Rainfall, Extraction and Standing Water Level Part 2


Rainfall, Extraction and Standing Water Level         Part 2

The Scott Evans Position

In his presentation to the Parliamentary Inquiry, Evans sought to provide an overview of the Southern Basins and an introduction to its management. He expressed the management approach in the following way:

 The management concept was that if you can take water from the lens but not cause it to vary differently from the natural variation that you would expect to see in the lens, you are probably taking it at a ‘sustainable rate’, whatever the definition of sustainable is. That was the intent.” 

 

Prior to stating this position Mr Evans had presented a graph comparing “the number of rainy days and how heavy the rain fell each day.

 image001


 

He continued:

            “On the left side of this graph or the left-hand side of this graph is simply for a particular year, the number of days of greater than 10 millimetres of rain, so a heavy   rainfall event. The colour of the gradation in the graph is the intensity of the rain – up to 35 millimetres in that day – and the white is 10 to 15 millimetres. So here might be 10 days of rain in that year. On the right side of the graph indicates how the water level changed from year to year to different lenses. The purpose of this graph is to show that, whenever you got a positive recharge, the water level went up from the beginning of one year to the beginning of the next. It occurred primarily when you got more than 10 days of greater than 10 millimetres of rain. . . . . .where you got heavy rain events there is a correlation to recharge – and there is no real correlation between the size of the recharge (this peak here is a lot bigger due to 13 days of rain compared to this peak here, which is not as big, where they had 21 days of rain).”

 

Discussion

 

Evans’ understanding of rainfall, extraction and standing water level is more sophisticated than that of Ben Bruce (see part 1). Here Evans presents the idea that “rainfall” as measured by daily rain gauge readings does not correlate at all with standing water level. He has worked out that only “heavy” rain is sufficient to recharge (or change the standing water level) of an aquifer. He is about to use (stumble on?) the concept of “effective rainfall” but falls short in a way completely unbelievable for someone who is tertiary qualified. He maps the days of “heavy” rain and compares it with annual rain gauge readings.

 

The unaccountable fault is Evans treating each “heavy” rain day as being equal! Five days of rain each between 10-15 mm is equal to each of five days of 25-30mm rain!! No wonder there is NO correlation (no matter how hard one looks) between these two variables. “Where you got heavy rain events there is a correlation to recharge – and there is no real correlation between the size of the recharge (this peak here is a lot bigger due to 13 days of rain compared to this peak here, which is not as big, where they had 21 days of rain).”

 

It would appear that Evans wants to go no further than registering that only “heavy” rain has an effect on recharge. He then goes on to acknowledge that the magnitude of the effect on recharge is NOT based on the number of days of “heavy” rain. Think about it – how could it be?

 

The next graph is based on the one presented by Evans. By taking each year in turn and taking each “heavy” day and converting it to actual rainfall in mms (eg 5 days of 10-15mm became 5 x 12.5mm [– mid-point between 10-15mm] = 62.5mm; and 4 days of 20-25mm became 4 x 22.5mm = 90mm) we can construct a graph of “effective” rain. Now look at this graph and compare it with the rainfall graph presented by Evans.

 


 

 

The important feature is not that “heavy” rain results in recharge, but to note that on a number of occasions the annual rainfall bears no relationship to recharge! This underlies the importance of NOT using annual rainfall figures as a gauge for recharge.

 

Now let us return to our model of an underground tank buried (say 2 metres) below the surface. (see part 1). Whenever it rains, water enters the tanks and it starts to fill. The rain can be slight or heavy, water will enter and the standing water level will change (in this model it will always go up). A simple groundwater basin is the same as the tank but with holes in the bottom – it leaks. But even if the aquifer leaks, the effect of rainfall (provided it is more than 10mm in a day ie it is “heavy”), is to change the water level. Presumably the water level rises BUT immediately starts to fall because of loss to leakage. The ultimate water level (after a period of time, eg a year) will depend on the number and timing of “heavy” rainfall events.

 

If a groundwater basin receives no rainfall whatever, its water level (assuming like all aquifers that it leaks) will go down. Any rain entering (provided it is a “heavy” event) will at least slow the water level going down, ie it will go up only if temporarily. More and frequent “heavy” rainfall will not just slow the lowering of the standing water level, it will raise it at least for a time.

image002 

 

According to Evans: “that if you can take water from the lens but not cause it to vary differently from the natural variation that you would expect to see in the lens, you are probably taking it at a ‘sustainable rate.” This raises the question: what is the natural variation? Is it measured daily, or monthly, or during “winter months” or yearly? How is it possible to plot the natural variation if there is a combination of leaking and extraction (ie water leaving the aquifer) and it is then correlated with annual rainfall (but NO attempt is made to correlate it with “effective” rainfall– the actual measure of water going in?

