Since last Sunday’s feature story ‘Australia’s Salinity Crisis, What Crisis?’, I’ve received comment that it is difficult to understand the different models and theories explaining dryland salinity. The dominant theory has been the rising ground water theory which Dr Brian Tunstall suggested was complete “bunkum” on Sunday.
In my opinion the model has some application, but lets start with a basic description of the theory:
If you dig a hole in the sand at the beach, or a bore in your backyard, chances are you will strike water at some depth. This water is often referred to as ‘ground water’.
The ‘rising ground water’ theory is essentially based on the idea that if you remove lots of trees from an area or irrigate an area, then more water will percolate down than would occur naturally and the ground water will eventually rise. If there is a lot of salt in the landscape the rising groundwater will be salty.
The theory is applicable to many irrigation areas and I have previously written about how Murray Irrigation Ltd, in the NSW Riverina, has dramatically reduced the area at risk of salinity working from this model (click here for that blog post).
I have also acknowledged the value of salt interception schemes along the Murray River (click here for an article recently published by Online Opinion). These schemes are based on the idea that if the rising ground water is intercepted, and the water evaporated and salt collected, the amount of salt entering the Murray River will be reduced and salt levels will fall.
But a potential problem with salt interception schemes is that they can draw groundwater from a distance away, and in this way potentially suck the soil profile dry of water.
It really depends on whether the groundwater is confined or whether the ground water covers a much larger area and may be flowing underground along, for example, old river beds.
A fellow called Geoff emailed the following comment yesterday:
“As I see it and please correct me if I am wrong, there has been a blanket campaign to lower water tables to combat salinity. In reality, some areas need to lower their water tables while others have no water table problems. In fact these areas need to increase the water infiltration to leach the root zone salt down the profile.
Chisel plowing, stubble retention, avoiding excessive grazing are all well established and accepted ways of increasing this water infiltration by increasing the organic matter and bacterial activity in the soil. And, dare I say it; clearing trees followed by careful soil husbandry would be the preferred option in many areas.”
It is worth remembering that many people in rural and regional Australian rely on groundwater for ‘stock and domestic’ as well as irrigation and that groundwater is not necessarily salty. Groundwater is mostly a very valuable resource and while the National Land and Water Audit gave the impression it is everywhere increasing in abundance, the reality is quite the opposite (click here for a Land column I wrote on this issue).
In summary the rising ground water model has some application, but I don’t believe it has general application outside of irrigation areas in eastern Australia. I am less familiar with the situation in Western Australia. The model probably has limited application through most of Queensland and NSW and yet it has been applied inappropriately across this landscape including through the National Land and Water Audit, and specifically at places like Dick Creek (click here for BrianTunstall’s explanation as to why Dick’s Creek is a soil heath rather than rising ground water issue).
Professor Pannell, from Western Australia, has a different view. He has posted comment at his website defending the rising groundwater model and suggesting it has general application including in eastern Australia. He also supports Wendy Craik’s view that the drought has lowered water tables. But hang on, which drought? Despite all the hype, the rainfall record for the Murray Darling Basin as recorded by the Australian Bureau of Meterology does not suggest the last few years have been partiucularly dry:
The last very dry year was 2002 and that wasn’t unusually dry in the scheme of things.
Professor Pannell writes:
“Contrary to the claims expressed on the [Sunday] program, there is copious evidence in support of the rising groundwater model, including a catchment in WA [Western Australia] where groundwater and stream salinity levels have been monitored ever since the land was cleared. There are numerous areas where establishment of perennial vegetation has lowered watertables and thereby mitigated salinity (e.g. Burke’s Flat in Victoria, the Denmark River in WA).
Powerful recent evidence in the Murray-Darling Basin has been the decline in saline discharge in many areas, due to extended periods of below-average rainfall. For example, in a site at Kamarooka (northern Victoria), there was formerly a large area of saline discharge, but the recent dry period has lowered saline groundwaters to 2 metres or more below the surface for the first time in 50 years. This widely observed recent phenomenon is completely consistent with the groundwater model of salinity, and (unless I’ve misunderstood it) completely inconsistent with the soil-health model. The same is true of the fall in salinity in the Murray River, which was rightly emphasised in the program.
… I’d also be very interested to know how the alternative model explains the onset of salinity affecting roads and buildings in the middle of rural towns, or occurring within remnant native vegetation (where soil health is presumably pretty good). It seems to me that these things can only be explained by rising groundwater.”
