Journal of Hydrology, 74 (1984) 295--309 295 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands [5] SIMPLIFIED DETERMINATION AND SENSITIVITY ANALYSIS OF SOIL-MOISTURE RETENTION CURVES AND HYDRAULIC CONDUCTIVITY EPAMINONDAS SIDIROPOULOS and STAVROS YANNOPOULOS School of Technology, Aristotle University of Thessaloniki, Thessaloniki (Greece) (Received September 30, 1983; revised and accepted March 16, 1984) ABSTRACT Sidiropoulos, E. and Yannopoulos, S., 1984. Simplified determination and sensitivity analysis of soil-moisture retention curves and hydraulic conductivity. J. Hydrol., 74: 295--309. A simple algorithm is presented that estimates the parameters contained in the Van Genuchten analytical soil-moisture characteristic curves. No graphical estimations are involved and convergence is rapid. The values obtained from the simple algorithm are introduced as starting values in a Newton--Raphson minimization procedure which yields more accurate evaluations of the parameters. The very good approximation offered by the simple algorithm is responsible for the small number of subsequent Newton--Raphson iterations. Based on the final results of the minimization scheme an analytical sensitivity analysis is carried out with respect to changes induced in the basic parameter "residual water content". Soils with a wide range of hydraulic properties were subjected to the above analysis. It is demonstrated that for lower water contents the sensitivity of the hydraulic conductivity becomes by 2--5 orders of magnitude larger than it is for higher ones. INTRODUCTION In order to solve the equations of moisture transfer in soil it is necessary to know the hydraulic conductivity K as a function of water content 0 or pressure head h. Values of hydraulic conductivity must be obtained experimentally (Hillel, 1980). However, direct hydraulic conductivity measurements, either in laboratory or in situ, are time consuming, expensive and require extensive preparation. Critical reviews of various measurement techniques have been presented, e.g. by Klute (1972) and Hillel (1980). For the above reasons various models have been suggested that permit relative hydraulic conductivity prediction based solely on soil-moisture retention curves, which are much easier to obtain. In this connection methods have been developed based on a statistical model due to Childs and 0022-1694/84/$03.00 © 1984 Elsevier Science Publishers B.V.