MOVEMENT OF WATER IN RESTRUCTURED SALINE AND SODIC CLAY TOPSOILS UNDER A RAINFALL SIMULATOR

The nature of water movement through freely draining saturated and fie ld moist aggregates of saline sodic clay topsoil was studied using 200 mm long columns filled with soil aggregates. Water containing tritium as a tracer was supplied either by means of rainfall simulator or dir ectly to the surface of the soil under a negative pressure head of 500 Pa. The proportion of macropore and micropore flow was elucidated. Th e micropores of the aggregates were shown to convey very little water (0.013 mm h(-1)) and hence, even at low rainfall intensities water was expected to move down through the macropores. In practice, at a low w ater application rate of 0.6 mm h(-1) drainage did not begin from the base of the column until the aggregates had become fully saturated due to mobile water in the macropores being continuously absorbed into th e micropores. The results, however, indicated that extensive rapid byp assing does occur at medium and high rainfall intensities (> 2.3 mm (- 1)), with the result that a, large proportion of the water falling on the unsaturated plough layers of clay soils is drained before the tops oil becomes saturated. The soil absorbed water continuously during the application of the equivalent of a wetter than average winter's rain (400 mm), the rate of absorption being directly proportional to the am ount of salt leached. Tritium, used as a tracer, was found to be prefe rentially absorbed by the clay during the leaching process, the concen tration in the soil water rising to 1.8 times that of the applied trit iated water.