Critical analysis of the ®eld determination of soil hydraulic conductivity functions using the ¯ux-gradient approach K. Reichardt 1,a,* , O. Portezan 2,b , P.L. Libardi 1,a , O.O.S. Bacchi 1,a , S.O. Moraes a , J.C.M. Oliveira b , M.C. Falleiros b a CENA/ESALQ, Univ. of Sa Äo Paulo, C.P.96, 13400±970, Piracicaba, SP, Brazil b Dept. of Physics, State Univ. of Londrina, C.P.6001, 86051±970, Londrina, PR, Brazil accepted 9 December 1997 Abstract The determination and the use of soil hydraulic conductivity functions obtained from internal drainage experiments, performed in ®eld soil pro®les, using the Darcy±Buckingham ¯ux gradient approach are discussed. Details of the calculation procedures indicate the sensitivity of the methodologies used, and of the most commonly adopted model for K() relations: K()K 0 .exp[.( 0 )]. It is shown that the unit total water potential gradient cannot prevail in the whole soil pro®le during internal drainage after in®ltration and that, as a consequence of this assumption, several methods of hydraulic conductivity determination yield K() relations which are depth dependent, even for homogeneous soils. The dif®culties of using K() relations in modeling soil water dynamics due to their very strong exponential behaviour are also discussed. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Hydraulic conductivity; Unit gradient 1. Introduction The description of soil water dynamics using the Darcy±Buckingham ¯ux gradient approach involves the determination and the use of soil hydraulic con- ductivity K vs. soil water content  functions. Under ®eld conditions the determination of these K() rela- tions can be performed in previously saturated soil pro®les submitted to internal drainage, covering soil surface to prevent evaporation losses. Some of the earliest contributions on this subject were made by Youngs (1964); La Rue et al. (1968); Black et al. (1969) and Davidson et al. (1969), using Darcy± Buckingham's equation in the form: q K: @h @z  1   (1) where q is the soil water ¯ux density, h is the matric soil water pressure head and z is the vertical position coordinate. The term in brackets is the hydraulic potential head gradient @ H/@ z taken as the sum of the matric potential gradient @ h/@ z and the gravita- tional potential gradient @z/@z1. Davidson et al. Soil & Tillage Research 48 (1998) 81±89 *Corresponding author. E-mail: KREICHAR@CARPA.CIAGRI.USP.BR 1 CNPq fellow. 2 CAPES fellow. 0167-1987/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0167-1987(98)00093-2