Evaluation of the Impacts of Deep Open Drains on Soil Root Zone Salinity at Narembeen in the Wheatbelt of Western Australia

The present extent of salt-affected land, one million ha in the wheatbelt of Western Australia, is expected to increase to between three and five million ha if current trends continue. Drains, as an engineering option to mitigate dryland salinity, have been trialled in many catchments in the wheatbelt but few formal evaluations of their impacts on root zone salinity have been carried out. This research quantifies the impacts of deep open drains on root zone salinity in the drained areas of the Wakeman sub-catchment near Narembeen. Four sites were selected, two in the drained areas (Latham and Pini) and two in the proposed drain areas (Bailey and John Deluis). Binannual soil sampling, up to 1.6 m depth at several distances from the drain (up to 200 m), was carried out for four years to determine root zone salinity and pH profiles. The study found that the soil root zone salinity remained low (below thresholds for barley and wheat) at the drained sites throughout the four year monitoring period. Pre-drain soil surface layer salinity was very high during summer at both the Bailey and John Deluis sites. After drain construction, the soil surface layer salinity decreased and pH increased. The improvement in the root zone salinity was observed up to 100 m away from the drain. The results suggest that deep open drains improve soil surface layer salinity due mainly to leaching of the salts following watertable reductions and significant rainfall events. Results are confounded by climate variability and further analyses are planned.