Impact of Infiltration Rate on Residual Air Distribution and Hydraulic Conductivity

Abstract

The dynamics of water infiltration into soil have a strong influence on the subsequent distribution of air trapped inside pores. We present results of two infiltration experiments conducted on an artificially prepared sample under ponding and drip irrigation boundary conditions, with concurrent neutron imaging of the sample. A cylindrical sample was packed with two grades of sand and disks of fine porous ceramic in an axially symmetrical geometry. The configuration of the sample provided a number of interfaces between regions of higher and lower hydraulic conductivity. Infiltration was started in dry media. The bottom boundary condition was seepage face. Water was applied on the sample surface during the experiment with drip irrigation at a water application rate about one order of magnitude lower than the minimum flux reached during the ponding experiment. Despite this low application rate, ponding eventually occurred on the top of the sample due to an unexpectedly low infiltration rate. Neutron tomographic imaging revealed massive air entrapment in the coarse sand regions of the sample during slow infiltration under drip irrigation conditions. In contrast, during the ponded infiltration experiment, the air was mostly flushed out from the coarse sand regions by gravity‐driven water flow due to greater hydraulic head. Neutron imaging showed that the capillary barrier effect, air entrapment, and entrapped air redistribution were responsible for the observed low infiltration capacity of the sample during the slow‐infiltration‐dominated drip irrigation experiment. It is reasonable to assume that similar phenomena can occur in natural soils having highly heterogeneous structures.