Acotanc: Engineering Solutions to Salinity

Engineering Solutions to Salinity

Author: David Williamson
Organization: Agritec Soil Consultants
37 C Rushton Street
Burswood WA 6100
Phone: +61 8 94463203

Problems of salinity in Australia have come about through the geological history of Australia plus decades of mismanagement using incorrect farming practices. The situation will only improve when economic factors are considered in conjunction with land rehabilitation and improved farming practices.

The issue, fundamentally, is to try and work with a sustainable landscape. We are certainly not operating sustainably now. It is obvious in all sorts of ways. What are the sort of issues that need to be considered in that? I am just identifying a few here. Obviously natural resources is the fundamental resource that we draw on when we are doesn't matter if it is urban or rural...but in the rural situation, natural resources are basically soil, water and air. With hydrology, there is consideration of surface and subsurface water. Generally, it is only a consideration of surface water and none or very little of the subsurface. Salinity, fundamentally, is a groundwater problem.

One of the key issues that often gets forgotten with salinity is the fact that something like 52% of the divertable water resources in the southwest of the state cannot be used for drinking water. Some of it is still used for irrigation, but just about 46% is beyond use even for irrigation. That tends to be forgotten because people are seeing the salinity on the ground, rather than seeing what is happening in the water. The water looks nice and clean, but is too saline to use.

The whole question of biodiversity is very strongly studied. You have both indigenous and exotic plants which are both very important as far as salinity is concerned. You have the whole question of productivity, whether we want to have it or not. If we are going to live in this land then economics and productivity and social coherence are all a part of the whole ecosystem. You can't just remove people out of it. The Aboriginal indigenous people were here before; we have just moved in and messed it up a bit more.

I like to ask the question: does the agricultural industry specifically have the capacity to move into a sustainable environment? And what are the conditions? Firstly, I think it requires a willingness to make a lot of broad scale changes. Our farming practices come out of a European context. As Professor Gilkes of UWA likes to show people, in fact our soils are very, very old, worn out even before we started to try to use them. That is often forgotten, and it is a significant component in the whole issue.

One thing that farmers keep saying to me in the last couple of years since I have had more interaction with them is, there is no way they are going to make these broad scale changes unless there is an improvement in their farm profitability. They just haven't got the capacity to make the changes.

Another issue they have brought to my notice recently, but I think I have known it for a long time, is that there is a real need to have demonstrations of what, in fact, works and is it effective. Farmers are saying, "Yeah, we have heard all these things, we read them in books, but show me. Show me somewhere it has actually worked." In most cases they are small, experimental plots. Virtually nothing has been done at the scale of even a small catchment. There have been attempts to do that, but they have tended to fail at the human level, rather than at the field level. Nevertheless, they are just not there for farmers to look at.

Finally, there is a real need for awareness amongst the farming community, and the whole community, that there is a real complexity in management. It is not just a simple matter of doing something and everything else will disappear. That frightens a lot of farmers.

OK. What are the basic requirements for salinisation to occur in a soil and a stream? There are three of these. The first one it that there has to be a storage of salt in the system. From numbers that we have, we are looking at thousands of tonnes per hectare, for instance, in 450 mm of rainfall. There is a lot of country that fits into that category.

Secondly, you need a supply of water to mobilise that salt in the system. What we know is that the leakage under agricultural crops in the non-irrigated areas is around 4 to 10 % of rainfall. That is a very useful number to have.

Thirdly, you need a mechanism for redistributing the salt from one position in the landscape to another. These are basically subsurface movements into the aquifer systems or some hydrogeological structure in the system that allows water and salt to be moved.

If you could eliminate one of those three requirements, you would obviously eliminate the problem. So the question is, what are we able to do as far as controlling salinisation? I'm giving you a test now. Which one of these would you choose? The first one is just removing all the salt that is stored in the system. The hint that I am giving you is, the current discharge of the salt, the rate at which it is being removed, even though it looks ugly on the landscape, is hundreds to thousands of years to empty that store.

The second one you could choose, possibly to stop the water leakage beneath crops and pastures, the concept of switching the tap off. It has been dripping for a while, let's switch it off. There are parts of this that are important. Soil degradation problems are very important in the whole issue because they limit root growth. Limiting root growth limits water use.

The third possibility, maybe managing waterlogging and perched water tables in soil systems. You can see all of these are in some way underground. You can't see them. Farmers understand to a degree. I think the urban community have no idea of anything that is going on underground, generally speaking. So, you are really looking at something that it is hard to get people to take seriously. The third possibility is to change the plumbing for the groundwater flow. Basically, that is just to enlarge the size of the plug-hole. If you take the bathtub concept, what we had was a system that was in equilibrium; the amount of water coming in and the amount going out was the same. We have now come in and added a bit more water at the top and the bath has been steadily filling up. It is now leaking out at the surface--can we enlarge the size of the plug-hole at the bottom so that we drain a bit more out?

