The Murrumbidgee River catchment located in the southern end of the Murray Darling Basin (MDB) is Australia’s most economically important agricultural area. Decreased rainfall and reduced river flows have featured throughout the MDB in recent decades. It is important to consider finer scale hydroclimatic impacts on individual catchments to analyse both river flows and rainfall to make more spatially focused regional assessments of water availability. Here, digitized weekly river height data at Hay on the Murrumbidgee show reduced variability over the last decade and zero winter (JJA) flood level exceedances (6.7 m) since 1991. In contrast, the flood level was exceeded every few years in JJA from 1874 to 1991.

There has been no major new regulatory infrastructure upstream of Hay since at least the 1990s. Using bootstrapped yearly intervals, the winter months June-August exhibit no significant decline in catchment rainfall (Burrinjuck and Blowering dams), since the 1990s, to match the lower river heights at Hay. However, there is a significant decline in late autumn (April-May) catchment rainfall since the 1990s. Insufficient late autumn catchment wetting reduces run-off during winter even though winter rainfall has not declined significantly. An accentuating factor is likely to be the observed mean temperature increase, as an increase in temperature increases evaporation, thereby reducing run-off into rivers and dams.

Wavelet analysis of the JJA rainfall of the Burrinjuck/Blowering dams catchments reveals a 2-7 year, ENSO-like periodicity in rainfall, with more coherence towards August rainfall. This is in accord with the well-known relationship in southeast Australia between ENSO and rainfall.

The water availability from the Murrumbidgee at Hay over recent decades continues to be affected by changes in catchment rainfall and run-off, suggesting the need for a new review of water availability in the Murrumbidgee River system and also of other rivers in the southern MDB.