Salinity and Evaporation in the River Murray Basin, Australia

The River Murray discharges only 4% of annual precipitation delivered to the Murray Basin. Flow gauging and water composition data over an 18 year period (1971-1989) indicate that median annual transport of chloride out of the basin was 2.6-4.1 times that delivered annually by the atmosphere to surface runoff plus inflow from the tributary Darling River. The additional salt flux out of the River Murray was apparently derived primarily from saline ground water plus mobilization by irrigation drainage of salt stored within the unsaturated zone. During March 1989, towards the end of a dry summer, the deuterium composition of water near the river mouth was delta-D = -4 parts per thousand, compared with delta-D = -43 parts per thousand in the largest headwater catchment. Most of this heavy isotope enrichment was the result of evaporation from reservoir and river surfaces plus inflow of irrigation drainage water. The trend in deuterium values along the river axis suggests that a large component of the chloride increase in the middle third of the river was derived from irrigation drainage whereas the further increase of chloride in the downstream third of the Murray was derived primarily from influx of saline ground waters. Evaporative enrichment appears to play a major role in the seasonal and geographical trends of stable isotope composition in the River Murray. The total deuterium enrichment observed indicates that integrated evaporation losses in the basin are similar in magnitude to transpiration losses during the months of intensive irrigation.