Unsaturated Soil Moisture Research Articles

Infiltration analysis and perturbation methods: 2. Horizontal absorption

Perturbation methods are employed to analyze the horizontal absorption of moisture in unsaturated soils. The present study applies to absorption under the general diffusivity function, subject to constant boundary concentration. The concept of the wetting front moving with a finite velocity plays a fundamental role in this set of papers. The solution emerges as a series of terms that can be explicitly calculated in terms of the integrals involving diffusivity functions. The examples of exponential and constant diffusivity functions, as well as the case of Yolo light clay, are computed to one or two terms to exhibit the advantages over, and the good agreement with, other published data of Parlange, Philip, etc. The method possesses great potential, and in a subsequent paper the study of vertical infiltration will be presented by utilizing the perturbation mechanism.

Influence of Hysteresis on Moisture Flow in an Undisturbed Soil Monolith

AbstractThe moisture dynamics of an undisturbed soil monolith were studied during a lysimeter experiment. Daily measurements were made of the soil suction at 10 depths. Also daily measured were the precipitation, the seepage, and the evaporation from the monolith during a 3‐year period. For selected periods, a drying (desorption) curve and a wetting (sorption) curve of the soil moisture characteristic were determined from field data. Also the capillary conductivity was determined with use of daily monolith observations. With use of these hydraulic functions, the unsaturated soil moisture flow equation was solved numerically for one‐dimensional vertical flow. In order to determine the effect of hysteresis on the suction distribution in the monolith, calculations were performed either with the desorption curve or with the sorption curve without scanning between these curves. Neither of the two curves leads to complete agreement between observed and calculated soil suction values; the desorption curve usually gives too high values, the sorption curve too low values.

Infiltration analysis and perturbation methods: 1. Absorption with exponential diffusivity

Simple perturbation methods are employed to analyze the horizontal absorption of moisture in unsaturated soils. The special case treated assumes the diffusivity to be an exponential function of the concentration and the concentration at the boundaries to be constant. The solution emerges as an explicitly determined power series in the Boltzmann variable.The resulting profiles are compared with some others found in the existing literature. A discussion about the relevance and advantages of this type of analysis forms the concluding part of the paper.

Rainfall Excess Model from Soil Water Flow Theory

The complex computer-dependent solution of the partial differential equation for unsaturated soil moisture flow (often called Richard) equation has been used to develop a relatively simple parametric description of the performance of a variety of soils under various rainfall input rates and patterns, and initial moisture contents. This equivalent parameterized description is expressed in terms of an infiltration model, termed SEQM, that is dependent on accumulated soil water. The SEQM model is incorporated in a kinematic model of 6 x 12 ft. plot rainfull-runoff response, and its predictions are compared with those of an empirical model of infiltration (INFIL) developed by the USDA Hydrograph Laboratory. The results emphasize: the practical utility of the model and its potential advantages over comparable empirical models; and the inadequate sensitivity of equipment currently used to measure rainfull and runoff from small plots.

The infiltration envelope: Results from a theoretical infiltrometer

The theoretical partial differential equation for unsaturated soil moisture flow is solved by a versatile numerical scheme designed for accurate simulation of infiltration from various patterns of rainfall. This model is used to study the independent effects of soil type, initial soil moisture, rainfall rate and rainfall pattern. The results of the investigation are expressable as a simple parametric model for vertical infiltration. Infiltration from a suddenly ponded surface is shown to be an asymptotic limit to increasing rainfall rates. A single dimensionless formula is found to accurately describe the infiltration decay curves for all soils, initial conditions, and rainfall rates tested, and another dimensionless relation predicts time to ponding under arbitrary rainfall patterns as a function of infiltrated rainfall depth. The effect of initial soil moisture is shown to be well described by a simple linear effect on the normalizing time in the dimensionless system. Uses and implications of these results are discussed.

Measuring Unsaturated Soil Moisture Flow With a Meter

AbstractA meter for measuring unsaturated soil moisture flow was constructed and tested in the laboratory. The meter was made from a single porous plate mounted in a thin brass cylinder. Flow through the meter was calculated from measurements of hydraulic head loss in the porous ceramic plate. The unit was calibrated in silt loam, silty clay, and loamy sand soils under nonisothermal but steady state conditions. It responded to water vapor flow induced by thermal gradients, as well as to liquid flow. Transient tests made in the silt loam soil indicated that it is possible to calibrate the meter under such conditions. Design analysis indicates that increasing the length of the cylinder reduces the sensitivity of the meter's calibration to changes in soil moisture content.

An Instrument for in Situ Measurements of Soil Moisture Flow and Suction

AbstractA soil moisture flux transducer was built and tested in the laboratory under steady‐state conditions in a column of Portneuf silt loam soil. Initial data indicate that the instrument may be developed into a useful field research tool. Its principal advantage is that measurements of moisture flow may be made without any prior information concerning the unsaturated hydraulic conductivity of the soil. Rather, one needs to know only a soil moisture convergence factor which is dependent on the state of the soil and the design of the transducer. While this convergence factor is dependent upon the soil moisture content, the dependence appears to be nearly an order of magnitude less than the dependence of hydraulic conductivity on soil moisture content. It may prove possible to develop the unit for installation in the field to provide continuous measurements of the unsaturated soil moisture flow in the tensiometer range with errors no greater than those arising from the natural heterogeneity of the soil.

Thermistor Hygrometer for determining the Free Energy of Moisture in Unsaturated Soils

IN any investigation involving the movement of moisturo through soils, the measurement of the free energy or total potential of the soil moisture still remains one of the chief problems. The basic requirement of any satisfactory technique for such measurement is that it should depend on some property which is uniquely related to the free energy of the soil moisture and is in no way affected by the soil type. One such property is the relative vapour pressure or humidity in equilibrium with the soil moisture as given by equation (1), where h = free energy (cm water); R = universal gas constant; T = abs. temp.; M = molecular wt. of water; g = 981 cm/sec2; p/p0 = relative humidity.

Flow of Soil Moisture in the Unsaturated State

Conclusions and SummaryIt was assumed when this problem was undertaken that there existed a function f of the moisture content that could be multiplied by the permeability constant k of the Darcy equation to make it applicable to movement of moisture in unsaturated soil. It would appear, however, from the experimental data obtained for a typical soil, that in addition to a pressure gradient due to variation of soil moisture content from point to point in the soil, there may be a gradient due to the dynamic process itself. It is perhaps unfortunate that f is not a function of ρ alone, but this should not constitute an insurmountable obstacle in applications. Modifications may be made to simplify the analysis at the expense of some precision. The experimental data do indicate that the permeability decreases rapidly with the moisture content, and this is in agreement with the observed phenomena that irrigation water moves very rapidly when first applied but slows down very appreciably as it moves out from the source. Even though the problem is difficult, requiring a rather complicated mathematical approach, it is an important one and deserves further consideration. An attempt should be made to eliminate the geometrical variable R from equation (41) by use of a relationship such as that of equation (25) from which R can be obtained as a function of r, ρ, and ∂ρ/∂r. This will permit the expression of f in terms of ρ with the gradient of ρ as a parameter, both of which may be measured in the soil. Other soils should be studied and attempts should be made to find approximations that will render the principles involved useful to irrigation engineers.