A new application of the upward infiltration method to quantify preferential flow hydraulic conductivity on undisturbed soil cores

Abstract

Soil water preferential flow, PF, is the phenomenon where water moves along certain pathways, while bypassing a fraction of the porous matrix. This work presents a laboratory procedure to estimate the hydraulic conductivity of the soil water preferential flow, KPF. Using an upward infiltration curve followed by an over-pressure step, KPF is calculated as the difference between the total hydraulic conductivity, KT, measured with the constant head method, and the soil matrix hydraulic conductivity, Ks, calculated from the inverse analysis of the transient upward infiltration curve. Previous to this analysis, a new approach to detect and remove the effect of possible overpressures due to the soil core placement on the sorptivimeter on the hydraulic properties estimate was presented. This consisted on searching the best fitting between the experimental infiltration curve after removing successive early time steps and the corresponding simulated function. The method to estimate KPF was validated on four different 5-cm height soil columns packed with 2-mm sieved clay loam and loam soils traversed longitudinally by a closed/opened pipe of 1.5 mm and 2.5 mm internal diameter, respectively. This same experiment was also repeated on the same soils without artificial preferential channel. Next, the method was applied on 20 undisturbed soil cores collected from two fields with different grazing intensity and soil tillage management (conventional, CT, reduced, RT and no-tillage, NT, practices). Results showed that correction made to remove the infiltration curve jump due to an overpressure step allowed more accurate and realistic estimates of the soil matrix hydraulic properties. On the other hand, both KT and Ks, which could be satisfactorily calculated from a single upward infiltration curve followed by an overpressure step, where satisfactorily employed to quantify KPF. While no significant effect of grazing intensity on KPF was observed, the CT KPF was significantly larger than those obtained in RT and NT.