In fact a bit has been written about ‘lateral flow’ and ‘soil health’ to explain impacts on roads and other infrastructure from salt, click here for a piece by Ken Tretheway and Rob Gourlay.
Jennifer Marohasy BSc PhD is a critical thinker with expertise in the scientific method.
Jennifer, the BOM chart gives an average rainfall over an enormous area stretching from the Snowy Mts to QLD, whilst some may say that the drought has lowered water tables obviously from the BOM chart not everywhere is in drought and aquifers may be charged by rain events at the other end of the country.
Certainly in NSW and I believe in VIC there has been a long drought but I believe that all of SA has had the best rain for years.
“The ‘rising ground water’ theory is essentially based on the idea that if you remove lots of trees from an area or irrigate an area, then more water will percolate down than would occur naturally and the ground water will eventually rise. If there is a lot of salt in the landscape the rising groundwater will be salty.”
To me this sounds like a gross over-simplification. Certainly it would foster in your readers’ minds the notion that water “percolating down” ought to take salt with it, ergo, the salt has to be coming from somewhere else. And that therefore. removing vegetation ought to result in soil salinity reduction.
Well, you have a point – all that extra water that’s in the soil has to be coming from somewhere. The options are, I suppose, directly overhead (which would have the initial effect of washing salt downwards) or through an aquifer from elsewhere. Before we dismiss the “rising ground water theory, maybe it would be wise to consider, for any given location, what the balance of these two flows – salt depletion of the soil through rain water leaching downwards, versus salt replenishment from increased aquifer inputs is. We might also consider the other physical, chemical and biological processes at work.
For example, diffusion of solutes (Na+ and Cl- presumably), down concentration gradients. Raise the water table above an underground source of salt, and guess how this well knonw phenomenon of physical chemistry would affect the salinity in the raised water table and the soils above the salt reservoir. I know, common sense tells you that water flows downwards under the influence of gravity, and it ought to take solutes (such as salt) with it, but what do reason, logic and basic science tell you?
What they tell me, is that unless you have some pretty aggressive flushing downwards, processes such as capillary action and other – ahem – electrochemical effects – that is, effects depending on the electrostatic attractions of charged ions (such as Na+ and Cl-) for each other, and small regions of electrical charge within the complex mixture of stuff of various kinds that is soil, are going to trump gravity, in the same way that capillary action will trump gravity in the lab, allowing you separate a mix of several amino acids into a neat string of purple ninhydrin stained dots on a sheet of chromatography paper.
Alternatively, you have some sort of energy input into the system, sufficient to lowrer the water table and keep the salt down in the lower sub-soils (trees actively pulling water out of the soil would achieve that). But unless you have either strong downward flushing or active lowering of the water table, diffusion is going to get you.
What’s the alternative theory? If you’re going to knock down the “rising ground water” theory of soil salinity, it’s generally considered good form, in science, to have an alternative, testable theory to put up against it. One you can actually demonstrate by experiment.
Finally, in your presentation of ‘the rising groundwater’ theory, you combine two mechanisms that the 2004 Parliamentary committee treated as quite distinct, and distinct problems – drland salinisation through depletion of tree cover and salinisation through irrigation. In the latter case, if you start off with salinated irrigation water, common sense, reason and logic would all be in agreement on the most probable cause of a rise in soil salinity.
Damn, but this nuance stuff is tedious!
Jennifer,
Sorry to have to disagree with you, but this is an issue where the evidence is overwhelming. There certainly have been cases where people have misapplied models, or jumped to false conclusions, or distorted things, but that does not alter the huge body of evidence of many different sorts in support of the rising groundwater explanation for salinity.
You certainly surprised me by claiming that there has not been below average rainfall to lower watertables in the Basin. In December I toured northern Vic to look at salinity issues, and there was hardly a person we met who did not comment on the extended dry period they’ve had, and all of them related it to the falls they’ve observed in local groundwaters. I went to the BOM site to check if they are all deluded, but happily for them, they are not. I have posted maps of the rainfall anomolies (deviation from average) for Vic and NSW here: http://cyllene.uwa.edu.au/~dpannell/rainanom.htm
Almost all areas have below average rainfall for the past 36 months. It is even worse in NSW than in Vic.
I know nothing about Dick’s Creek, but if it is true that the degradation there is not due to salinity, this is hardly proof that the rising groundwater model is generally wrong!
