Water Resources Management 17: 133–145, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 133 A Note to the Variable Sorptivity Infiltration Equation I. ARGYROKASTRITIS ∗ and P. KERKIDES Department of Natural Resources Management and Agricultural Engineering, Sect. of Water Resources Management, Agricultural University of Athens, Iera Odos, Athens, Greece ( ∗ author for correspondence, e-mail: jarg@aua.gr, fax: 210 529 4081) (Received: 21 November 2001; in final form: 13 February 2003) Abstract. A simplification for the variable sorptivity infiltration equation of Poulovassilis et al. (1989) is proposed. The resulting equation has three parameters S x , c and K 0 . From these, S x and c are considered as fitting parameters and K 0 as a physical one. The new empirical infiltration equation is tested for precision, parameter time-dependence and applicability for soil surveys. The test was carried out by comparison with reference solutions i.e. infiltration data obtained experimentally, ana- lytically or numerically for two different head conditions at the infiltration surface. A good agreement is observed for all examined cases. The dependence of the fitting parameters S x and c on the initial and boundary conditions, as well as the error that arises by taking into account different values of them, are examined. In fine textured soils parameter c seems to be very small, so that one can easily suppose that the proposed equation reduces to the well-known Philip’s infiltration equation (Philip, 1957). Key words: infiltration, infiltration equations, porous media, sorptivity 1. Introduction A large number of empirical and semiempirical equations (Green and Ampt, 1911; Kostiakov, 1932; Horton, 1940; Mezencev, 1948; Philip, 1957b; Gosh, 1980; Par- lange et al., 1982, 1990; Fok, 1986; Swartzendruber, 1987; Poulovassilis et al., 1989) have been proposed for the description of the relationship between the cu- mulative infiltration i z and the infiltration time t , during the process of vertical infiltration of water into the soil mass, under ponded conditions. Common ways for the evaluation of the proposed equations is by comparing the agreement between i z − t relationships taken from reference solutions, i.e., analytical, experimental or numerical and those, which have been calculated by the infiltration equations. This paper proposes a simplification to the ‘Variable Sorptivity infiltration equa- tion’ of Poulovassilis et al. (1989) and makes an attempt to evaluate the resulting equation by using all the above mentioned ways. So, i z (t) relationships of two porous bodies as these were obtained numerically by Poulovassilis et al. (1989), as well as experimental, analytical and numerical reference solutions presented by