Deep drainage and soil salt loads in the Queensland Murray-Darling Basin using soil chloride: comparison of land uses

Increases in deep drainage below the root-zone can lead to secondary salinity. Few data were available for drainage under dryland cropping and pastures in the Queensland MurrayDarling Basin (QMDB) before this study. Modelled estimates were available; however, without measured drainage these could not be validated. Soil chloride (Cl) mass-balance was used to provide an extensive survey of deep drainage. The method is 'backward-looking' and can detect low rates of drainage over longer times. Soil Cl and other soil properties were collated for a number of soils, mostly Vertosols and Sodosols, for paired native vegetation, cropped and sometimes pasture sites, from historical data and new soil sampling. Large amounts of salt and Cl had accumulated under native vegetation (Cl mean 25t/ha, range 654, in 2.4m depth), due to low rates of drainage. Steady-state Cl balances for native vegetation gave average drainage of 1.2mm/year at wetter, eastern sites and 0.3mm/year for Sodosols and Grey Vertosols in drier, western areas. Chloride profiles were mostly of a shape indicating matrix/piston flow. One site (Hermitage fallow trial) appeared to be affected by diffusion of Cl to a watertable. The Cl profiles from 14 longer term cropping sites (18-70 years), mainly used for winter cropping/summer fallow, indicate: (i) large losses of Cl since clearing (mean 50%, range 13-85% for 01.5m soil); and (ii) drainage rates from transient Cl balance are a relatively low percentage of rainfall but are considerably higher than under native vegetation. Drainage averaged 8mm/year and ranged from 2 to 18mm/year. This variation is partly explained by rainfall (R = 0.63) (500-730mm/year) and soil plant-available water capacity (R = 0.77) (80-300mm). Deep drainage increases with increasing rainfall and with decreasing available water capacity. Drainage under pasture was less than under cropping but greater than under native vegetation. The deep drainage water (leachate) was of poor quality and will increase salinity if added to good quality groundwater. Leachate at nine sites was too saline to be used (undiluted) for irrigation (>2500mgCl/L) and was marginal at the remainder of sites (∼800mg Cl/L). Cropping areas in the QMDB have the precursors for secondary salinity developmenthigh salt loads and an increase in drainage after clearing. The Vertosols and Sodosols studied occur in 90% of croplands in the QMDB. Salinisation will depend on the properties of the underlying regolith and groundwater systems