Uncertainty of Hydraulic Conductivity under Field Conditions and at Fixed Pressure Heads and Water Contents

Unsaturated hydraulic conductivity ( K ) can be expressed as a function of soil water content (θ) or pressure head ( h ). Methods to determine K often give highly variable results, thus making it difficult to report values for K in a reliable manner. Due to the sensitivity of pore conductivity to pore radius, it can be expected that K expressed as a function of h shows less variability than expressed as a function of θ. To test this hypothesis, K was determined at several values of θ and h using the instantaneous profile method at 48 locations in the surface and a subsurface layer of a Typic Hapludox in Brazil. Results were analyzed considering the field values of K at the time of observation ( K t ), at fixed values of θ ( K θ ), and at fixed values of h ( K h ). Results show that the coefficient of variation of K θ is higher than that of K h , especially in the surface layer, which suggests that, if one has the choice, it would be preferable to estimate soil water movement using measured h values, and using K ( h ), instead of measuring θ and using K (θ). We also show that the variability in K t increases as the soil gets drier, while K θ and K h show more variability for wetter soil.