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

Tolmie, P. E., Silburn, D. M. and Biggs, A. J. W. (2011) Deep drainage and soil salt loads in the Queensland Murray-Darling Basin using soil chloride: comparison of land uses. Soil Research, 49 5: 408-423. doi:10.1071/SR10172


Author Tolmie, P. E.
Silburn, D. M.
Biggs, A. J. W.
Title Deep drainage and soil salt loads in the Queensland Murray-Darling Basin using soil chloride: comparison of land uses
Journal name Soil Research   Check publisher's open access policy
ISSN 1838-675X
1838-6768
Publication date 2011-07
Sub-type Article (original research)
DOI 10.1071/SR10172
Volume 49
Issue 5
Start page 408
End page 423
Total pages 16
Place of publication Collingwood, VIC, Australia
Publisher C S I R O Publishing
Collection year 2012
Language eng
Formatted abstract
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 Murray–Darling 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 25 t/ha, range 6–54, in 2.4 m depth), due to low rates of drainage. Steady-state Cl balances for native vegetation gave average drainage of 1.2 mm/year at wetter, eastern sites and 0.3 mm/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 0–1.5 m 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 8 mm/year and ranged from 2 to 18 mm/year. This variation is partly explained by rainfall (R2 = 0.63) (500–730 mm/year) and soil plant-available water capacity (R2 = 0.77) (80–300 mm). 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 (>2500 mg Cl/L) and was marginal at the remainder of sites (~800 mg Cl/L). Cropping areas in the QMDB have the precursors for secondary salinity development—high 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.
Keyword Cracking clay
Cropping
Dryland salinity
Native vegetation
Pasture
Steady-state mass-balance
Transient mass-balance
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: Official 2012 Collection
School of Agriculture and Food Sciences
 
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