It is true that the push to lower groundwaters has been somewhat indiscriminate. But again, that doesn’t mean that the explanation is wrong. It just means that treatments need to be better targeted. This has been a strong message by me and others in recent years.
So below average rainfall for 36 months constitutes a drought and everyone telling us its a drought means it really is a drought?
I reckon there is probably less inflow because landholders have worked out how to hold water on farm and reduce their groundwater table etcetera
and that by talking up ‘the drought’ we are distracting attention away from this issue.
I also think it is somewhat amusing that the MDBC is so concerned about future rises in groundwater, while also predicting that there will be a drought ‘forever more’ because of global warming.
Jennifer, Why are you distorting what I said? I didn’t mention the word drought. You don’t need a drought to get falling water tables, just below average rainfall. Look at the Ferdowsian et al. (2001). http://cyllene.uwa.edu.au/~dpannell/j60ajsr.pdf
The many people who made those comments to me were not fishing for drought assistance, they were discussing salinity!
We don’t need to argue or conjecture about this. The Ferdowsian et al. model allows you to distinguish between rainfall and other influences on groundwater levels. I’ve seen it used in Victoria and the fall in groundwaters does relate to rainfall.
Your shot at the MDBC is also a bit cheap. Everyone, including the MDBC, acknowledges that if there is lower rainfall, salinity problems will lessen. But given the huge uncertainty about what the consequences of global warming will be, if any, it is reasonable not to take your eye off the salinity ball.
Hi David,
I probably could have added a few :-)’s.
But are you seriously suggesting that it is the recent drier conditions more than the salt interceptions schemes and drainage management plans that have significantly contributed to a drop in salt levels?
Groundwater levels have been dropping across NSW for about 15 years and we have had some wet years in between?
And hasn’t the drop been spectacular in the NSW Riverina irrigation area, http://www.jennifermarohasy.com/blog/archives/001288.html .
Jennifer: “are you seriously suggesting that it is the recent drier conditions more than the salt interceptions schemes and drainage management plans that have significantly contributed to a drop in salt levels”. No I certainly am not suggesting that at all. They’ve both had an effect on river salinity. I don’t know which has been greater. My point is just that the lowering of saline discharge, directly associated with lowering of groundwater due to below-average rainfall, is compelling evidence for the rising groundwater explanation for dryland salinity.
Agreed.
This water table salinity discussion has greater implications than academic argument. Click up the Wilderness Society’s campaign on Land Clearing in NSW May 15th media release and you will read—
‘Land clearing is the number one cause of dryland salinity’ and
‘When native vegetation is removed the rain moves down to the water table, causing it to rise and force the soil salt to the surface. This situation is almost impossible to reverse and ruins not only the native life but devastates the agricultural value of the land’.
I accept these statements are true for some areas of NSW but I have a problem with the use of this reasoning to anchor their anti land clearing campaign in western NSW. Is the country west of Nyngan really at threat of a rising water table caused by land clearing?
Geoff
I’ve been told that:
“The bogus Queensland salinity maps have been used to restrict tree clearing. All the appeals have been successful (without any proceeding to trial), which might suggest that the government doesn’t have any confidence in its maps. However, there are hundreds of landholders that were knocked back and that don’t have the skills or resources to progress a Planning and Environment Court appeal.”
Lets face it Jen hardly knows what shite drips from her mouth.
Unless I missed it, no mention has been made of the role of evaporation to increased salinity. Where salinity cannot be attributed to rising water table, is not the cause just evaporation of surface water?
Irrigation by river water carries some salts, and as commented in the TV program (transcript), adding fertilizer adds more salts. Ground water will also carry a variety of salts. As the water evaporates, the salts are left behind!!
Paul Goard you are right on the money in talking about evaporation as being a major source of salinity. The average evap. rate across the Murray Darling Basin is near 2m per annum.
As the rate of evaporation is a function of the windpseed over the surface, by the difference in humdidity (Saturated-measured) then the losses over the total area is rather large.
Naturally the concentration of salts just keeps increasing.
Perhaps some of my betters could comment on the facts as I know them.
1. Fresh water floats on top of saline water?
2. Rainwater carries a small amount of salt?
3.Water in the soil will travel along on top of an impervious layer until it comes to the surface at the break of a slope?
and finally If irrigation and fertiliser use are the harbringers of salinity, why aren’t suburban gardens salt encrusted deserts?