Salinity conference

Hydrology and modelling systems
Dr Glen Walker

Glen Walker is a groundwater hydrologist with CSIRO Land and Water (currently on secondment to Rural Solutions SA). He has 18 years experience in dryland salinity, working on a number of issues from Mallee recharge, assessment of salinity options through integrated modeling, stream salinity, vegetation water use and health in saline areas and salinity hazard and risk assessment. He has been a winner of the WEWoods award for excellence in salinity research awarded by the National Dryland Salinity Program.

A number of the points that I'll touch on have already been discussed before, and I will make some comments that may be different from what I would otherwise have talked about.

I guess I'll just go back through the need for prediction and look at salt as a risk factor, as compared with others.

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I guess the point that has been made today is one that not only applies to salt mapping or salinity mapping but to all our mapping technologies, that we don't want to be just an observer, we actually want the information being used in a lot of ways, and that we are looking to use it in the decision making.

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I think the other point, as part of that, is the way a scientist sees a natural resource management planning cycle. A lot of the previous discussion on spatial technologies tended to focus on what I would call the risk assessment: what is the current extent, the trends et cetera? Potentially, a lot of the mapping technologies go into other areas. In the South Australian geophysical exercise we were looking at a set of options, things that go towards planning and implementation, and there are probably other aspects there. So in terms of looking where mapping technologies come into a number of things, we probably need to get a bit broader than what I what I refer to as risk assessment but actually is part of the whole planning cycle.

As we go through that planning cycle more and more, then the more we are relying on prediction, the more we are relying on trying to understand the processes, and an important part of that is the dynamics and time scales that are associated with it. So to some extent, with looking at early parts of the cycle, there may be aspects that we can get away with, in terms of looking at the size of the problem, without actually necessarily knowing the process understanding. I guess this is where the hydrology comes in.

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The first one I want to talk about is salt itself as a risk factor. I actually found that there were conflicting statements in the report, and I guess part of this is just trying to look at the balance.

To look at this map, which is a salinity hazard map and is obviously a fairly coarse type of salinity hazard map: most salinity hazard maps around probably use a similar definition to this of what hazard is, but it is actually quite different from what this morning's discussion talked about as hazard. This is not a map of salt per se, it is not a map of saline water. It is a map of some sort of inherent characteristics of the landscape for where salinity is likely to occur. I would just make that point. That may be technically not a hazard, but a bit like Stefan I would tend to say, 'Well, what is it that we actually want to solve?' and then we call it that, whether it fits in with hazard or risk.

In there, obviously, as we go towards the higher rainfall areas, there is a lack of salt.

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Here are some data just from a number of Victorian unregulated catchments. This is flow weighted mean stream salinity versus rainfall, so 800 to 1200 mm rainfall. I am just making the point that one of the biggest determinants of salinity in the landscape, particularly the mobile salt that I have tried to capture here, is the rainfall pattern. I guess the question, as a corollary to that, once we get below 800 mm and that threshold might vary a little bit from place to place, is: how much information do we actually need to know about the salt store, for looking at hazards and risk assessments?

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That is really dependent on sensitivities, and here I am schematically saying that, depending on the sort of assets that we are worried about as at risk, it may be more important for some times than for others. For example, in a lot of agricultural situations, the actual salinity of the groundwater, once there is enough salt in the system, is probably not ultra-important, whereas other issues may be, such as water resources – the more salt there is, the more likely the sort of impact is and therefore they are much more sensitive to those issues. So, broadly, once we start getting into below the 800 mm there is probably enough salt around to know how some of these things might impact.

The third point I would make is that although people might argue about the figures, generally salt moves pretty slowly. We are talking about things that are moving with the groundwater in terms of metres per year, and so if we are looking at an impact on an asset like a water resource within 30 or 100 years, we are talking of things within hundreds of metres of that asset. So if we actually have to put a lot of investment into mapping where salt is, we can be a lot more targeted about it. I guess it is how much effort we put in at the sorts of resolutions and scales that we need to; we need to think through those problems. I am saying that, in terms of the salt mapping, we can be a lot more targeted about it from looking at it from an asset point of view. And that is not quite different from other risk factors.

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What I was wanting to focus on now, as we actually go towards the lower rainfall areas, is that we are starting to talk of a lot of other factors coming in too, which are to do with either mobilisation or risk factors. Most of these things most people have covered there. I have a list of things in which salinity is then one of a whole lot of risk factors, and the same sorts of comments as I have just made apply to all of them. There is some level of coarse information that we need, but there is information that we need to spend a bit of effort mapping, when it comes down to when we can actually do something in regard to high asset values. This comes back into the last discussion, on when it is economic to do things.

The other point I would make – and this is a point that Peter had made in his first talk, and I am just reiterating it – is that mapping technologies can fit into a number of those areas. In fact, I think in the South Australian exercise that we were involved in we would have said that the main things of some of the technologies was actually to fit more into these mobilisation factors than into the salt factors. I think trying to work out where those mapping technologies will contribute is an important part of this exercise.

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What I won't go into is how we actually look at combining those sorts of risk factors into any objectives; I want to just go back and reiterate that what we actually want to come out is some list – whether or not these are exactly right – of the sorts of questions that we are trying to answer. And in terms of the question of whether a hazard/risk map is useful, I actually tend to think we put the data to answer the questions, rather than getting too bogged down with what hazard or risk is. That has always tended to be my perspective.