|Van Genuchten, Martinus|
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 5/11/1999
Publication Date: N/A
Citation: Interpretive Summary: Many laboratory and field methods exist to determine the soil-hydraulic properties, including especially the unsaturated hydraulic conductivity. These properties are important for determining rates of infiltration and water flow in soils. Most methods to obtain these properties are relatively time-consuming and costly, and often are limited to a relatively narrow range of water contents. One fairly simple laboratory method for simultaneous estimation of both retention and unsaturated hydraulic conductivity data for the past thirty years has been the evaporation method. Wind (1968) introduced an iterative graphical procedure to calculate the water retention characteristic from average water content and pressure head readings at several locations in a homogeneous soil sample. An alternative method of analyzing the transient flow processes is to use parameter estimation techniques. Methods of this type typically involve the ecoupling of a numerical model for variably-saturated water flow with a parameter optimization algorithm. The objective of this study was to evaluate the potential of parameter optimization techniques for simultaneously estimating the water retention and hydraulic conductivity relationships from an evaporation experiment. Advantages and limitations of the method are discussed in detail. Results should be of interest to soil scientists, hydrologists, and agricultural engineers dealing with the unsaturated zone of soils.
Technical Abstract: The objective of this study is to demonstrate the potential application of in-situ soil water extraction to estimate soil water retention and unsaturated hydraulic conductivity parameters. The Levenberg-Marquardt algorithm in combination with the HYDRUS-2D flow code is used to inversely estimate the parameters of the hydraulic functions from transient soil matric potential and cumulative soil solution extraction measurements. Experiment was carried out in the field of Yolo silt loam. A series of vacuum extraction pressures was applied to a ceramic soil solution sampler, and cumulative soil solution extraction volume and matric potential heads at various locations near the extraction device were monitored during solution extraction. A power function determined from measured tensiometric data was used to calculate the matric potential at the lower boundary. The upper boundary was defined as a zero flux boundary. Cumulative extraction volume and matric potential data were included in an objective function that was minimized to estimate the parameters describing the hydraulic functions. We determined that the inverse solution was sensitive to the hydraulic resistance of ceramic cylinder of the extraction device. The optimized parameters were well estimated after we included in the objective function three independently measured soil water retention data points determined during the extraction experiment. Comparison of the optimized soil hydraulic parameters with those determined independently with the instantaneous profile method indicated that the in-situ estimation using a multi-step soil-water extraction technique can provide accurate soil hydraulic data.