SOIL-MACROPORE AND LAYER INFLUENCES ON SATURATED HYDRAULIC CONDUCTIVITY MEASURED WITH BOREHOLE PERMEAMETERS

The borehole permeameter technique can produce erratic saturated hydra ulic conductivity (K(sat) in soils with macropores and abrupt layers b ecause operating theory assumes homogeneous and isotropic conditions. Dye application during an infiltration test demonstrated water movemen t in macropores and erratic K(sat) or matric flux potential (phi(m)). To evaluate the effects or micropores, cracks, and layered soil on K(s at) and phi(m), a finite element solution of the Richards equation was used to simulate infiltration from a borehole (0.03-m radius and 0.50 m deep) with a constant head (H) of 0.05 or 0.10 m. Borehole infiltra tion (for 2 h) was simulated for a Rozetta silt loam (fine-silty, mixe d, mesic Typic Hapludalf) with four configurations: homogeneous, layer ed, a cylindrical macropore centered at the borehole base, and a crack intersecting the borehole wall. Simulated flow rates were increased b y 29% (with H = 0.05 m) and 21% (with H = 0.10 m) when a cylindrical m acropore (4 mm by 0.10 m) was located at the borehole base. Respective increases were 25% (H = 0.05 m) and 20% (H = 0.10 m) when a crack ext ending 0.1 m laterally intersected the borehole wall. Three methods we re tested for calculating K(sat). The simultaneous-equations approach (SEA) using either the Guelph or the Philip model for a homogeneously configured borehole estimated K(sat) within a factor of 2 from input K (sat), but the Laplace analysis method overestimated input K(sat) by a factor of 5 to 12. The fixed alpha value (alpha = K(sat)/phi(m)) meth od with either the Guelph or Philip model estimated K(sat) close to in put K(sat) when a proper alpha value was chosen, but the proper alpha value differed by soil and model. A negative K(sat) was computed using the SEA with the Guelph model when macropores intersected the base of a borehole; negative K(sat) or phi(m) were produced when cylindrical macropores laterally intersected the borehole wall, depending on the v ertical locations of the macropores. Soil with layered hydraulic prope rties also produced unrealistic K(sat).