Comments on: Wishful thinking about forests and water

By: buddh•ism ad•junkt › Urbanism, Sprawl, and Water

buddh•ism ad•junkt › Urbanism, Sprawl, and Water — Mon, 13 Aug 2007 18:07:30 +0000

[...] while the long-time regional identification of forests as guarantors of water supply may be an ecological falsehood, it clearly has some sort of historical experience behind it: killing of the forests may not [...]

By: Royal hydrology

Royal hydrology — Sun, 12 Aug 2007 10:27:44 +0000

[...] that!).� But I beg to differ.� Whatever the benefits of forest cover, increased water supply is not one of them. In fact forests are big users of water and tree plantations (given their rapid growth in the early [...]

By: Cambodia » Blog Archive » Ko Chang journal: Cambodia to Ko Chang

Cambodia » Blog Archive » Ko Chang journal: Cambodia to Ko Chang — Fri, 13 Apr 2007 07:43:04 +0000

[...] I think it would be a good way to make a positive contribution and enjoy a holiday of a very … Comment on Wishful thinking about forests and water by Deathpower … […] read Andrew Walker’s post “Wishful Thinking About Forests and Water” over at the excellent [...]

By: New Mandala » Roadside hydrology

New Mandala » Roadside hydrology — Fri, 13 Apr 2007 04:40:13 +0000

[...] for the comments on my previous post about forests and water. Forest hydrology is a complex issue and I hope that the paper I refer to in [...]

By: Bystander

Bystander — Fri, 13 Apr 2007 01:10:17 +0000

I am curious as to how the evapotranspiration numbers are arrived at in these studies that you cite. Did they do real in situ measurement? Or do they just guesstimate from what is known about the number and types of trees and their density and other more easily measureable quantities? Also, do they also account for recondensation? Water loss from the soil during evapotranspiration which happened much more during the day because of the photosynthesis-capillary pull is not necessarily all lost to a bottomless sink. I can imagine that the increased humidity from ET will lead to a feedback which has an inhibitory effect on further ET. Also, during the night, a lot of the lingering moisture is going to condense and drop back to the soil. Anyone who has done some camping in the forest of northern Thailand should notice that recondensation is quite considerable. You can get quite soaked by it. A well known case of this kind of effect is the coast redwoods of Northern California. These trees are very effective at capturing moisture from the air. Many studies have been done on these.

I don’t think this ET number will be an easy thing to arrive at with any kind of uncertainty unless somebody did some very extensive modeling or measurements. So, I’m not quite convinced of the given explanation that attribute the observed effect to increased ET in forest.

And what about these:
“the absolute level of dry season flow does not necessarily decline and it may increase.”
Is that another way of saying there is no correlation or the data is too noisy?

Anyway, I guess these days I am extra-cautious about any studies that claim certain yes/no things about the climate and the environment. The devil is always in the details.

By: Andrew Walker

Andrew Walker — Thu, 12 Apr 2007 22:20:31 +0000

Paul – you should write this up. There are plenty of places with good long term land cover and rainfall data and I can’t recall too many studies that so confidently link logging with reduced rainfall. Would be nice to see a detailed case study.

By: Paul Sidwell

Paul Sidwell — Thu, 12 Apr 2007 21:45:05 +0000

Logging does cause reduced rainfall and riverflow, but the real effect is felt 50 to 100 years later, after governments, NGO reports, and the actions of our granparents have been forgotten. The short term effects can include significant increases in flow from run-off, and subsequent reduction in waterflows as replanted forests soak up rain, so it is easy to make convincing reports that show beneficial effects of logging. But, to rephrase an old saying: “we are here for a long time, not a good time” and our children will know it. I grew up in an area of Victoria that had the tallest trees in the world, mountain ash up to 450 feet, all cut down to create suburban fencing for Melbourne (!!) in the 1880s. In the century that followed rainfall dropped from 86 inches a year to now less than 40, specifically in that area. No “literature seach” conducted within a generation of that logging would have revealed that reality.

By: Deathpower in Cambodia » Blog Archive » Interested in Water and Forests?

Thu, 12 Apr 2007 17:36:37 +0000

[...] read Andrew Walker’s post “Wishful Thinking About Forests and Water” over at the excellent New Mandala [...]

By: Erik Davis

Erik Davis — Thu, 12 Apr 2007 16:54:26 +0000

I have no agenda to pursue here, but am left (by both this post and the paper) feeling that the total situation is not adequately addressed here. No ‘total situation’ could be addressed of course, but there are deeply connected issues which seem left out. I am not a hydrologist, so please forgive the naive questions: First, in what way is the dry season water flow being maintained or increased, if not in forest systems? What are the impacts of this mode of retention and flow? And finally, is there a connection between these flows and soil erosion?

Thanks for your continued work on these important water and land issues. It is indeed vital that uplanders stop being blamed for, well, everything relating to the environment.

By: John Francis Lee

John Francis Lee — Thu, 12 Apr 2007 12:42:23 +0000

I have followed up a bit on tropical forest hydrology and discovered that you have not, in fact, “cherrypicked” your sources but have fairly reported the consensus among the people who spend their time earnestly studying the subject.

So I am chastened and apologize for having assumed the worst case concerning your motivation.

I still love forests and hope they return to Thailand. I am confident that if they do we will all appreciate their value and therefore approve their price.