2011 update 37 year Perth dams catchment rainfall trend – nearly a Thousand GL of water wasted over 15 years by not managing catchments

In 2011 the Perth region has enjoyed an above average rain year – as shown in my graphic of 37 years of Perth dams catchment region May-October rainfall.
37 years of Perth dams rain
This second graphic shows the constant decline in “catchment efficiency” CE since the late 1990’s – which was about the last time water authorities did much to control understorey regrowth in catchments which drastically cuts back inflows to streams. A calculation for each of the last 15 years – assuming that a CE of 5.6% could easily be achieved without harming wildlife values – shows that a total of 998GL has been lost from 1997-2011. I have not costed this – but the extra 66GL PA would have made it unnecessary to embrace seawater desalination in such a rush.
2010 update 36 year Perth dams catchment rainfall trend
Stunning ignorance and lies surround Perth water supply policies
2009 update – Perth dam catchments rainfall still normal, Govt building $Billion seawater desalination plant #2
There never was a rain shortage to justify seawater desalination for Perth’s water supply

12 thoughts on “2011 update 37 year Perth dams catchment rainfall trend – nearly a Thousand GL of water wasted over 15 years by not managing catchments”

  1. Warwick.

    You talk as if not clearing undergrowth in forests reduces water supply to the dams. My understanding (from Peter Andrews who has written extensively on this) is that the more vegetation in the cachment, the better from a water management viewpoint, since the vegetation assists holding more water back in the landscape. This water is releases slowly and regularly so that you get more constant stream flows.

    That is, the issue is only one of how fast the water is released. He says that if there is less vegetation in the cachment, then when storms develop, the water rushes off the land in a flood, depriving the land of the use of that water, which downgrades the whole landscape. Especially if the floods are big enough to cause water to flow over slipways. In that event, the water is lost entirely.

    Management of fire risk is another issue entirely that needs to be considered separately.

  2. Gidday mondo,
    The well known process of thinning forest to increase streamflows does not have to mean clearing and “water rushing off the land” – with those negative connotations.
    When “…the vegetation assists holding more water back in the landscape.” – result is increased evaporation/transpiration – = reduced water for streamflow.
    If PA says “This water is releases slowly and regularly so that you get more constant stream flows.” Then I do not think studies show that – as I said above – when increased veg holds back water then incr evap/transpiration will result = less water for streams.
    Catchment thinning is a process of aiming for a balance between retaining bio-values yet increasing or maintaining streamflows for the human species.
    Correct me if I am wrong but I understood PA to be a horse dealer, farmer, author – not a scientist.
    I have read his book which I enjoyed but some passages purporting to describe earth processes if read carefully – do not stand up to close scrutiny – in my humble opinion.
    all the best mondo.

  3. When eucalypt forest catchments experience catastrophic wildfire e.g. the 1939 Black Friday fires in Victoria, the sequence of regrowth has the following impact on runoff:

    * firstly there is an increase in runoff for the first few years,

    * followed by a massive reduction in runoff during the twenty years plus period of vigorous regrowth, and

    * finally after many years a return to normal long term runoff levels.

  4. Warwick,

    Thank you for updating on this issue.

    Your information on water catchment management was an eye opener.

    Keep up the good work

  5. Warwick
    What do you think of the idea of clearing part of a catchment and covering it with geo-fabric to stop erosion. This would enable tou to catch close to 100% of the runoff. While the bush is an excellent filter for the run off , it would seem to be very inefficient from a collection point of view. It may seem like a hare-brained scheme ,but not as silly as trying to turn coal into water(via de-sal)

  6. In earlier times it was not uncommon for isolated towns to concrete or tarmac a little catchment – get near 100% runoff for their town supply. It might be possible for Perth to “geo-engineer” a small percentage of their total 3500sq km of catchment to get a vastly increased runoff – the debate has never been had. If it were possible without setting off unacceptable salinity increases – could be a novel solution. Then 80-90% of the bush could be left alone – as it is now to placate the Greens.

  7. It should be possible to work out average rainfall over those 3500sqkm to work out volume of rainfall vs volume in the reservoirs. The difference would be the amount lost to evaporation,soaking into subsoil etc.
    Realistically there is a finite amount of bushland that can be reserved for reservoirs and as our cities grow it is going to be a problem.
    It is very frustrating when we get big rains and the reservoirs don’t seem to fill up at all. The rain has to fall in the right sequence for it to have any affect.Summer rain for example is pretty useless in terms of filling reservoirs.

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