Stable Isotopes as an Indicator of Evaporation in the River Murray, Australia

The stable isotope composition of water (δD and δ¹⁸O) and chloride concentrations were measured monthly at 26 stations along the River Murray, Australia and several major tributaries from December 1988 to June 1989. The first four months had low natural runoff plus large irrigation diversions from storage reservoirs. The following three months had much higher (up to a factor of 5) river discharge. Deuterium and oxygen 18 compositions had a maximum range of 50‰ and 9‰, respectively, from headwaters to mouth. Chloride concentrations increased downstream with a maximum range from 1 to 180 × 10⁻³ kg m⁻³. These trends can be attributed to (1) evaporation from storage facilities and river channels, (2) inflows of irrigation drainage water enriched in heavy isotopes, and (3) inflows of saline groundwater. Water losses by evaporation from the river surface and irrigation areas amounted to about 40 ± 15% of releases from reservoir storage, based on the integrated enrichment of deuterium, which is comparable to the total transpiration flux within the basin. About half of the chloride acquired by the Murray as it traversed the middle third of the basin during summer months appears to have resulted from return flows of irrigation drainage water.