Infiltration and water movement in an in situ swelling soil during prolonged ponding

Abstract

Infiltration, swelling, and water movement were studied during ponding on a swelling clay soil. The soil was uniform in texture and clay mineralogy to 2 m depth. Most structural heterogeneity, caused by gilgai and shrinkage cracks, had been removed by grading, cultivation, and pre-ponding irrigations. Measurements were made of infiltration, moisture content, soil water potential, hydraulic conductivity, bulk density, vertical soil swelling, and the effect of overburden on tensiometer readings. Infiltration was rapid and water penetrated deeply during the first ponding day. Thereafter, qualitative agreement was found between measured infiltration and that expected from theory from 1 to 45 days after ponding. From 45 to 120 days after ponding, the development of a time-variable flow restriction near the soil surface prevented the attainment of a final, steady infiltration rate. During ponding a transient water table developed, moisture profiles were distinctly hydric, and seepage to a deep water table or aquifer was not negligible. Core sample values of hydraulic conductivity agreed with those obtained from mean flux and potential gradients, although conductivity and infiltration rate varied greatly from place to place. Measured swelling compared favourably with that calculated from bulk density changes. The maximum measured soil swelling, in the rather narrow range of moisture contents involved, was 25 mm. This is consistent with reported data on similar soils. Mean values of á = Ù/P near saturation at 0.2 and 0.4 m depth were between 0.20 and 0.25, indicating that the effect of overburden potential on flow was not large.