Sunday, January 21, 2007

Sound economic analysis made me a tree-hugger

Over a decade ago I wrote a paper (‘Forest Rotation and Streamflow Benefits’, Australian Journal of Forestry, 1994) that looked at optimizing the total water supply plus timber benefits from logging in a water catchment – specifically the Thomson River catchment.

When trees are of intermediate size and growing fast they cut into water yields simply because they drink a lot. When they are small their small absolute size means that their drinking has little effect while when they are old they don’t drink much because they are not growing quickly.
Thus if you are thinking about harvesting trees you want to avoid having forest stands that lie in an intermediate age group. Either cut them very young or leave them alone and don’t cut them at all to gain sustained water supply benefits. (I always enjoyed obtaining this result just by drawing a few graphs using Excel and tree growth-water yield data – the optimization task itself is a complicated, non-concave problem so drawing graphs turns out to be the simplest and best approach). Generally I favored not logging at all in the Thomson.

According to The Sunday Age this morning the solution I suggested seems right.

‘MELBOURNE is losing out on a million litres of drinking water every year [as reader ChrisL points out this is a ludicrously low, presumably a typo] from continued logging in the city's main catchment area. And it comes at a cost to the taxpayer of at least $147 million — the difference between the royalties paid by the logging industry to the State Government and the value of the lost water, according to economic consultants commissioned by Melbourne Water.
As the city heads towards stringent stage 4 restrictions, a host of scientific studies indicate the Thomson Dam, which supplies about 60 % of Melbourne's water, is losing up to half the potential run-off in the highest rainfall area owing to effects of logging……
It is estimated that if logging was stopped, water yields in the catchment would increase by 20,000 megalitres within two decades.
Despite the existence of studies dating to the 1950s, in 2004 the Bracks Government decided to conduct more research into the reduced water yields
caused by logging. It is scheduled to be completed in May 2008.
All the scientists spoken to by The Sunday Age questioned the need for further studies, saying the numerous existing reports, many of which were commissioned by the Kennett and Bracks governments and on which this article is based, were sufficient’.

Of course the Bracks Government seeks further studies because it is fearful of the electoral implications of giving the forest industry a well-deserved kick in the rear-end. In the meantime we continue to face costly water restrictions, lose beautiful forests of mountain ash and other species and pay a huge implied subsidy to the forestry industry.


chrisl said...

Harry , There is a typo or a beat up in the first line of the Age report.A million litres is only five household's yearly use. Not much in either volume or $

hc said...

Thanks Chris. Its not a beat up but it is a typo. The claimed loss further in the article is 20,000 megalitres and the Thomson supplies 60% of Melbourn'es water.

I had a quick look around but couldn't get a correct figure - when I do I'll correct it.

chrisl said...

Harry.Have they ever done a study on clearing some catchment land.( Before you say erosion the cleared land would be covered with geo-fabric,recycled of course) A study would require you to work out the volume that fell on an area compared with the volume that actually ran into a creek and thus into a dam. If you look at the trees and undergrowth in the Maroondah Catchment (Black Spur) it would take an ENORMOUS amount of rain for any to filter down into the catchments.

hc said...

I have no idea Chris. But the requirement that the ground be covered is a pretty stiff one. Without it there would be massive siltation & erosion issues as you suggest. Might there also be effects on microclimates?

chrisl said...

When you think about it the current method of water harvesting is very inefficient. It needs an enormous amount of land and the reality is we have run out of places to locate a new dam.Before we are asked to drink our own urine-or worse somebody elses- I think some tree thinning or clearing ought to be tried.
Did you study the effect of tree thinning but not allowing replacement trees to grow?

hc said...

Chris, No I didn't but I have studied it it other contexts. It tends to stimulate the growth of trees that remain which might be adverse for water yields.

But these forests have conservation value - I cannot believe that clearing underbrush would be that desirable. Even if you do harvest for timber the underbrush will protecty soils and in some cases provide a replacement forest.

derrida derider said...

chrisl, I understand that having a mature forest doesn't so much reduce the inflow as smooth it. This is really important in dry spells - creeks will continue to flow into the dam.

Here in the ACT, sub-alpine peat bogs are an especially important mechanism for this smoothing.

chrisl said...

Harry A couple of years ago I toured the limestone caves in WA. The water levels in the caves were dropping alarmingly which meant no more growth of stalactites. The theory was that there was too much growth in the bush above the caves, so they tried some controlled burning .... and the result... I don't know, I never went back.