Field studies of water and salt movement in an irrigated swelling clay soil. II. Profile hydrology during ponding

Two plots on a saline sodic cracking clay soil, to one of which gypsum was applied at 10 t/ha, were instrumented to 4.5 m from a pit, in order to observe wetting patterns during extended inundation. On the gypsum-treated plot the wetting front was diffuse. Water content increased simultaneously within quite large depth intervals, and two distinct wetting phases were detected to a depth of 2.7 m. The change in water potential with time showed the same pattern. Once steady-state flow had been reached, tensiometer-pressure potentials were positive except between 0.55 and 1.50 m. In contrast, the untreated soil exhibited a better defined wetting front which moved slowly down the profile. Only in the upper 0.55 m were two distinct wetting phases discernible; tensiometer-pressure potentials were generally lower than for the gypsum-treated soil, and remained negative throughout the ponding period except at 0.25 m. This behaviour, and the estimated values of hydraulic conductivity, suggest that, in the ameliorated soil, water penetrated and moved through the profile mainly in macropores associated with structure, and that, in the unameliorated soil, such flow was largely prevented by a region of low hydraulic conductivity between 0.25 and 0.55 m.