An assessment of borehole infiltration analyses for measuring field-saturated hydraulic conductivity in the vadose zone

Single-borehole infiltration tests are widely used for in-situ measurement of field-saturated hydraulic conductivity, Kfs, in the vadose or unsaturated zone. United States Bureau of Reclamation (USBR) methods are used extensively in geologic and water management engineering to analyse single-borehole infiltration data, while borehole permeameter (BP) methods are preferred in the environmental and agricultural sciences. Little is known about how well the USBR and BP analyses correspond; hence, the objectives of this study were to compare and provide usage recommendations for some of the most common constant head and falling head USBR and BP analyses in terms of range of validity and accuracy for Kfs determination. HYDRUS-2D numerical simulations of variably saturated, axisymmetric flow for a range of specified Kfs values, porous media capillarities (as represented by sorptive number, α*, and change in water content, ∆θ) and outflow geometries (as represented by length, L, and radius, a, of borehole discharge zone) were used to generate perturbation-free borehole test data for use in the USBR and BP analyses. The numerically generated test data consisted of steady borehole discharge rate, QS, versus steady ponded head, H, for the constant head borehole tests, and ponded head, Ht, versus time, t, for the falling head borehole tests. The constant head USBR estimates of Kfs were generally accurate (≤ 25% error) when H/a ≥ 10, α* ≥ 12 m− 1, a ≥ 10 cm and L/a ≥ 10, but could overestimate or underestimate by more than an order of magnitude when the above parameters were outside those ranges. The falling head USBR analysis appeared to always overestimate or underestimate Kfs (> 25% error), and the overestimates could be more than an order of magnitude when early-time Ht vs. t data were used. The constant head and falling head BP analyses, on the other hand, provided accurate Kfs estimates (≤ 26% error) for all test scenarios. The BP analyses were therefore recommended for all tested borehole configurations and porous medium characteristics, while the USBR analyses were recommended only for constant head tests when H/a ≥ 10, L/a ≥ 10, α* ≥ 12 m− 1, and a ≥ 10 cm.