Measuring Soil Hydraulic Properties Using a Falling-Head Borehole Permeameter.

An enhanced falling-head analysis is presented for measuring field-saturated hydraulic conductivity (K_fs) and sorptive number (α*) using cased boreholes finished in the unsaturated zone.  The analysis extends the original vertical discharge solution of Philip (1993) to include a “well screen” of variable length (L) and radius (a), and vertical, radial and combined vertical-radial discharge options.  Numerical optimization via Excel “Solver” is used to calculate K_fs and α* from two or more head vs. time data points.  HYDRUS-2D simulations of falling-head borehole infiltration are used to show that the Philip (1993) “flow efficiency correction”, C = π²/8, and “gravity factor”, G = r₀, should be replaced with C = 1 (no flow correction) and G = 0 (negligible gravity effect).  The enhanced solution determines K_fs with ≤ 20 % error for all discharge geometries, and it determines α* with ≤ 20 % error provided that 1 ≤ L/a ≤ 4.  The new enhanced analysis improves the accuracy and utility of the falling-head cased borehole permeameter.