A FIELD ASSESSMENT OF THE SIMPLIFIED FALLING HEAD TECHNIQUE TO MEASURE THE SATURATED SOIL HYDRAULIC CONDUCTIVITY

Abstract

The Simplified Falling Head (SFH) technique to measure field saturated soil hydraulic conductivity, Kfs, has received little testing or comparison with other techniques. Different experiments were carried out to i) determine the effect of ring size on the measured conductivity; ii) compare the SFH and Pressure Infiltrometer (PI) techniques in a clay loam soil; and iii) evaluate the approach used in the SFH methodology to estimate the * parameter. Sampling a relatively large number of sites allowed to detect statistically significant relationships between the Kfs values obtained with rings differing in diameter (0.15 and 0.30 m, respectively). The ring size effect was substantial (factor of discrepancy between Kfs results of more than an order of magnitude) for low Kfs values (~ 2 mm h-1) but it was practically negligible (factor < 1.7) for high conductivities (Kfs > 350 mm h-1). Data supporting the hypothesis that the detected ring size effect was an effect of the different total area sampled with the two rings were obtained. A test carried out with a small ring contained enough information to approximately predict the Kfs value that would be obtained with a larger ring. The SFH and PI techniques yielded statistically similar means of Kfs but substantially different coefficients of variation, that was particularly high for the SFH technique. The duration of the infiltration run, appreciably shorter for the SFH than the PI technique with the Two Ponding Depth approach, probably influenced swelling phenomena of the field soil during the run and this circumstance determined the detected discrepancies. The two techniques should be considered complementary, being usable to determine Kfs at the beginning (SFH) and at a later stage (PI) of a ponding infiltration process. Using * values directly measured by the tension infiltrometer or estimated on the basis of a general description of soil characteristics did not affect significantly the results of the SFH technique. Therefore, the approximate criterion to estimate * was appropriate. In conclusion, this investigation gave support to the use of the SFH technique for a rapid and reasonably simple approximate determination of Kfs. Developments should consider a transient, three-dimensional infiltration process established by a ring inserted a short distance into the soil.