V. Rasiah

Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Science

First Advisor

R.A. Kohl


The soil water pressure head, h, versus soil water content, 9, and the hydraulic conductivity, K, versus θ, relationships appear as functional coefficients in the non-linear partial differential water flow model. Before the flow equation can be used to simulate the flow of water through soils, the h versus θ and K versus θ relationships must be established. This requires the estimation of the parameters that describe the h versus θ and K versus θ functional relationships. In-situ water retention and hydraulic conductivity measurements were determined from the knowledge of initial and boundary conditions and water content profiles during drainage. This information obtained in the greenhouse was used to estimate the parameters of the non-linear h(θ) and K(θ) empirical functions. The estimates were obtained through the first and second order least square best fit procedures for the logarithmically linearized h(θ) and K(θ) functions. The estimates, when combined with the flow model, SWATRE, estimated soil moisture content profile, θ, which did not agree well with the observed data. In the second method, the flow model was linearized using the Taylor series expansion. The same parameters mentioned above were estimated using two iterative procedures, Marquardt (1964) and Taylor. The simulations, θ, for the fitted estimates from both procedures, agreed well with the observed data. The convergence of the estimates in Marguardt's maximum neighborhood method of iterative fitting was more stable than that in the Taylor method. Marquardt's method converged more slowly. Similar procedures were followed for fitting the root uptake function parameters. The parameters in the root uptake function, estimated and fitted individually for each compartment, produced better solutions than when single function parameters were defined for the whole profile. To obtain a satisfactory solution from the flow model, including root uptake, the parameters in the h(θ) and K(θ) functions were estimated for the range of θ in which the root water uptake took place.

Library of Congress Subject Headings

Soil permeability
Roots (Botany)
Soil moisture




South Dakota State University