In discussions about water allocation in the Murray Darling Basin it is generally assumed that runoff now is equivalent to what it was at the time of European settlement. Consider, for example, the following table from a Murray Darling Basin Commission Facts Sheet.
At the same time there is a perception that there are a lot fewer trees now than there were at the time of European settlement. As Ian Mott points out in the following comment, first posted at ‘Banking in the Macquarie Marshes’, if there are fewer trees now, then there is more water now:
“If you really want to correct the misconceptions that threaten you water allocations then you should correct the fallacy that the pre-irrigation runoff into the marshes was 460,000 megalitres which has since been reduced to 395,000ml by irrigation.
The pre-settlement runoff into the marshes would have been much less than 460,000ml and most likely less than 395,000ml because much of the upper catchment has been cleared for pasture. And this has substantially increased the catchment yield.
But don’t expect the MDB Mafia or the Land and Water audit people to concede this willingly. The work of Robert Vertessy and the CRC for Catchment Hydrology makes it very clear that the switch from forest to pasture increases water yield.
So your group has to determine the exact amount of clearing that has taken place in your catchment and overlay the rainfall data so you can find out the real historical water footprint for the marshes.
There is not the slightest room for doubt that the volume of water taken out of the system by irrigators, given that extractions are only 14% of current runoff, is less than the improvement in yield produced by clearing.
My understanding is that the upper Macquarie is not subject to widespread thickenning like the mulga and brigalow country so cleared land has been more likely to stay cleared.“
Jennifer Marohasy BSc PhD is a critical thinker with expertise in the scientific method.
This is a point that could well apply to the whole MDB, as I had independently realised during preparation for my water use article on OLO
David Tribe
Ian, not only does one have to account for tree clearing but also woodland thickening in uncleared areas which would tend to decrease runoff, have you conveniently forgotten your own postings on this point?. If there significant areas of overgrazing in the catchment this would increase runoff to to poor ground cover and decreased infiltration rates. Finally changes in CO2 from 280-380ppm should also be impacting, possibly increasing flows especially during wet periods. The translation of runoff to rainfall is non linear, where is the analysis to show that there is no change in the pattern of rainfall (even with the same annual rainfall runoff can vary appreciably). Selection of the “baseline period” for historical flows and the “analysis period” for modern flows could give results quite at varience with the data presented in the table.
So we have to simulate the lot – we need the historic land use, CO2 increases for transpiration effects, and dare I suggest convert all this from megalitres of flow to the historical frequency distribution of inundation periods in the marshes. Sound like a job for some very clever landscape modellers.
(And not sure Jen should accept any MDB numbers – if they can’t get it right on salt .. ..)
I would like to make a couple of comments in relation to the historical runoff above the Macquarie Marshes.
Runoff is largely determined by land use.
The relationship between runoff and changing landuse above the dam has probably been quantified but below the dam dryland farmers are intent on increasing the infiltration of rainfall using no till techniques. They are maintaining greater ground cover with more conservative stocking rates, better rabbit control and so on, also increasing infiltration.
Environmentalists are intent on increasing the woody plant cover which may also increase infiltration.
I suspect that this changing land use will increasingly lessen runoff and that it will be of the magnitude to challenge the effect irrigators have on the river flows.
It has the potential to nullify the river flow benefits of water buybacks.
My suspicions come from practical experience and my bit of goggling has not uncovered any science that quantifies runoff with changing land use.
Have I missed something?
You are quite right Geoff, modern farming practices can increase infiltration and retention thereby increasing water holding capacity. This would have to be offset against previous land use to determine nett effect.
Modern grazing practices can also have an effect on grasslands.
Forests hold water and release it through evapotranspiration. Mature forests differ from harvested forests, regrowth and regeneration can be high water users during various phases.
There is some concern that timber plantation will use up much of the available water and this is of concern to catchment areas.
There has been a lot of work done to gauge the nett effect of water between forest and grassland, both in volume and velocity of flow.
“Annual evapotranspiration is generally greater for forested than for non-forested catchments and tree plantations will increase catchment evapotranspiration compared with pastures or crops.”
http://www.catchment.crc.org.au/pdfs/technical199912.pdf
However using forest data may be inappropriate; “The high rainfall data sets used in the Holmes and Sinclair relationship make it of little use for floodplain wetlands in arid or semiarid regions, although it may apply to those in temperate regions. It is not considered appropriate for tropical regions”
http://www.lwa.gov.au/downloads/publications_pdf/PR000212.pdf
Grassland management can make or break catchment areas, “…Set stocking at low stocking rates is particularly detrimental and should be avoided..
..Land-use planning must be linked to land capability, and a landscape-based plan is an essential management tool. Grazing management should aim to maintain the diversity of species within grasslands, as well as their capacity to use water in summer, and their ability to generate runoff.
This requires a flexible and strategic approach to grazing. High grazing pressure for short periods followed by extended rest periods may be the most appropriate management strategy for diverse grasslands”
http://www.regional.org.au/au/stipa/2001/p-15.htm
Richard D, you will note my referenceto an absence of widespread thickenning in the upper Macquarie catchment.
This issue was covered in a detailed report with four main yield scenarios in “The implications of adjusting runoff values from land clearing on water yield in the Murray-Darling Basin” Ian Mott 2005. This analysed clearing data by bioregion as supplied by Benson J. to the NSW Vegetation committee that developed the latest legislation in that state.
It is too big to post here but a summary can be seen at http://ianmott.blogspot.com/
The Reports Conclusions were;
The volumes of water that constitute the natural flows of the MDB have been wrongly assumed to be unchanged from pre-settlement days. The very substantial increases in runoff that are a by-product of clearing have been deliberately ignored.
