Evaporative losses from the Darling River, NSW, during drought from 2002-2005

Hughes, CE; Meredith, KT; Cendón, DI; Hollins, SE

Evaporative losses from the Darling River, NSW, during drought from 2002-2005

Date

2009-12-03

Authors

Publisher

Australasian Environmental Isotope Conference

Abstract

The Darling-Barwon River system faces extreme pressure from drought and over extraction of water from its catchment. Isotopic tracers can be used to assess water gains and losses and to distinguish evaporative losses from groundwater recharge or pumping. Stable water isotope (δ2H and δ18O) and major ion data was collected monthly at nine stations from Mungindi, near the Queensland border, to Burtundy, above the confluence with the Murray River, between 2002 and 2005, as part of the Global Network for Isotopes in Rivers (GNIR) monitoring programme [1]. This data has been used to investigate groundwater exchange, evaporative and pumping losses along the Barwon/Darling River [2,3] during an extended dry period with relatively minor flow events. Individual flow events were found to be isotopically distinct but the local evaporation lines (LEL, Fig 1(a)) that develop after these events have a very similar slope indicating similar climatic conditions across this region. During low flow conditions, salt concentrations increase systematically, δ2H and δ18O become enriched and deuterium excess becomes more negative indicating significant evaporation. Flow events input isotopically depleted fresh waters to the system and as a result the deuterium excess returns towards the local meteoric water line. Fig. 1(b) shows this progressive enrichment and rapid depletion following flow events for Wilcannia during this study period. These stable water isotope data can be used to calculate the amount of river or lake water lost to evaporation, and an approximate scale of evaporative losses for the Barwon/Darling River system from 2002 to 2005 is shown in Fig. 1(a). During extended dry periods evaporative enrichment of δ2H and δ18O indicates increasing evaporative losses as water moves downstream with up to 80% evaporative losses for water in the lower reaches of the Darling River and in the Menindee lakes system. In other reaches of the river there is evidence that the hydrological response of the river to drought has increased saline groundwater discharge into the river system resulting in detrimental affects on water quality [2]. Discrepancies between isotope based mass balances, which can improve the quantification of evaporation losses and groundwater inputs, and standard flow gauging based models can be used to detect ungauged losses and gains [3]. These may include irrigation or domestic water use, bank storage or groundwater recharge. These are examples of how isotope tracers can help to quantify the hidden hydrological fluxes that impact on flow particularly during drought.

Keywords

Evaporation, Rivers, New South Wales, Australia, Droughts, Stable isotopes, Tracer techniques, Water, Groundwater recharge, Rivers

Citation

Hughes. C. E., Meredith. K. T., Cendón. D. I., & Hollins, S. E. (2009). Evaporative losses from the Darling River, NSW, during drought from 2002-2005. Paper presented to The 10th Australasian Environmental Isotope Conference and 3rd Australasian Hydrogeology Research Conference, Resources and Chemistry Precinct, Curtin University Perth, Western Australia 1st – 3rd December 2009. (pp. 22).