It has been suggested that before irrigation runoff into the Macquarie Marshes was 460,000 megalitres and that this has been reduced to 395,000ml by irrigation.
But Ian Mott has argued that pre-settlement runoff into the marshes would have been much less than 460,000ml and most likely less than 395,000ml because much of the upper catchment was once forested. Because it has since been cleared for pasture, runoff would have substantially increased.
Yet the opposite appears to be the case.
In an earlier thread Chris Hogendyk expained that: “Inflows to Burrendong (on the catchment of the Macquarie) for the 68 months from December 2000 to July 2006 was approximately 1700 GL which is the same as the driest similar period on record that occurred from December 1934 to July 1940. The next driest period was December 1903 to July 1909 that received approximately 1950 GL.
“The first data set are actual observations whilst the latter two are modelled. Out of interest, for every 10 megalitres that is captured by the dam, 4 megalitres come into the system as down stream tributaries.”
Interestingly rainfall history as plotted by Warwick Hughes suggests that it was drier during the late 1930s.
This is Warwicks comment on the charts: “These show you some rainfall history for the region from the Bureau of Metereology high quality rain dataset and you can see the obvious cycles in all charts.
“Trangie data is the closest HQ station to the Macquarie Marshes and it shows that in the recent past conditions were similar to dry times times in the 1990’s and 1980’s, if you go back to the 1960’s rainfall was obviously less and even lower in the late 1930’s thru 40’s and earlier again WWI years into the 1920’s look the driest of all.
“The other graphic, also of HQ data, from Mudgee and Bathurst, could be a fair proxy for long term trends higher in the catchment.”
So it has been dry, but not that dry, and with fewer trees, why have inflows been so low lately?