Field Testing Parameter Sensitivity of the Two-Term Infiltration Equation Using Differentiated Linearization

Knowledge of soil hydraulic conductivity in the vadose zone is important in many agronomic, engineering and environmental areas. Transient tension infiltrometer experiments can be used to estimate the hydraulic conductivity, K0, corresponding to a given pressure head by the single-test (TST) method that uses the coefficients C1 and C2 of the two-term infiltration equation. The “Differentiated Linearization” (DL) method has been proposed to estimate these coefficients when a layer of contact material is used for the experiment. A field test of the DL and TST methods was conducted on a sandy loam and a clay soil. Eliminating the early-time influence of the contact layer was easy when the sorptivity of the contact material was 10-12 times higher than the soil sorptivity. In the other cases, a transition zone, complicating application of the DL method, appeared between the decreasing and increasing portions of the data set. Therefore, applicability of the DL method required high differences in capillary forces between the contact material and the soil. The estimates of K0 varied by up to 650% with the duration of the experiment and by less than 50% with the time interval between readings at the water reservoir. Sensitivity of K0 to the experiment duration was particularly remarkable in the sandy loam soil for short durations. Considering a minimum duration of the experiment of approximately an hour caused estimates of K0 to vary by a maximum of 40% with the duration of the experiment.