This assumption has allowed the volumes and frequencies of water that have been used by wetlands and flood plains within the natural range of variation to be grossly exaggerated.
This assumption has allowed the volumes and frequencies of water that have flowed to the sea within the natural range of variation to be grossly exaggerated.
Most, if not all, of the water being diverted from the Murray-Darling Basin is drawn from a man made surplus yield that the science is clearly capable of predicting but which the policy/science interface either refuses to recognise or has selectively applied.
This has allowed the policy process to ignore the fact that most water capture takes place in the middle levels of the catchment in major storage facilities. And this obscures the fact that vast areas of the Murray -Darling Basin above these storage facilities are in robust good health from significantly enhanced volume and frequency of flows.
And the failure to recognise this man made surplus has seriously distorted the current degree of departure from the natural range of variation in river flows and has seriously distorted the character, scale and extent of adverse environmental impacts in the Murray-Darling Basin.
The assertion that natural flows within the Murray-Darling Basin have been seriously over-allocated or seriously over-utilised has no basis in fact.
This is not to say that water is not wasted in the basin. At least 2000GL is wasted from evaporation of fresh water from a converted tidal estuary and the maintenace of fresh water “values” in that artificial system. But this “wasted” water has still delivered all the ecological services that nature expects of it as it flowed the length of the river system. But it could also make a much greater economic contribution.
The issue of man made surplus water yields is a “relevant consideration” within the meaning of Section 5 of the Commonwealth Administrative Decisions (Judicial Review) Act 1977, which, by way of informing questions of best practice governance, may not be ignored by any policy process.
This material is of such magnitude and relevance that any discussion of water usage or balance in the Murray Darling Basin that omits this material is worse than bad science or policy, it is a misrepresentation of fact to the policy process that may also constitute official misconduct.
The Government has a clear duty to refrain from taking any actions on decisions that have already been taken without informed consent, and under inappropriate circumstances, and to fully investigate the matters raised in this report.
The CRC for Catchment Hydrology paper by Zang et al is at; http://www.catchment.crc.org.au/pdfs/technical200108.pdf
Rog, the quote from the National Land and Water Audit site would be be credible if the LWA actually recognised the issue. They still don’t. All their stuff on water flow and yield in Victoria assumes that current flows are “natural flows”. There is no adjustment for altered yield.
And the most important yield changes in the MDB are in the highlands where the rainfall is between 1100 and 600mm. For the marshes it is not the yield in the marshes themselves but up stream in the higher rainfall zones.
The sincair stuff is 1986, I seem to recall, while Zhang et al is much more recent.
In general there appears to be a global consensus that forests reduce recharge of groundwater.
Therefore clearing land should increase water flows to wetlands.
Could someone tell me why the Macquarie Marshes is littered with dead trees, infested with Cumbungi and had a dribble of water flowing through it in the middle of the 2002/2003 drought?
Marshes do have only a dribble of water through them in droughts, Michael. Thats why they call them droughts.
Thanks,Rog and Ian, for the references to various reports/papers on water use and runoff.
Assuming other factors such as rainfall intensity remain constant, current and future land use does point to lesser runoff.
I cannot see where anybody has had a stab at predicting the magnitude of this other then in the case of hypothetical reforestation.
I think this is important because figures show 31.5% of runoff north of the Murrumbidgee/Murray is presently used. If runoff decreases by say 15% and environmental flows increased, what is the future for the irrigators and indeed much of west of the Dividing Range [Well named isn’t it] in central/northern NSW and Q’Land.?
I thought that the available data was insufficient to make any other than a general assessment relating to the Macquarie Marshes.
Irrigators may well be the most efficient users of water, and irrigators can also be graziers (albeit intensive).
Extensive tree planting may also take valuable water away from the marshes.
Geoff, these people are fully capable of quietly capturing every benefit that is produced by farmers and then assigning every cost onto those same farmers. That is the nature of parasites, they will take, and take, and take until the host is dead. They will just go looking for another host while chanting some mantra about “partnerships for intergenerational equity”.
By their deeds shall ye know them.
“Marshes do have only a dribble of water through them in droughts, Michael. Thats why they call them droughts.”
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These are a semi-arid, lower floodplain wetland system right? I was surprised by the presence of any water at all in the middle of the drought, that was my point. I presume river regulation upstream was to blame.
I was extremely dissappointed by the Macquarie marshes and kept on driving when I went to visit the area back in 2002. They looked like a pig infested, shit mess of cumbungi, dead trees, weeds and stagnant permanent water. Looked awfully stressed and unhealthy, but certainly not because of lack of water!
Sorry Michael, I took it the other way. But you are right, it is most likely this fetish for maintaining artificial water flows during times when rivers would have been bone dry, that has played the major part in habitat destruction.
For this ‘landscaping water’, (you couldn’t correctly label it as ‘environmental flow’) play a key role in concentrating ferals and excessive native animals, especially ‘roos, in the very places where they will do the most damage.
This water does not act as a sustaining refugia in drought but merely operates to prolong the suffering of populations that routinely undergo up to 90% declines in populations in drought.
These natural population declines are the flip side of the enhanced reproductive capacity of natives and ferals in good years. But the bimboscenti do not carry out any culling of these species when circumstances demand so large populations are left long after farmers have either destocked their herds or have switched to suplementary feeding.
The evidence is very clear, especially in the mulga lands where ‘roos don’t eat mulga that is pulled for grazing stock, that the critical damage that is being inflicted on overgrazed pastures, and public reserves, is being done by the public’s negligently managed native and feral animal herds.
Access to artificial water supplies, at stock watering points and by non-seasonal creek flows from regulated sources, plays a critical role in keeping animals alive and overgrazing when nature would have killed most of them off much earlier.
But that is par for the course in the brave new green utopia.