| Popis: |
The falling-head cased borehole analysis of Philip for determining the field-saturated hydraulic conductivity ( K fs ) and sorptive number (α*) in the vadose zone was extended and assessed. The extension allows several water discharge configurations (vertical, radial, combined vertical and radial), variable length ( L ) and radius ( a ) of outflow screen, variable reservoir radius, and use of Excel Solver to calculate K fs and α* via numerical optimization. The assessment used simulated borehole drawdown curves from HYDRUS-2D to determine the accuracy with which the Philip and extended solutions determined K fs and α*. Using the Philip “flow efficiency correction,” C P = π 2 /8, and “gravity factor,” G P = equivalent sphere radius r 0 (maximum gravity effect), caused the Philip and extended solutions to overestimate K fs or α* by amounts ranging from a few percent in strong-capillarity soils to several orders of magnitude in weak-capillarity soils. Replacing C P with C E = 1 (no flow efficiency correction) and G P with G E = 0 (negligible gravity effect) in the extended solution resulted in accurate K fs determinations (≤20% error) regardless of discharge geometry, screen configuration, drawdown range, or soil capillarity. Consistently accurate determination of α* required use of the C E and G E coefficients, 1 ≤ L / a ≤ 4, and avoidance of small borehole drawdowns. Solver-based determination of K fs and α* using several heads distributed along the drawdown curve was less sensitive to measurement error and lack of data-model fit than the two-head approaches of Philip and others. The extended analysis improves the accuracy and utility of the falling-head cased borehole permeameter. |
