Dryland salinity

Dryland salinity is a particular problem of some agricultural areas in the southwest of Western Australia. It was caused by the removal of deep-rooted native perennial vegetation and its replacement by shallow-rooted annual crops and pastures. Because the new crops and pastures use less water, this caused the water table to rise, bringing with it salts that were already present in the soil. They got moved to the roots of the new plants and, of course, crop and pasture plants don’t like growing in salty soils. These three diagrams illustrate the process of developing dryland salinity. Initially, on the left there’s a lot of vegetation. It’s using up all of the rainfall that falls. And the groundwater table is kept low.

Then some of that is removed. The groundwater starts to rise. And in the third panel, the groundwater has almost reached the surface. It’s quite shallow and it’s seriously affecting the vegetation.

In 2000, there were around 2 million hectares of land that was affected by dryland salinity to some extent across Australia. And it’s pretty good that this may grow to around 3 million hectares or perhaps even more over the coming century if we continued on our existing trajectory. Although climate change may change that. So managing dryland salinity is not an easy thing to do. There’s a number of options. One is to use deep-rooted plants, such as the ones that we cleared from the land originally to try and increase water use. So perennial postures, lucerne or alfalfa is one example. Or to practice farm forestry, move away from cropping and go back into farm forestry is another.

Although that might not be successful in an environment like the one we’re in today where the rainfall is probably too low to support commercial farm forestry. Another option is to protect the remaining native vegetation that is here. There’s not a lot of it in many parts of our agricultural zones, but it’s worthwhile trying to make sure that what we have got isn’t lost through overgrazing or other reasons. Another option is engineering solutions. Some farmers have established deep drains to try and lower water tables locally so that they can crop or have pastures between the drains have been successful in some cases.

And then other farmers have tried to live with the problem by growing salt-tolerant species, generally forest species for livestock. We don’t really have suitable salt-tolerant crops yet for the level of salinity that we face in this region. But some farmers have noted success of that. So let’s talk about salinity from an economic perspective. Salinity is a problem that has both public and private dimensions. Some of the impacts of salinity affect the farmer and the farmer’s own land. But some of it is an externality that affects issues offside. Research has shown that the biggest impacts overall are probably on the farmer’s own land in terms of the area affected.

But there is, of course, some offside effects in some cases as well. So let’s focus on the private impacts and benefits of management for the individual farmer. So analysis has shown that in many situations, the cost of preventing salinity by perennial vegetation, particularly woody perennial vegetation, trees, can outweigh the costs of salinity. If you’re going to rely on trees to solve the problem then depending on the local hydrogeology, you may have to establish 50% or even more of the farm back into woody perennials or trees. And that would, depending on how much of the farm was going to be lost, but typically, some farms face a threat of maybe losing 5% to 10% of their land to salinity.

So if those trees are not commercial– and they’re generally not commercial in this sort of region that we’re in today– then why would you sacrifice 50% of the land to protect 10% of it? It’s not going to be an economic proposition. So it’s not surprising that quite relatively few farmers have actually taken action on this sort of scale that would, particularly through planning of perennials, that would be sufficient to really turn around the trend of salinity.

So this graph shows the marginal cost and the marginal benefits of different levels of salinity abatement. This graph is a bit like a supply and demand curve. And it’s about the supply and demand of abatement– so abatement meaning reductions– in the future risk of salinity. So the upward sloping curve, of course, is the supply curve, the marginal cost curve of abatement. The downward sloping curve is the demand curve or the marginal benefit curve. And where they intersect you can see what is the optimal level of abatement for a particular farmer. So these are benefits and costs as perceived by the individual farmer.

However, if we move to a different farmer who had the same marginal benefits, but higher marginal costs we’d get a different optimum. So in this graph, we can see because the marginal cost of abatement is higher– perhaps, the trees don’t grow as well or it’s more expensive to obtain suitable treatments– then the optimal level of abatement is lower. And if the marginal cost of abatement is high enough as it is in this diagram then the optimal level of abatement from the farmer’s perspective could be zero. Better off just living with the problem than trying to prevent it. And at least for some farmers in Western Australia that seems to have been the reality. OK.

Let’s turn to the public externality version of the economics. So some of the impacts are off-farm. And we’ve referred to those already as external costs. And these include impacts on infrastructure. So there’s roads that are already being affected and many more kilometers of roads that may be in future. There are effects on water resources. Salinity levels have risen in many rivers of Southern Australia with effects on irrigators and on potable water users. And there are environmental impacts. So in Western Australia, there are around 450 species of plants and 400 species of animals, mostly arthropods, that live only in areas that are at threat of future salinity. So their existence, their survival, may be threatened.

If we were include external benefits from salinity abatement in that graph that we showed a little while ago then, of course, the optimal level of abatement would increase. Whether and how much it would increase depends on how large the external benefits of abatement are and how high the private costs of abatement are. So in this graph, I’m showing the example where originally, the optimal level of abatement was zero. But if I include in there the marginal benefits from a public or external perspective, it raises the marginal benefit line. And we now get an intersection with the marginal cost line. And we have a new optimum, a new level of abatement that’s optimal that’s greater than zero.

It’s not very much greater than zero. How much greater than zero it is will depend on the individual case how large this marginal benefit increases and how low the marginal costs are. So in summary, dryland salinity, an Australian condition, results from clearing deep-rooted perennial vegetation and replacing it with shallow-rooted annual crops and pastures. And we have done a lot of that. 90%, 95%, or even more in some regions has been cleared. And that’s led to rising water table, which has mobilized salts and caused reductions in the performance or even an inability to grow crops and pastures in certain cases.

There’s a lot of salinity management options that are available, but for some of them at least, the economics are not very attractive. And in an environment like I am in today, the economics of some of the salinity management or salinity prevention methods are quite negative. And so farmers have had to live with it or farm around it.