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Soil-Macropore and Layer Influences on Saturated Hydraulic Conductivity Measured with Borehole Permeameters

Soil Science Dep., Univ. of Minnesota, St. Paul, MN 55108

J. B. Swan

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

J. L. Nieber

Dep. of Agricultural Engineering, Univ. of Minnesota, St. Paul, MN 55108

R. R. Allmaras

USDA-ARS, Soil Science Dep., Univ. of Minnesota, St. Paul, MN 55108

*Corresponding author.

ABSTRACT

The borehole permeameter technique can produce erratic saturated hydraulic conductivity (K_sat) in soils with macropores and abrupt layers because operating theory assumes homogeneous and isotropic conditions. Dye application during an infiltration test demonstrated water movement in macropores and erratic K_sat or matric flux potential (_m). To evaluate the effects of macropores, cracks, and layered soil on K_sat and _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 infiltration (for 2 h) was simulated for a Rozetta silt loam (fine-silty, mixed, mesic Typic Hapludalf) with four configurations: homogeneous, layered, a cylindrical macropore centered at the borehole base, and a crack intersecting the borehole wall. Simulated flow rates were increased by 29% (with H = 0.05 m) and 21% (with H = 0.10 m) when a cylindrical macropore (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 extending 0.1 m laterally intersected the borehole wall. Three methods were 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 value ( = K_sat/_m) method with either the Guelph or Philip model estimated K_sat close to input K_sat when a proper value was chosen, but the proper 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 _m were produced when cylindrical macropores laterally intersected the borehole wall, depending on the vertical locations of the macropores. Soil with layered hydraulic properties also produced unrealistic K_sat.

Received for publication January 14, 1992.