the range of 0 to -1000 cm-water. Pressure equilibration is faster than with other techniques, since approximately half of the sample desaturates during a given step. A drawback, perhaps the main sacrifice for speed, is that the pressure coordinates of the measured points on the retention curve are not known in advance. For many applications, this is inconsequential. This technique is suitable whenever faster equilibration times are required, such as when sample size becomes a limiting factor to the speed of pressure equilibration. This method is attractive for use with large, undisturbed samples, e.g. borehole samples, where repacking is undesirable in order to preserve natural structure, although it can be applied equally well to repacked samples. Due to the limited range of matric pressure that can be attained by this method, supplementary methods, such as a combination of evaporation and either the filter paper method or water activity meter, may be required to determine drier points on large samples. For smaller or repacked samples, the pressure plate technique or other methods can be used to define additional points.
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