SHALLOW SUBSURFACE DRAINAGE IN AN IRRIGATED VERTISOL WITH A PERCHED-WATER-TABLE

Waterlogging and salinity are reducing the productivity of irrigated a griculture on clay soils in south east Australia. We compared five dra inage treatments: (1) undrained control (Control); (2) mole drains (Mo le); (3) mole drains formed beneath gypsum-enriched slots (GES) (Mole + GES); (4) shallow pipe drains installed beneath GES (Shallow Pipe); (5) deep pipe drains (Deep Pipe). The experiment was set out on a vert isol and our measurements were made during the growth of an irrigated onion crop. Over the 3 months before the spring irrigations commenced, the perched water table on the Control was less than 400 mm below the soil surface for 27% of the time, whereas the shallow drainage treatm ents (Treatments 2, 3 and 4) reduced this time to less than 4%. During the irrigation season, the perched water table on the Mole + GES trea tment rose above 400 mm for 3% of the time. The perched water table on the Mole treatment was above 400 mm for 14% of the time, compared wit h 19% of the time on the Control. The Deep Pipes were less effective i n reducing the depth to the perched water table, both before and durin g the irrigation period. Mole drains increased the gas-filled porosity above the drains. However, the gas-filled porosity remained below rep orted levels for optimum root growth. Although the drains effectively drained excess water, and lowered the water table, the hydraulic gradi ent was insufficient to remove all of the water from the macropores. G ypsum enriched slots above the mole drains increased the gas-filled po rosity in the slots but the drainable porosity in the undisturbed soil appeared to be inadequate for optimum root growth, even though some d rainage occurred near the slots. Discharge from the shallow drainage t reatments averaged 58 mm for each irrigation, and was considerably mor e than the amount required to drain the macropores. The mole channels were in reasonably good condition at the end of the irrigation season, with at least 70% of the cross-sectional area of the channel open. Sh allow subsurface drains increased onion yield by about 38%. For each d ay the water table was above 400 mm, the yield declined by 0.23 tonnes per hectare. Farmer adoption of shallow subsurface drainage will depe nd on the long-term economic benefits (influenced by the longevity of the mole channels and yield response) and the need to develop more sus tainable management practices.