Tension Infiltrometers: Correcting for Small Diameter to Obtain Accurate Sorptivity and Hydraulic Conductivity Values

Abstract:

Because tension infiltrometers apply water through a disk of finite size, the subsurface flow they produce is not one-dimensional. For disks larger than about 200 mm in diameter, only a small portion of the applied water goes to lateral spreading, so it can be accounted for by one of several commonly-used formulas that correct for the multidimensional effects. For small disks, however, lateral spreading constitutes a large fraction of the total flow, and these formulas are no longer valid. The calculated values of sorptivity, S, and hydraulic conductivity, K, are inaccurate, sometimes overcorrected so severely as to produce a negative number for K. Measurements from small infiltrometers therefore require a different correction for lateral spreading of infiltrated water.

We developed several new formulas to produce S and K values from measured or simulated cumulative infiltration. Importantly, the new formulas only require information from measurements of cumulative infiltration and soil moisture content, independent of soil type or other characteristics not known from the infiltrometer measurements. To test them we conducted numerical experiments using the Richards’ equation-based code VS2DRTI to simulate disk infiltration for diverse media and a range of disk sizes, including the widely-used 45-mm diameter. We compared hydraulic property values calculated from simulated infiltration with formulas from Haverkamp et. al. (1994), Hussen and Warrick (1993), and a modified version of Zhang’s (1997). For comparison, we also calculated both S and K from the parameterized properties that were supplied to the VS2DRTI code. The best-performing formulas use a multiplicative factor to correct the cumulative infiltration for finite disk size, thereby producing results that approximate the ideal case of an infinite disk. These in turn allow S and K to be computed with standard one-dimensional infiltration equations. The results provide an algorithm based in unsaturated-flow theory that produces reliable values even for small disks.