Infiltration rate and soil temperature were measured during a 5-d period in a 6.1- by 6.1-m field plot ponded with approximately 40 mm of water. The infiltration rate varied during a 24-h period, with a maximum at approximately 1700 h and a minimum at about 0700 h every day. Variations in infiltration rate approached +/- 30% of the mean rate (14.5-26.9 mm/h) and followed the variations of temperature at the soil surface. Temperatures at the water-soil interface varied from 14.9 to 31.5 degrees C, with a maximum change during a 24-h period of 15.7 degrees C. A conceptual model was proposed to account for the observed fluctuations in infiltration rate. The model proposed a hydraulically restricting surface layer, overlying an unsaturated subsoil of greater permeability. Changes in the hydraulic conductivity of the surface layer, reflecting changes in the water viscosity in response to diurnal temperature changes, caused the infiltration rate to vary throughout the day. A numerical model was constructed and used to demonstrate that the conceptual model could explain most of the observed behavior at this site. Ramifications of temperature-dependent infiltration affect infiltration and hydraulic-conductivity measurements, leakage from irrigation canals, and performance of rapid-infiltration basins.
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