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Land clearing caused drop in rainfall in South West of Australia

Rain rain go away, let’s chop a forest down today?

Mark Andrich and Jorg Imberger compare the rainfall patterns in different regions of southwest Western Australia. The areas where the most land was cleared show the greatest decline. They estimate that as much as 50 – 80% of the observed decline in rainfall is the result of land clearing, which doesn’t leave much to blame on CO2.  The paper came out in 2012.

This fits with other researchers working on the Amazon who estimated chopping down the forests could reduce rain by as much as 90%. Once again: it’s not so much that trees grow where the rain falls, but that the rain falls where the trees grow, and the taller the trees, the better.

So the good news for Greenies is that we ought to plant more trees (and I’m all for that). But driving a Prius, building windmills, and using solar panels won’t do much for our rainfall. (It’s so strange anyone thought it would. The witchdoctors have them completely bamboozled.) The Abbott government’s plan to plant trees to sequester carbon may work, but by accident, not because of anything to do with CO2.

Oh the irony. The evil climate skeptics want more trees, while the good and earth-loving gullible Greens want a forced financial markets of fake goods (sounds more like bank-loving!).

If you are a rainfall analyst, WA (where I live) is a bit of a prize spot because, unlike most of the world, the flora was mostly chopped down after long-term rainfall data started being collected. So it’s possible to analyze the effect clearing has on rainfall patterns. The rainfall has declined by 30% since 1970 in the inland areas of southwest Western Australia, as climate activists like to remind us at every opportunity. Instead of being a prime example of a global warming disaster, it turns out that southwest WA is a bit of a poster-child to show the effects of land clearing.

The many ways land clearing can affect rainfall

To gloss over a complicated array of effects: clearing land increases the albedo (which means the surface reflects more light), and there are lower transpiration rates (the air is drier and there is less latent heat flux in the boundary layer). Trees affect something called the Biotic Pump (see here as well), and produce volatile organic compounds (VOCs) that seed cloud nuclei, so there are less cloud seeding particles if there are less trees.

Taller trees break up the surface, and without them the surface profile is flatter, so the wind flows faster. Inland WA is a pretty flat region, mostly around 300 m above sea level, and trees as high as 100m tall would make a big difference to the profile. Indeed rainfall increases by 40mm for every 100m in altitude between Fremantle (the port) and the hills.  This is known as the orographic effect. We don’t just want trees, apparently, we want tall trees.


Figure 7. Rainfall zones. Moving from east to west, Zone 1 includes the uncleared region east of SWWA, Zone 2 includes the wheatbelt, Zone 3 includes the hills region, and Zone 4 covers rainfall station locations along the coast. The station location details and rainfall data are available from BOM [2012]. The station locations are as follows: 1.1 Bullfinch; 1.2 Lake Carmody; 1.3 Ravensthorpe; 2.1 Northampton; 2.2 Beverley; 2.3 Duranillin; 2.4 Broomehill; 2.5 Deeside; 2.6 Merredin; 3.1 Mundaring Weir; 3.2 Dwellingup; 3.3 Brunswick Junction; 3.4 Collie; 3.5 Nannup; 3.6 Wilgarrup; 3.7 Manjimup; 3.8 Pemberton; 4.1 Mandurah; 4.2 Cape Naturaliste; 4.3  Busselton; 4.4 Cape Leeuwin; and 4.5 Albany. All zone station location numbering corresponds with the rainfall at station locations shown  in Figure 8.

Zone 1 above is the most arid and furthest inland, but also the least cleared, and it hasn’t lost rainfall (though it didn’t have much to start with).  Zone 4 is the wettest area close to the coast. But Zone 3 is the hilly escarpment where the biggest trees live, which has the highest annual rainfall, and it shows the largest decline in rainfall.  Zone 2 (the wheatbelt) was drier to begin with and was more heathland and forest.

Note the scale changes. Zone 1 gets about 20 cm (8 inches) in winter which is the “wet” season.  Zone 3, the wettest, gets five times as much.


Figure 8. 9-year moving average of winter rainfall. Zone 1 has a slight increase in rainfall over time, Zone 2 rainfall declines, Zone 3 rainfall has the largest decline, and Zone 4 also declines, but by less than Zones 2 and 3. The zones are shown in Figure 7 and exact locations are available from the BOM [2012]. (Click to enlarge)

Most of the clearing happened from 1950 – 1980

In 1910 around 90% of the wheatbelt was covered in native vegetation. Clearing accelerated from 1950 to 1980 when 40% of the land was cleared. By 1980 a mere 20% of natural cover remained.

Andrich and Imberger calculate the dollar effect of deforestation on water resources: “if deforestation had been managed in a way that did not reduce rainfall at reservoirs or increase streamflow salinity, then SWWA residents could be paying as little as $765 M/year for their water (instead of $1,165 M/year).” The additional expenses on water work out to be around $250 – $300 dollars a year per household.


It is widely recognized that southwest Western Australia has experienced approximately a
30% decline in rainfall, in areas inland from the coastal margin, over the last forty years or
more. It is generally thought that this decline was due to changes induced by global warming,
but recently evidence has emerged suggesting that a substantial part of the decline may be
attributed to changes in land use. These changes involved extensive logging close to the coast
and the clearing of native vegetation for wheat planting on the higher ground. We present a
methodology that compares coastal and inland rainfall to show that 50 – 80% of the observed
decline in rainfall is the result of land clearing. Using an index of sustainability, we show that
the economic consequences associated with this change of land use on fresh water resource
availability are substantial, disproportionately affecting the environment and poorest
members of the population. Given that the effects of land-use change on rainfall have been
recently shown to be widely underestimated world-wide, the methodology is relevant to other
regions where land-use change may have caused rainfall reductions in the past.

