The Darling River in drought: where does all the water go and how can isotopes help us find out? Cath Hughes With input from Dioni Cendon, John Gibson, Stuart Hankin, Suzanne Hollins, Karina Meredith and David Stone & the invaluable collaboration of the NSW Dept of Water and Energy FIELD ASSISTANT AT TILPA PUB

Catchment area 650,000 km 2

Climate context Avg Rainfall = 500 mm/yr 1200mm/yr Avg Potential E = 2000 mm/yr Murray Darling Basin Annual Rainfall Murray Darling Basin Annual Rainfall Anomaly (base ) Recent drought years similar rainfall to first half of 20 th century – so why is this drought so severe? Murray Darling Basin Annual Mean T Anomaly (base ) Higher temperatures and potential evaporation Increased water diversions

Past and present river flows DNR NSW Drought Analysis 2006 OBSERVED FLOWS PAST FLOWS MODELLED FOR CURRENT DIVERSION CONDITIONS MEDIAN FLOW MORE THAN HALVED BY DIVERSIONS 5-10 FOLD INCREASE IN DIVERSIONS SINCE 1980’s – – – Natural median annual flow – – – Current median annual flow River flow modelling from the NSW Dept of Water and Energy shows the effect of dams, weirs and pumping for irrigation for last 90 years If all the diversions now in place were there during 1920’s to the 1940’s the flow in the river would have been halved Climate change and variability may affect water supply and demand but human water use and catchment management have a very big impact!

Cotton irrigation near Bourke Water diversions 1650 GL/year water use from catchment 310 GL/year water use from Barwon- Darling River reaches in this study 300 GL storage capacity in shallow dams (max depth <5m) Summer temps high 30’s to mid 40ºC leading to high evaporation rates Thanks to Google Earth Cotton irrigation storage dam

Saline groundwater at Glen Villa weir 19A FISH KILL AT GLEN VILLA

Where do isotopes come into the equation? Isotopes have the same atomic number but different numbers of neutrons in the nucleus – they can be stable or radioactive - natural or anthropogenic. In some water molecules either a 1 H or 16 O atom is replaced by its ‘heavier’ cousin – 2 H or 18 O. Heavy or ‘enriched’ molecules condense or rain out quicker and evaporate slower so they can be used to study hydrological processes. Deuterium or 2 H atoms were formed during the BIG BANG! Abundance 18 O= ppm, 2 H= ppm – expressed as ratio of the standard VSMOW in per mille ‰

So how do isotope ratios change in the hydrological cycle?

(Gibson et al. 2002) Darling Basin stable isotopes & evaporation Extreme evaporative enrichment in Barwon-Darling, tributaries and reservoirs The degree of enrichment can be used to determine how much water has evaporated

GMWL  D = 8  18 O Imagine the river is like a cup of water left out in the sun – in a river it’s not so easy to measure how much water evaporates but using isotopes we can… Evaporation can be seen in one place in time…  18 O (‰) /7/021/7/031/7/041/7/05 Daily discharge (ML) isotope values increase between floods at Wilcannia … or over distance as we travel down river flood front

Water sampling along the Darling River

The Darling River is dying of thirst… Big dams and water extraction for irrigation of cotton and other crops are taking the water the river needs. Weirs along the river allow water to evaporate, contaminants to concentrate and algal blooms to flourish… WHAT EFFECT WILL CLIMATE CHANGE HAVE?

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