About two weeks ago there was an article in the newspaper which dealt with a story about salinity. Prof. Ian Acworth from UNSW discussed a study of groundwater levels that he completed with Aleksandra Rancic form DECC.The key point was that they believed to have found that the expression of dryland salinity was more related to climate variability then to landuse change.
"By studying historical records for thousands of water bores across NSW, researchers from the NSW Department of Environment and Climate Change and the University of NSW have shown that salinity is traceable to rising groundwater levels."
Well we knew that, but I assume that is the reporter not getting it right. it started off much more controversial:
"CLIMATE and rainfall, not land-clearing, have emerged as the main drivers of salinity in south-eastern Australia, in a study that could overturn years of research."
Ian knows how to kick up a controversial story and this one definitely got the airwaves buzzing. Several people have contacted me and checked whether I read the story and wanted to comment. I promised I was going to write something on my blog about this.
I have argued for some time that the broad brush view, that all expressions of dryland salinity are human induced, is not very accurate and is mainly based on the Western Australian experience. However, I am not keen to replace one mantra with another. I think there is more to the story than is reported in the newspapers. I haven’t yet checked with Ian on the detail, but here is what I think is going on.
If you analyse the rainfall data for the southeastern Australia over the last century or so then there is a distinct increase in the rainfall in the 1950 – 1970/80. This is particularly true for SE Aus, but is also true for the Murray Darling Basin. I have commented on this last year: see here
Given the strong connection between the shallow groundwater and the rainfall, you would expect to see a response and, yes, this means that expressions of salinity will worsen after high rainfall periods and in dry periods (current) will disappear. This has been noted and commented on by others, for example Dahlhaus et al. (2008), so in itself the debate is nothing new.
The comments that were sent to me concentrated in particular on the question whether this ignores the years of salinity research (about 30 years worth) that have been completed and which have indicated a connection between landuse and salinity. I think it does and it doesn’t. There are a few key points to remember in this issue:
1. Landuse , deep drainage and responses in groundwater levels are strongly coupled and thus it is obvious that climate variations will be observed in the groundwater tables and that as such it is not surprising that groundwater tables respond to climate. What the salinity research has shown us is that they will respond more under certain land uses than under others due to differences in deep drainage
2. The research by Prof. Acworth and his colleagues probably covered the last 100 years (at the most) given the availability of groundwater data. Land use change has been happening much longer.
3. The salinity research has shown that there is a wide variety of groundwater systems, some of which respond very quickly to changes in landuse (and climate) while others respond much slower. The seminal work by Ian Jolly (Jolly et al. 2001) clearly indicates that most of the groundwater systems are not in equilibrium and that changes in discharge and groundwater levels due to landuse and climate will sometimes take centuries to work through the system.
4. There is a difference in pressure responses and groundwater volume responses and sometimes it is difficult to disentangle these two effects on larger scales. Climate influences will work stronger on pressure responses as their input tends to be a larger pulse compared to changes due to landuse effects.
I can’t stress too much that the best approach to salinity is not broad brush, but case by case. There is a wide variety of geological systems, salt stores and processes. However, general hydrological laws will apply:
A. Changing one component of the water balance will affect the other components
B. Salt moves with water, if there is a store and water moves salt will move with it.
References
Dahlhaus P, Cox J, Simmons C, Smitt C (2008) Beyond hydrogeologic evidence: challenging the current assumptions about salinity processes in the Corangamite region, Australia. Hydrogeology Journal 16, 1283-1298.
Jolly ID, Williamson DR, et al. (2001) Historical stream salinity trends and catchment salt balances in the Murray-Darling Basin, Australia. Marine and Freshwater Research 52, 53-63.