Rainfall is especially important in WA — about half our water comes from underground aquifers, and farmers across the Wheatbelt depend on rainfall in a make-or-break kind of way. Perth dam levels are often in the 20% range, and two desalination plants were built in the last decade to ensure the water supply. Right now, Perth dam levels are nearly 37% full — which is not as bad as it sounds, last year dams were only 31% full at the same time.

WA used to have very tall trees

There are still tall trees of course — like the glorious Karri trees which grow up to 90m tall, but there were probably a lot more of them.

Although the impact of the early indigenous humans was likely transformational, the low population density, lack of modern machinery, and long time scale for recovery probably allowed the vegetation to evolve without dramatic short term devastation. This continued until European settlement began in 1829.

By 1901, when Western Australia officially became  a state of the new Australian Federation, the non-indigenous population had reached 184,000  [ABS 3105.0.65.001, 2006], six times the original indigenous population density. In these  seventy years 4,900 km2 of land had been cleared according to the West Australian State  Library Collection [WASLC, 2001].

“In 1902 it was estimated that the SWWA contained the following forests (not including other minority species such as Red Gum): 32,000 km2 of Jarrah; 4,800 km2 of Karri; 28,000 km2 of Wandoo; and 16,000 km2 of York Gum [Fraser, 1904]. It was reportedly not unusual to find trees 300 ft (100 m) in height, 20-30 ft (7-10 m) in circumference and 40 tons in weight.”

WA used to surface nutrients (in the trees), now it has some of the poorest soil in the world

“The introduction of tractors with a ball and chain prior to the Second World War allowed land to be cleared quickly and by 1950, 68,000 km2 (30%) of the SWWA arable area had been cleared [WASLC, 2001]. However, it was not realized until the 1950’s that the Australian bush held almost all available nutrients above ground. As the trees were removed so too were the nutrients and trace elements that had once been recycled via leaves [Attiwill, 1966] and water was no longer adjusted within the soil by hydraulic lifting from the roots to the leaves [Whitehead and Beadle, 2004]. The native vegetation had evolved in a salt impregnated soil matrix [Peck, 1978] and as transpiration disappeared the water table began to rise, bringing the ground water nearer to the surface, where it intercepted the salt in the soil  matrix, turning the surface ground water saline, further impacting vegetation regrowth and reducing agriculture production [Peck and Hurle, 1973].”

Does CO2 cause weather patterns to shift south? Not really

It does not appear that the cold fronts (that bring the rain) have shifted southwards as some people (like the Climate Commission) claim. Long term rainfall from points on the coast (namely Cape Naturaliste, Cape Leeuwin and Dongara) shows little decline. Dongara — the most northern of these three long records shows a 12.5% decline after 1970, compared to the years 1884 – 1970.

Now here’s an odd paragraph about the decline in data in the last 15 years:

“Because coastal rainfall was stationary, the coastal analyses results do not support the hypothesis that global warming effects were contributing to rainfall decline; at least until around 2000 when data became less reliable.

It rather begs the question as to why data was less reliable after 2000 than it was before 1900? Data at Dongara has many “gaps from 1999 – 2004”. And at the two Capes, “data at both of these locations needs to be treated with caution as it was not quality controlled after 1997”. Hmm?

SW WA is a biodiversity hotspot – a wildflower wonderland

The Queen of Sheeba orchid

One of the special things about the southwest of WA is that it’s the opposite of places with rich soil and reliable rain where monocultures rule. Like other zones where nutrients are scare and conditions are variable, evolution produces weird and wonderful ways to fill a lot of little niches with an array of little creatures, and there’s a different plant for every moment. (Some 8,000 species, 75% of which are found nowhere else).

It could be argued that the main value of SWWA is its unique biodiversity, since the region is recognized as one of the world’s twenty five, and Australia’s only, biodiversity hotspot, with 1.4% of world-wide endemic plants [Myers et al., 2000]. Of the 4,333 endemic plant species (3.07%) are known to be critically endangered or extinct [DEWHA, 2006].

Total losses due to land clearing: $7.6billion

As well as agricultural losses due to salinity and falling rain, land clearing also impact on tourism, biodiversity, and freshwater fisheries (eg marron catches).

“Calculating a baseline value for tourism and biodiversity is more difficult than other  indicators. Nevertheless we estimate that the tourism, biodiversity and freshwater natural products ISF baseline indicator I6 is given by the sum of current tourism plus the value of biodiversity/tourism loss plus the loss of natural freshwater products, that is $6,950 + $590 + $20 = $7,560 M.”

For the global perspective, it could be said that the area has a population of about 2 million people but feeds tens of millions of people: SWWA produces less than 1% of the world’s wheat, less than 6% of the world’s apparel wool, and less than 1% of most other major products such as sheep meat and canola. (Kingwell and Pannell 2005)

Since climate models are pretty bad at predicting droughts, the theory that CO2 drives droughts could use a bit of a shake-up. The tree factor changes everything.


Andrich M., and Imberger, J. (2012) The effect of land clearing on rainfall and fresh water resources in Western Australia: A multi-functional sustainability analysis [Available here PDF]

Kingwell and Pannell (2005) Economic trends and drivers affecting the Wheatbelt of Western Australia to 2030,  Australian Journal of Agricultural Research, CSIRO, 56, 553-561


See also Tallbloke: New study lends support to Makarieva et al Biotic Pump theory

Hat tip to Michael. Thank you!

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