The question is, which one of those three would you choose? The economic factor is much larger than a lot of people are willing to admit. Certainly, at the scientific level we tend to ignore it because we are fiddling around with the mechanics of water movement and trying to answer which one of those three requirements we would select. I'm saying this again--I mentioned it earlier: in fact, to achieve ecosystem sustainability there really has to be significant improvement in farm profitability. At the moment, the only good thing that is probably happening is that beef prices are going up. Maybe wool is starting to show something, but I have rural friends who are having to sell their house in Perth to grow a crop to survive this year, because they have had a string of bad years. Droughts, frosts, and so on. They are not going to have money to make changes.

Secondly, there is a legacy from the past practices and government policy and we now have to consider how we are going to pay it. There is no question that the farmers are unable to pay this bill. Although the way the system is moving at the moment, there is an assumption, particularly with urban people, that the farmers are somehow going to pay that bill. They hear about the huge cost of fixing salinity and they just say, "Well, that is their problem." I don't believe that the community can just walk away from it like that. We are now reaping the reward, if you like, of our past practices.

I have been involved a study with another ex-CSIRO colleague of mine, looking at the economics of salinity management. A major conclusion that came out of the study was that, in order for you to have a favourable economic environment for managing salinity, you have to manage it in a way that restores the land. You actually have land rehabilitation. The only other option, which is the one that is actually being pursued, is to look at value-added land use. What other things can we do on the land that will bring in an economic benefit? The question of land rehabilitation, looking at ways the land can be brought back into production again, is being ignored, to a degree.

There needs to be a much more integrated salinity management strategy. First, you need to combine biological and engineering methods to control excess water. Up till now, it has been just looking at the biological methods, mainly only vegetation, and native vegetation at that. The engineering methods have been ignored, largely. There needs to be an integrated approach; not just one thing. There needs to be control of all the soil and water and land degradation problems which affect root activity. The changes are going to be made when we reinvent an agricultural ecological landscape. We have to get an ecosystem in Australia that is suitable for Australian soils and suitable to Australian climate. We have been farming this country for 130 years in broad scale, more so in the last 50 or 60 years. We are still using an ecosystem carried over from history rather than adapting to the Australian situation.

We are looking at vegetation...if we are going to take the second of those three options, where we tried to stop the excess water that is moving the salt around the landscape. Can we stop the leakage using vegetation? If we can use the available water, prevent the excess, basically we are looking at deep-rooted, perennial vegetation, trees, lucerne, and using it in some agricultural way. There has been a lot of work done with that. There have been a whole series of best management practices for sustainable farming that have been well-publicised and used by many farmers, having trees and agricultural crops together in an agroforestry system. However, one of the concerns is that the leakage is only stopped where those deep-rooted plants occur.

Then the whole question of redesigning agricultural landscapes. Some of this is happening, but it needs to be a lot more.... Is it feasible to use vegetation to stop the leakage? The last ten or fifteen years the conclusions have been drawn by a whole heap of us that the area needing deep-rooted vegetation is 60% to 90% of the whole landscape. The question is, where do you put the agriculture? Or can you include the deep-rooted vegetation within that agricultural scene and achieve that 60% to 90% of deep-rooted vegetation? Now, deep-rooted means a minimum of three metres of rooting depth.

Can we have success in managing soil degradation? The one soil degradation issue that is being addressed fairly well, particularly in the southwest of WA, is soil acidity. It is not a huge problem. It is not as problematic as in parts of the eastern side of the continent. But, farmers have been very willing to use liming techniques to try to deal with that particular issue. There is a whole suite of these soil degradation problems: soil structure decline, soil acidity, waterlogging, erosion and so on, which are all having a major impact on how efficient plants are at using water.

Another issue is the response time. If you are able to completely restore a catchment with trees, then you would find that you would only start to see some reduction of seepage that is causing salinity after about 20 years. Then it might be another 50 or 60 years before you see a complete reversal of the system. It is just the nature of the hydrogeology of the landscapes that causes that to happen. We have shown a partial reduction of leakage when agricultural perennials are used over large areas. There have been some results from Victoria which have shown quite good results. But it has only been partial reduction: the areas of saline seepage have only reduced, not disappeared, after about 30 years.

I think one thing that it is essential to realise is that if you are going to put trees and woody perennials into the system, then you only reduce the recharge where those trees are. They are not going to remove water from the groundwater system in a way that will act like a biological drain. If you have a strip of trees across the landscape, they are not going act like a drain. They will take a little bit of water, but they certainly won't stop a lot of water moving underneath. The woody perennials reduce recharge; they stop excess recharge. That is the real benefit they have.

