The Brigalow Catchment Study revisited: Effects of land development on deep drainage determined from non-steady chloride profiles

SummaryA large area of woodland in the brigalow bioregion in semi-arid central Queensland was cleared for agriculture from the 1960s to the 1980s. To assess the risk of salinity associated with land clearing, soil chloride (Cl) was monitored at the Brigalow Catchment Study (BCS), in brigalow (Acacia harpophylla) scrub, a cropped catchment and a pasture catchment before and after clearing in 1982. The monitoring sites include three landscape positions, two on clay soils and one on a sodic duplex (Sodosol), within each catchment. An earlier report of deep drainage, using the early soil Cl profiles and steady-state and transient chloride (SODICS) mass balance, was revisited after a further 13 yr and four more sampling times.Profile Cl mass changed little in 18.4 yr at scrub sites, justifying the use of native vegetation sites to represent pre-clearing Cl for paired cleared sites. Steady-state Cl mass balance (CMB) gave deep drainage of 0.13–0.34 mm/yr for nine pre-clearing scrub sites. Large losses of soil Cl occurred under cropping and smaller losses occurred under pasture. Transient CMB gave average deep drainage of 59 and 32 mm/yr for crop and pasture catchments, respectively, during the development phase (1981–1983) when the land was bare following clearing of native vegetation and prior to establishment of crops or pastures. In the 16.7 yr following establishment of agricultural land uses (1983–2000), transient CMB gave average deep drainage of 19.8 (range 3.3–50) and 0.16 (−2.2 to 1.4) mm/yr, respectively, in crop and pasture catchments. The drainage rate under pasture was similar to that under brigalow scrub. In the cropped catchment, drainage for modern farming systems (less tillage, more summer/opportunity crops) was about half that of older farming systems (wheat-summer fallow, more tillage, less stubble retention). Drainage was greater for the Sodosol than for the clay soils under cropping. Deep drainage occurred under cropping even though the soils are considered to have low permeability and the climate is semi-arid, with potential evaporation exceeding rainfall, on average, in all months.Increased drainage at cropped sites has driven a clear exponential loss of soil Cl, as predicted by the transient CMB theory. One cropped site is at or near a new steady-state and the others will reach a new steady-state 50–200 yr after clearing. The leachate is saline with an average Cl concentration of 7000 mg/L and would salinised any groundwater it entered. The salinity risk associated with the drainage is not well understood as yet and will depend on local hydrogeological conditions, which are poorly mapped in the Fitzroy. Effects of these losses of salts on sodicity, soil structure and permeability should also be investigated.