The objective of the present work is to investigate the flux&ndash, concentration (F(&Theta, )) relation, where &Theta, is the normalized soil volumetric water content for the case of one-dimensional horizontal flow, subject to constant concentration conditions. More specifically, the possibility of describing F(&Theta, ) by an equation of the form F(&Theta, ) = 1 &minus, (1 &minus, &Theta, )p+1 is examined. Parameter p is estimated from curve-fitting of the experimentally obtained &lambda, (&Theta, ) data to an analytic expression of the form (1 &minus, )p where &lambda, is the well-known Boltzmann transformation &lambda, = xt&minus, 0.5 (x = distance, t = time). The results show that the equation of (1 &minus, )p form can satisfactorily describe the &lambda, ) relation for the four porous media tested. The proposed F(&Theta, ) function was compared with the limiting F(&Theta, ) function for linear and Green&ndash, Ampt soils and to the actual F(&Theta, ) function. From the results, it was shown that the proposed F(&Theta, ) function gave reasonably accurate results in all cases. Moreover, the analytical expression of the soil water diffusivity (D(&Theta, )) function, as it was obtained by using the equation for &lambda, ) of the form (1 &minus, )p, appears to be very close to the experimental D(&Theta, ) data (root mean square error (RMSE) = 0.593 m2min&minus, 1).