METHODS FOR ANALYZING CONSTANT-HEAD WELL PERMEAMETER DATA

The constant-head well permeameter has proven to be a useful and versatile instrument for determining the in situ hydraulic properties of soils in the unsaturated (vadose) zone. The flow measurements are obtained under conditions of saturated-unsaturated, three-dimensional flow in the unsaturated zone. As a consequence, the steady-state flow rate out of the permeameter is determined by both the field-saturated hydraulic conductivity (K(fs)) and the matrix flux potential (phi(m)) of the unsaturated soil. Because both K(fs) and phi(m) contribute to the flow, calculation of these parameters from well permeameter data requires either the solution of two (or more) simultaneous equations, or reduction of the problem to one equation in one unknown if additional information is known or estimated. Use of the simultaneous-equations approach in heterogeneous soils can result in a high percentage of invalid (i.e., negative) K(fs) and phi(m) values. Negative results can be avoided and good estimates obtained, however, by using an independent measurement or site estimate of the ratio alpha* = K(fs)/phi(m).