We haven't found any results that adequately support biological methods to control this excess recharge. People have said to me, "Surely after 30-odd years working with salinity you have a solution to this problem." I said, "Yes, I have a solution: put all the trees back." They look at me stunned. The point of that is that it is not socially acceptable. People don't want to be told to get off their land. We have to try to work out a way of putting the two together.

There has been a lot of research towards treating the cause. We know that in the long run they are treating the cause as the way to go. But, how long can we wait to try to get a solution? In realistic terms, is there one we could have in the foreseeable future?

There have been a lot of trees grown in discharge areas, but the success there has been mainly aesthetic. Certainly, that land hasn't been brought back into production. Remember my economic colleague and I were saying that there was no question that you would have to have a system rehabilitated to agricultural production in order to achieve successful outcomes.

If you do nothing, this is actually the case for the upper Blackwood River system in the southwest, and this is the prediction you would have if you do nothing about it...the year 2000. The estimates are that this is what you would expect. You would end up with about 35% of the landscape becoming salinised. If you could reduce the excess recharge by 50% then you won't stop the increase, but you will reduce the rate of increase and end up with a lower figure here.

The engineering option, if applied, and where you are rehabilitating 170,000 hectares of about 270,000 hectares of saline land, then you could anticipate that the area of salinity would take a nosedive, and you would start to restore this land into production. If you aim to rehabilitate 250,000 hectares there, you make somewhat of a difference, not a huge difference, but you pick up the extra 80 in here. These two figures come out of the state salinity strategy, it is the way they have seen things moving and predictions for what they had. This one, unfortunately, was ignored in the salinity strategy, but I believe this one is being taken more seriously.

I will talk a little about the systems we are working in. When we want to apply engineering have aquifer systems through this landscape, and for a large amount of the wheatbelt of WA you have these sedimentary valleys in the system which are very important. This is mainly where the salinity occurs in this area, so these sedimentary valleys need to be taken seriously. And you have all the Rs (?) just identifying flows in the system, and so there is not just one simple system, there are a number of systems including water accumulating in this surface area here, these perched water tables, which are allowing leakage to occur and just fueling the system.

So, what are the options for managing salinity with engineering options? Firstly, one that is relatively easy to do and quite a number of farmers are doing it, and that is just managing surface runoff. It does have an impact on controlling salinity. Certainly, the perched water tables up in the top end of the system are a part of the recharge mechanism. If you can prevent that occurring, in various intercepting flow in these perched water tables, collecting the water into dams, you have the economic potential of being able to reuse that water for something. One of the problems is that if you don't control this surface runoff, then the surface water runs off into a flat valley situation somewhere and enhances the perched water table in the valley. You get a link between deeper saline water and the soil surface. This is a very significant means of salt getting to the soil surface. A lot of farmers are quite willing to carry out something in that area.

There is a lot of surface water that farmers can use. Again, it is a matter of diversifying the agriculture, the sort of things that this conference is about. You often need a bit of irrigation. The rural scene does have some fresh water that they can collect if they really want to take it seriously. It would be beneficial as far as the whole problem of salinity is concerned. There is a need to prevent surface inundation and waterlogging in the flat valley areas, and to use this in some way for agricultural production. Then there is a need to enhance the discharge of saline groundwater, and this is where artificial drainage becomes an issue, and where the need to keep the groundwater fresher in saline areas greater than two metres below the surface. That is the sort of numbers that come out of irrigation. This is done in irrigation areas, but it applies equally to non-irrigated landscapes as well.

Unfortunately, there has been general, ad hoc development of drainage systems, certainly in WA for dryland agriculture. There has been very little application of proper engineering design. The design of drainage systems has been known for fifty or sixty years. It has been applied widely in irrigation systems. Just because it is dryland, doesn't mean that what applies to irrigation systems doesn't apply. It does apply. There have certainly been some good results of managing surface water. There is an impact of using an open drain; they need maintenance and looking-after, they need to be repaired every now and again. They do have a short-term problem, which is simply that you have to make sure they are well-maintained. If they are not well-maintained, they won't work properly. If you go to a drainage system which involves borehole pumping, pumping from some depth, then there is an energy cost. You have to put energy in to continue the pumping.

