would be the special case of the Buckingham law applying to horizontal flow or for those situations, generally in dryer soils, where gravity effects can be neglected. It was nearly a quarter of a century after Buckingham's publication before realistic experimental determinations of the Buckingham capillary conductivity function were made (29) in the suction range traversed by tensiometers, and it was another quarter century before Gardner (9) de- * What is here called the Buckingham law for unsaturated flow, has at times past been identified with the Darcy law. The proportionality discovered by Darcy between flow and hydraulic gradient for the one dimensional case in saturated sand was extended by others to apply to three dimensional flow in saturated soil. It is clear that to apply a similar proportionality to the far more involved case of unsaturated flow in soil is a very considerable additional step involving many complications for which Buckingham gave the first clear explanation and discussion. 73 Gen.7 Field capacity is supposed to represent the water content of soil at a certain location in the profile at a certain time after wetting and is often represented as being a definite soil property. It is the author's prejudice that the concept of field capacity has done more harm than good (30). Buckingham's law states that the flux density at a location in the profile is equal to the product of the capillary conductivity and the hydraulic gradient. While the capillary conductivity is an intrinsic physical property having a value at each location, the hydraulic gradient m a y be largely determined by hydraulic conditions elsewhere in the profile. W e come quickly to the first conclusion that neither the liquid flux through a localized region in the profile nor the rate of water loss from the locality are determined alone by intrinsic physical properties of the soil at the locality, such as texture or retentivity. The very considerable search that has been made to find a laboratory moisture determination that can be generally used to predict the upper limit of the field moisture range has not been successful and will not be, because hydraulic conditions in the profile that affect drainage rate in the field are difficult to reproduce in the laboratory. It is recognized that certain retentivity values have a useful correlation with the upper limit of the field moisture range for certain soils, but any such correlations must be verified by field measurements. Field observations indicate there is no sudden change in the profile drainage rate and it is not possible in general to designate a time after which soil-water movement is negligible (26). One of the principal disadvantages of the field-capacity concept has been its flagrant misapplication to plant experiments in soil-pots. Controlled water regimes for soil-pot tests with plants should be based on matric suction measurement and not on moisture values related to the rate of profile drainage in the field where, in general, hydraulic boundary conditions are totally different. Officers of the Nice Congress of Horticultural Science asked an international panel of soil scientists to clarify and explain certain soil-water terms. The assignment was a difficult one, especially when it came to the term field capacity. So long as elementary soil and irrigation textbooks continue to put out basically untrue ideas regarding this term, this bit of lore will confuse soil scientists and the confusion is compounded when the lore is taken up by plant scientists. It might help to adopt a complete moratorium on the use of this term. It m a y have originated from a wish to know something quantitative about the upper limit of the water content of the root zone. If this quantity has agricultural significance or usefulness, it can now be based on theory involving the retentivity and capillary conductivity functions, along with appropriate initial and boundary conditions. The idea that soil has capacity to store water should be used only with due caution. The water content for a certain depth of root zone could be expressed in terms of equivalent surface depth of water, which then relates easily to precipitation and irrigation, but its time transient nature as related to liquid flux at the upper and lower boundaries of the root zone should never be neglected or forgotten. Also, because of the continuous nature of the retentivity and conductivity functions, the older soil-water classification terms of hygroscopic water, capillary water and ground water are hard to justify and should be declared obsolescent. SOIL-WATER DIFFUSIVITY Various mass transfer processes may be described in terms of a diffusion equation in which flux is proportional to concentration gradient. Childs (5) 75 Gen.7 applied this relation to soil water, but Kirkham and Feng (17) pointed out limitations. Childs and Collis-George (6) later proposed and Klute (18) demonstrated the usefulness of the nonlinear diffusion equation in which the proportionality factor D, n o w called soil-water diffusivity, is. concentration dependent. This equation has been found to be very useful for describing unsaturated liquid flux in soil. It turns out that D is equal to capillary conductivity divided by the (specific) water capacity. Rapid progress has been made in the application of the nonlinear diffusion theory for describing and predicting both unsaturated flux and water distribution in soil. Numerous papers on this subject have appeared in recent years. Early contributions were made by Klute (19), Philip (27), Klute, et al. (20), and Youngs (35). Water entry into soil has been extensively treated, especially by Philip (28). Gardner's work (14, 11) is notable in that he has applied the theory to actual soils. H e determined experimentally for a number of soils the functional relation of diffusivity to water content, and used the theory to predict flow and distribution of water in soil. In addition, he checked his prediction against measurements made on soils under controlled experimental conditions. H e considered in particular the drying of soil by evaporation from the upper surface. Gardner found for several soils that the relation between soil-water diffusivity and water content was exponential. Thus, graphing the logarithm of diffusivity against water content gives straight lines as shown in figure 4. It turns out that measured values of diffusivity provide a very useful means for indicating or describing the effect of a variety of variables on unsaturated liquid flux. Figure 5 shows that for Pachappa, the value of diffusivity during sorption is about 4 times the value measured during desorption. This m a y be an extreme case since measurements on other soils have generally given a smaller hysteresis effect. The results shown in figure 6, while preliminary in nature, indicate that the effect of temperature on diffusivity is relatively small and for m a n y practical purposes m a y be negligible. PACHAPPA TEMPERATURE - °C. Fig. 6. Diffusivity of Pachappa sandy loam when saturated (D„) and air-dry (D„) as functions of temperature. [Gardner (12)]. 5 10 15 20 25 30 35 WATER CONTENT-PER CENT BY.WT. Fig. 5. Diffusivity of Pachappa sandy loam as a function of water content for sorption and desorption at 25°C [Gardner (12)]. 76 Gen.7 The effects of salinity and exchangeable sodium status on water movement rates in soil have been known qualitatively for a long time, but means for describing these effects quantitatively have been difficult to apply. The usefulness of soil-water diffusivity for this purpose is shown in figure 7. CONCENTRATION - MEQ./LITER Figure 7 Fig. 7. Weighted-mean diffusivity for Pachappa sandy loam as a function of electrolyte concentration for several different values of exchangeable-sodium percentage. [Gardner, et al. (15)]. It is n o w quite clear that both static and dynamic characteristics of the soil-water system can be quantitatively expressed in terms of measured functions relating matric suction and capillary conductivity to the water content of soil. Soil-water diffusivity and (specific) water capacity are also very useful functions, which m a y be measured separately or derived from the retentivity and conductivity functions. In addition, useful water characteristics of soil profiles can be both predicted and expressed in terms of these functions when appropriate boundary conditions are taken into account. Both the Buckingham equation and the nonlinear diffusion equation represent empirical relations that make possible a quantitative treatment of unsaturated liquid flux in homogeneous isotropic soils under isothermal conditions. Both the proportionality factor and the driving force in these equations will eventually be broken down into an array of component functions related to basic structures, mechanisms, and elementary force relations. The fact remains, however, that these empirical relations when accompanied by pertinent information on physical properties will have considerable practical and engineering value and will provide information that in the past could have been obtained only by actual tests, which are expensive. Analyses of force and energy relations inside the adsorbed water film are already well underway (2, 22, 16). W h a t is called matric suction in this paper will doubtless eventually be expressed in terms of distinguishable components. 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