Laboratory evaluation of falling-head infiltration for saturated soil hydraulic conductivity determination

Falling-head one-dimensional infiltration procedures, such as the simplified falling-head (SFH) technique, yield estimates of saturated soil hydraulic conductivity, K_s, with parsimonious and rapid experiments. Factors that can influence determination of K_s by the SFH technique were tested in the laboratory on three repacked soils differing by particle diameter ranges (0-2000, 0- 105 and 105-2000 mm, respectively). Using the theoretically calculated depth of ponding on the infiltration surface, D, instead of the measured one had a small impact on the K_s calculations (means differing by a factor of 1.1-1.2, depending on the soil). For the finest soil, K_s decreased by 3.1 times as D increased from 40 to 135 mm but D did not affect K_s for the coarsest soil, yielding in general the highest K_s values. The abrupt increase of the infiltration rate close to the end of the run did not influence appreciably K_s calculations since it determined an increase in K_s by a mean factor never exceeding 1.1. The most frequent result of the developed procedure for estimating the α* parameter was failure of the experiment although the valid α* calculations were plausible, being higher for the coarse textured soil (17 m^–1) than the finer soils (9.2-9.3 m^–1). The depth of the wetting front at the end of the run was 1.1-1.2 times deeper than that calculated theoretically before the run, depending on the soil. In conclusion, the method used to determine D should not affect very much K_s determination but larger D values can yield smaller K_s values in fine-textured soils. Air escapes from the sampled soil volume when almost all water had infiltrated but this circumstance does not have a great impact on calculation of K_s. A falling-head one-dimensional ponded infiltration process is not recommended to estimate α*. The theoretical depth of the wetting front can approximately be predicted before the run. The SFH technique appears a rather robust method to simply and rapidly determine K_s.