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Title: Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data

Author
item RAMOS, T - AGRONOMICA NACIONAL
item GONCALVES, M - AGRONOMICA NACIONAL
item MARTINS, J - AGRONOMICA NACIONAL
item Van Genuchten, Martinus
item PIRES, F - AGRONOMICA NACIONAL

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2005
Publication Date: 5/26/2006
Citation: Ramos, T.B., Goncalves, M.C., Martins, J.C., Van Genuchten, M.T., Pires, F.P. 2006. Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data. Vadose Zone Journal. Vol 5:684-696

Interpretive Summary: Mathematical models are increasingly used to address a broad range of subsurface flow and contaminant transport problems. Simulations for soils are generally based on numerical solutions of the Richards equation, which describes water flow in the unsaturated zone between the soil surface and the groundwater table. The simulations in turn require knowledge of the hydraulic properties of the soils involved. These properties consist of the water retention curve, which relates the water content to the soil water tension (a measure of the dryness of the soil), and the hydraulic conductivity curve, which relates the conductivity (or permeability) to the water content. While a large number of laboratory and field methods are available to measure the hydraulic properties, most techniques remain not only time consuming and costly, but also approximate in view of the enormous variability of field soils. One popular recent field method for measuring the hydraulic properties is the use of a tension infiltrometer. This device allows one to measure the infiltration rate into soils when water is applied at a slight tension to produce conditions slightly less than saturation. An inverse procedure can next be applied to estimate the hydraulic properties from the measured infiltration rate, possibly in combination with additional measurements of the soil water content in the laboratory. Tension infiltrometers have been found very useful for characterizing the water flux of macroporous field soils exhibiting preferential flow when saturated or close to saturation. The main objective of this study was to further test the inverse tension infiltrometer modeling approach by using the method to characterize the hydraulic properties of four field sites in Portugal. The field-measured properties generally were found to agree closely with independent laboratory data. However, the unsaturated hydraulic conductivity was estimated less accurately, although good estimates of the conductivities at full saturation were obtained. This study confirms that numerical inversion of tension infiltrometer data provides a relatively simple and reliable method for determining the water retention and conductivity curves of unsaturated soils. Results are important for scientists and engineers trying to estimate the unsaturated soil hydraulic properties for subsequent use in models predicting water flow and solute transport processes at the field scale.

Technical Abstract: Many applications involving variably saturated flow and transport require estimates of the unsaturated soil hydraulic properties. Numerical inversion of cumulative infiltration data during transient flow, complemented with initial or final soil water content data, is an increasingly popular approach for estimating the hydraulic curves. In this study, we compared Mualem–van Genuchten (MVG) soil hydraulic parameters obtained from direct laboratory and in situ unsaturated hydraulic conductivity measurements with estimates using numerical inversion of tension infiltration data of four coarse- to medium-textured soils in Alentejo (Portugal). The laboratory methods used were suction tables, pressure plates, and the evaporation method as applied to undisturbed soil samples collected from the surface horizons of four different soil profiles. Field measurements were taken with a tension disk infiltrometer using consecutive supply pressure heads of -15, -6, -3, and 0 cm. Six MVG parameters (the residual soil water content, the saturated soil water content, three empirical shape factors, and the saturated hydraulic conductivity) were estimated from the field data by numerical inversion using the HYDRUS-2D software package, and compared with values estimated from the laboratory data. Macroporosity was also determined. The laboratory- and field-measured water retention curves were found to agree closely for most experiments as reflected by relatively high values of the coefficient of determination, the modified coefficient of efficiency, and the modified index of agreement (always >0.9949, 0.8412, and 0.8931, respectively). The unsaturated hydraulic conductivity curves were predicted less accurately, although good estimates of the saturated conducivity were obtained.