An internal drainage experiment was conducted to examine soil water content variability in space and in time. It is shown that the flux-gradient model used to describe water flow in field soil profiles, based on the Darcy-Buckingham equation, yielded results of extreme variability and questionable validity. The problem lies in the representativehess of a site due to soil variability, horizontally and vertically in time, which added to the character of the hydraulic conductivity versus soil water content relation, which in many cases can be approximated by exponential functions, leads to coefficients of variation up to 170% in the estimation of soil hydraulic conductivity values.
Falleiros, M. C., Portezan, O., Oliveira, J. C. M., Bacchi, O. O. S., & Reichardt, K. (1998). Spatial and temporal variability of soil hydraulic conductivity in relation to soil water distribution, using an exponential model. Soil and Tillage Research, 45(3–4), 279–285. https://doi.org/10.1016/S0933-3630(97)00021-4