HYDRAULIC CONDUCTIVITY MEASUREMENTS OF THE SPATIAL VARIABILITY OF A LOAMY SOIL

The spatial variability of the parameters K(fs) and alpha of the exponential form of the hydraulic conductivity given by K(psi) = K(fs) exp (alpha psi) where psi is the water pressure head in the soil has been examined on a bare agricultural soil, by performing surface infiltration measurements with the Guelph Pressure Infiltromer. Thirty two measurements were made at each node of a 4 m x 8 m grid. Both the early-time transient and steady-state flow rates, for three hydraulic heads (6, 16 and 25.5 cm) imposed at the soil surface have been analyzed. It is shown that the best estimates of the field-saturated hydraulic conductivity, K(fs) and of the alpha parameter (alpha) are given by calculations based on the steady-state regime simultaneously applied to the three heads. The results indicate that both, K(fs) and alpha are better described by a lognormal than a normal distribution. They also show that the parameters are autocorrelated up to about 25 m and to 20 m respectively and negatively spatially correlated together within a distance of 24 m. These findings tend to invalidate some assumptions classically used in the stochastic models of unsaturated water flow in porous materials.