 

Unless this is done there is no hope of getting any insight into “natural variation” in an aquifer that is being used for water extraction. Of course, by deliberately ignoring, (even denying) any impact of extraction on changes in water level it is impossible to pin down the “natural variation”. However, ignoring the effect of extraction allows the water managers to continue extraction and “explain” any impact of change in water level on rainfall variation. Saying the same thing in a different way: the variation in rainfall is enough to explain the change in water level and extraction has nothing to do with it!

 

However, looking for the “natural variation” which can then be used to guide extraction to get it at “sustainable rate” is not asking the right question. What is the natural destination (or end-use) of the water being ear-marked for extraction? If it is NOT extracted it sits in the aquifer and slowly over time “leaks” out the bottom. If this is to the sea, managers want to get at it and save it from being “wasted”. However, reducing the water flow out to sea can result in seawater intrusion unless of course the amount extracted is controlled very carefully (on the basis of what is happening underground and not the immediate regional demand).

 

The ecological problem is that any water extracted that lowers the standing water level below a few meters of the surface has an impact on native vegetation and the surrounding springs, wells, swamps, etc. Any extraction at all will have an impact. Until there is a much better measure of what these impacts are in relation to every aquifer, then managers are inclined to keep extracting until there is an outcry.

 

The management strategy at present is to meet community demand for water but drain the basins as slowly as possible. Any attempt to maintain the “ecological sustainability” of a basin while extracting water for a public water supply is a contradiction. It just can’t happen. The amount to be extracted to meet public demand is just too great, unless of course the annual “effective rainfall” is huge compared with the demand.

 

The only “sustainable rate” of extraction is one where the basin water level returns periodically to the level it was at when extraction (for the public water supply) was first started. This has never occurred in the history of the basins on EP. The stop gap measure put in place by the WAP was to instruct the water managers that the storage volume of water was not to be used for extraction, only a percentage of the “recharge”. By not measuring the recharge by changes in water level year after year and moderating the extraction to bring the storage volume back to some agreed level, the basins are being slowly but surely depleted. Polda is the obvious case, while there is concern that Uley South has been compromised and will soon show irreversible decline in water levels too.

 

The temptation now is to look at Uley South and argue “it has maintained its water levels pretty well over the years therefore we can go on taking up to 6-7GL/yr”. This is only true if Uley South has not in fact had any interconnections with other basins. With the virtual demise of both Uley Wanilla and the recent demise of Uley East, whatever interconnections that existed are now probably close to zero. To base future management of extractions on these possibilities without further data is a recipe for disaster for the future of Uley South.

 

A FINAL OBSERVATION

 

At some point a judgement must be made about the level of impact and the region’s demand for water. Should this judgement be accepted community-wide, many think the problem is now solved – but it isn’t. The aquifer is now being managed when there is no longer any chance of it ever getting “full” if extractions continue. But more than that, if the aquifer leaks or water must be allowed to move through to the marine environment to stop any sea-water intrusion, the amount of water available for extraction can only be equal to the amount that would normally be lost to in other ways, eg springs, wells and swamps and not just some arbitrary %age of “recharge”. Of the natural or expected losses, the only one being monitored to any extent is a well. If the level in wells keeps dropping then too much water is being extracted. While this can be tolerated or compensated for a time, at some point extraction MUST come to a halt – the aquifer will be dry (eg Polda Basin). The only management decision to be made is – do we extract to the point of leaving no water in the aquifer, or do we try and maintain the aquifer at some lower-than-“full” level by juggling the rate of extraction?

 

The trouble with this latter option is that there is very little water is actually available for extraction. Above a certain amount and it will continue to decline, below a certain amount it will start to rise. Keeping the water level at some pre-determined, agreed upon level means that annual extraction rates will be “controlled” by water level changes in the previous year. Trying to put in place a system that allows greater certainty can only guarantee less water. Certainty comes at the cost of volume. Huge extractions year after year cannot occur unless water is taken from the storage volume, which will in time diminish to close to zero and the idea of “maintaining” a less than full aquifer is defeated.

 

The two underlying weaknesses of the approach being made by our water managers and the various authorities are: (1) deliberating ignoring the history of the basins, and (2) allowing members of the community to be directly and fully involved in water management planning and decision-making. Until these two constraints are acknowledged and rectified, dealing with other issues will not bring a result that can be supported by the community.

 

Overall, the science based arguments, and the science based documents such as Status Reports are so lacking in consistency and rigour that the real situation is obscured and do not allow ecologically sustainable management to be effectively implemented. Much of this situation can be clarified by using increasingly sophisticated models that estimate the volume of water in a basin each year, and management decisions based on maintaining it.

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