What are some of the issues regarding the relevance of engineering management? Probably the one that has caused the most consternation amongst the agencies and government is how do you dispose of this drainage effluent? Where are you going to put it, what are you going to do? I will raise an issue that is developing, and will happen in the future fairly soon, we hope. There really isn't good information about what it might cost to drain land. When you are talking to a landholder, you can't say it is $750, because it might be around $2000 per hectare. There isn't good experience here to determine exactly what those figures might be. So farmers are a little wary of the sort of cost it might be. Generally speaking, if you design a drainage system, and the irrigators put a huge amount of effort in to designing good drainage systems, fundamental information of soil and hydrogeological information is needed. Generally, we don't have that information through large areas of the southwest. Certainly, there is very little information about those broad, flat valleys in the central and eastern wheatbelt. They are just starting to map them. The details of their hydrogeology is very poorly understood, unfortunately.

Many deep, open drains require good understanding of the soils and the hydrology of that system, and what is the current hydrological situation. The negative talk about drainage has generally been related to these two components: the hydraulic conductivity of the soils is low and the gradient of the water--the force that is driving the water system, is also low. So they have generally said open drains won't work. I have seen them, quite to the contrary, where they work very well. It is a matter, again, of understanding the design criteria and having the appropriate information before you start digging your drains. One of the problems with open ditches is sometimes they can require quite a large amount of land in order to achieve rehabilitation. In fact, that may be a negative if the amount of land they are using is quite large. Pumping bores need power to operate. This can be quite a significant operating cost. In fact, the operating cost for pumping systems is generally greater than their initial installation. You need to be aware of that.

Take this example: the upper Blackwood catchment is about 1.3 million hectares. Wagin, Kojunup, Katanning, Bunbury are included. Most of that whole area has a lot of those broad, flat valley situations before you get into more undulating country. There are quite a number of shires involved. Dumbleyung Lake and a series known as the Wagin Lakes are here. There are some lakes with very significant ecosystems east of Katanning. There are about 273 hectares of saline land and Ag Department estimates suggest it could be up near 600,000 in the next twenty years. It's a bit frightening. The economic study we have done of the area says that the rehabilitation to cropping of 70% of that saline land gives us a benefit/cost ratio of about 1.4. In other words, for every dollar put in, the farmer can expect to get 1.4 dollars back. A pretty good situation for doing something about it. Again, these numbers are based on information we have to date. That information needs to be considerably improved, to be sure that is the figure. We don't expect it to change a lot. There are rural towns involved in this area, all having salinity problems. There are roads and other infrastructure that are affected. The quality of the water in the Blackwood River is terrible. There is some irrigation done from it, but not much--not as much as could be done.

The concept is for us to work at looking into a drainage system. We have these perched water tables and we can look at those with ditches and open drains. We have the sedimentary systems in the broad, flat valleys, and we are looking at using pumping bores in those. Those areas are quite extensive and they are all generally saline.

The third lot is the deeper systems underneath the slopes of the undulating part of the landscape. There are probably combinations of pumping bores and ditches that could be used there.

One of the major problems is effluent disposal. What are you going to do with the water once you have pumped it out? That has caused the government to ban drainage. That is why they have banned drainage in rural areas. This water can be put into streams, put into evaporation basins, it can be conveyed to the ocean via a canal or pipe. It could be used for industrial purposes or some aquacultural industries. This could be value-adding.

This is the system we have been looking at, to run a canal through, picking up drainage water from landholders. Near Duranilling it would be put into a major conveying canal which would take the water through the Collie catchment, through a hydroelectric station, and eventually out to the ocean. It is a scheme that has been on the drawing boards for a couple of years. We run against all sort of bureaucratic opposition in the process. There are a whole series of rivers that fit into that system and a series of lakes. The Collie River could potentially dump saline water into this canal. At the moment, the new Collie power station is taking its effluent water in a pipe out to the ocean. We found this quite interesting. The government were criticising something like this, but they had their own pipeline taking waste water out to the sea. And nobody tells us how much it costs to do that. The final main canal is about 270 km long, a long canal. The volume of water is about 380,000 megalitres, a lot of water.

Evaporation basins could be used. If you are taking this amount of water, you need an evaporating surface of about 40,000 hectares. This would use about 15% of the land we are trying to recover. There is obviously a cost there. You could discharge to streams, but all of us understand the problems of doing that, environmental costs. Aquaculture is certainly a possibility, but it doesn't use a lot of water, and you still have the disposal problem.

A series of other things come out of this project: environmental problems along the route; ocean outfall; economic feasibility; engineering challenges; managing flood flows; public participation in the planning process. One of the add-on values is the potential to produce about 15 megawatts of power as the water goes down the Darling scarp. What we do see is the potential for the system from the Blackwood to be applied to the Williams-Hotham-Murray system. Sections of the Avon, possibly the Moore River, and the Kent, further south, which is in fact a water resources recovery catchment. You wouldn't have a hydro component there but that would be an excellent catchment to work with.