Soil water data collected from three different fields are analyzed by two techniques (temporal analysis of the differences between individual and spatial average values; and Spearman's rank correlation) to search if time‐invariant characteristic statistical properties of the probability density functions can be assigned to individual locations. A grass field was equipped with 17 neutron access tubes and surveyed 24 times during a 2 1/2 yr‐period. In another field planted with olive trees, nine neutron access tubes were installed and quarterly measurements were performed during two consecutive years. The latter field cropped in wheat was gravimetrically sampled on a regular spatial pattern five different times and was routinely surveyed during a 1‐yr period at four selected locations by using a neutron moisture meter. All data show the existence of a very significant time‐stability of particular individual locations characterized by the same parameter in the statistical distribution of the observations taken over the field. It is shown that some locations conserve the property to represent the mean and extreme values of the field water content at any time along the year. This stability seems to be explained to a large extent by relationships between soil texture and water content.

Six models, employing different ways of discretization of the nonlinear infiltration equation were compared in terms of execution time, accuracy, and programming considerations. All models yielded excellent agreement with water content profiles measured at various times in a sand column. The two explicit models, the θ‐based CSMP model and the h‐based explicit model, used between 5 and 10 times more computer time than the implicit models. Results obtained with the two models which used the Kirchhoff integral transformation were no better than those obtained with the two h‐based implicit models. The implicit schemes with implicit, or explicit evaluation of the hydraulic conductivity and water capacity functions appear to have the widest range of applicability for predicting water movement in soil with both saturated and nonsaturated regions. Excellent agreement was obtained between water content distributions, infiltration rates, and cumulative infiltration volumes calculated with the implicit finite difference model and Philip's quasi‐analytical solution.

No abstract

Transient two‐dimensional water flow is studied in relation to the recharge of a water table aquifer. The approach is based on the physics of water transfer in the complete domain defined by both the saturated and unsaturated zones of soil. Experimental data were first obtained in a slab of soil (3 m in length, 2 m in height, and 5 cm in thickness) in which the changes of water content and water pressure occurring in the flow domain were measured throughout an artificial recharge event. These were compared with the predictions of a numerical model based on the continuity of transfer between the unsaturated and saturated zones. The validity of the model is proved by the excellent agreement between simulated and experimental results. Furthermore, comparison with the results obtained by a viscous flow analog reveals the errors associated with the traditional free surface approach, which neglects transfer in the unsaturated zone.

Quick response water potential and nondestructive water content measurements indicate that during transient, nonhysteretic flow of water following a rapid change of water content at one end of a horizontal column of soil, the soil moisture characteristic curve is a unique function during sorption from a particular initial water content, but during desorption there is a multiplicity of curves. These transient curves depend on the magnitude of the imposed water content change and also on the speed with which static equilibrium is achieved after the change.The consequences of this behavior are discussed in terms of diffusion theory, and it is concluded that it would be unwise to apply this theory to desorption situations until the quantitative significance of this behavior is determined for other materials.

Experimental results dealing with flow processes involving hysteresis effects in the suction‐water content relationship were obtained on a vertical column of soil submitted to the following successive controlled flow conditions: (1) redistribution of water following a constant head infiltration in an initially air‐dried column. (2) constant head infiltration followed by redistribution, and (3) constant flux infiltration followed by controlled evaporation and redistribution. Water contents were measured by using gamma ray attenuation, and water suctions were obtained simultaneously from 10 pressure transducers distributed along the soil column, each one being connected to a tensiometer. A data acquisition system permitted fully automated measuring sequences. All the data were punched on tape and treated with a computer. Examination of the changes of water content and water suction at a given depth permitted us to obtain precise information on the hysteretic behavior of Ψ (θ). This analysis showed that the scanning curves for a single reversal can be defined uniquely with reference to the transition water content θ*, that the independent domain theory appears to be inadequate for describing soil water hysteresis completely, and that k(θ) can be considered practically unique.

Previous experiments concerning the desorption of a horizontal column of uniform soil suggested that the moisture characteristic was not unique throughout the column. Further desorption experiments have been carried out on a vertical column. The results are consistent with those obtained previously, and give further information on the dual dependence of ψ on θ and on ∂ψ/∂t.

For sound land management, it is important to understand the temporal changes that soil hydraulic properties undergo. Estimation of the unsaturated characteristics of the hydraulic conductivity, sorptivity, and mean pore radius was performed using tension‐disk infiltrometers in two different soils of the Mediterranean region: a sandy soil (Xerochrept) and a heterogeneous, stony, and sandy soil (Alfisol). Both soils were cropped with maize (Zea mays L.) and underwent conventional tillage and different irrigation practices, namely furrow irrigation and gun irrigation. The mobile water content was also deduced from soil samples extracted underneath the disk following a period of infiltration with the tension infiltrometer filled with a Cl− tracer. Results are presented here of measurements made after plowing and again at the end of the growing season. During the growing season, the sandy soil under furrow irrigation showed a significant decrease in its hydraulic properties. This followed an increase in the bulk density and was a result of sealing of the small interconnected pores at the soil surface. Strong nonlinearity in the hydraulic conductivity was found for the stony soil, yet there was no significant change in this nonlinearity during the growing season. However, from the tracer results, it is stated that for this soil, the structure of the fine fraction changes from a well‐interconnected microporous network to a poorly connected one. This results in an increase of the mobile water content during the growing season. It is shown that a good understanding of the porous network can be obtained from tension infiltrometers and can explain changes in both the hydraulic conductivity and the sorptivity. These changes were also partially corroborated by the mobile water content measurements obtained from tracer observations under the disk.