Hydraulic resistance in puddled rice soils: measurement and effects on water movement

A technique is presented to measure in-situ infiltration rate and pressure head gradients within a puddled rice soil. Measurements at 33 sites within a small 0.1 ha plot indicated unsaturated flow conditions in the non-puddled subsoil. The least permeable layer within the profile, as determined by the greatest gradient in pressure head, was found to be at the interface of puddled topsoil (0–15 cm) and non-puddled subsoil. Average thickness of this layer was about 5 cm. Hydraulic resistance of the least permeable layer at all measurement sites was lognormally distributed with an average value of 209 days, and 95% confidence limits at 156 and 262 days. Corresponding average hydraulic conductivity was 0.36 mm day −1 with 95% confidence limits at 0.27 and 0.45 mm day −1 . Measured pressure heads in the subsoil showed good agreement with values predicted from comparison of measured hydraulic resistance in the puddled surface soil and non-puddled subsoil. Puddling reduced infiltration rate by a factor of 500.