ATS2545 Lecture Notes - Lecture 13: Murray–Darling Basin, Murray Basin, Groundwater Recharge
25 May 2018
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Lecture 13 - Murray Basin: groundwater recharge and origin of saline
groundwater
Management Problems in the Murray Darling Basin
• Great reliance on groundwater
• Disturbance of groundwater caused by loss of native, deep-rooted vegetation
adapted to the natural environment
• 60% of Australia’s water use is in MDB
• 75% of all irrigation water used is in MDB
• ~75% of all MDB river flow is diverted for us
• Major issue: sustainability of groundwater
Groundwater Recharge in Dry Climates
• Drylands are at same time most reliant on groundwater and pose the greatest
measurement difficulties
• Recharge can be found from data on the rainfall (input) and evapotranspiration
loss (output)
• Rainfall is low and very variable
• Evaporative losses are very large
o Difference between theme: only positive water supplies vary briefly
during and after wet weather
o Actual recharge: only mm per year (very difficult to measure)
▪ Yearly average figures (mean rainfall, mean evaporation) are
too crude for estimation of groundwater recharge
▪ Rather, data would have to be derived during major periods of
rain
• Patterns of water arriving at ground depend on: vegetation and Stemflow
effects on rainfall above the canopy and litter layers
• We need a way to analyse recharge over longer periods of time - decades
Solutes as Tracers in Hydrology
• Chloride flux used to estimate groundwater recharge rate, R
o Common tool: follow water movement in landscape using a tracer
• Chloride ion, Cl-, can be used since it arrives in rain (from salt condensation
nuclei)
o Rare in soils and rocks
• Data
o Annual rainfall, P
▪ Chloride ion concentration in rain (average) Cp chloride
concentration in groundwater Cr
• Analysis
o Assume that flux to the ground surface all passed downward through
the soil
o P x Cp = R x Cr
o Example:
▪ At Walpeup in the mallee, increasing amounts of Cl- in
groundwater samples taken at depth, reaching 9-12 kg m-3
▪ Maximum concentration was found 1-2 m deeper beneath
cropland than under intact mallee
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