As the dry season draws on, claims are often made that low flows in the region’s rivers and stream are a result of deforestation. Recently the Lao prime-minister got in on the act:
“If we don’t protect the trees, water in rivers will dry up. As we can observe in every dry season, many rivers now dry up,” PM Bouasone said. He said that protecting watershed areas was vital. “If we don’t do it, in the future we will have only concrete dams with no water,” he said. He added that Laos wanted to be the electric ‘battery’ of Asia . “If we don’t our protect forests, our objective of becoming Asia ’s battery might not come true,” he said… Logging has a massive impact on the biology and water level of rivers.
This thinking is often informed by the view that forests are very effective catchment “sponges” - storing water from the wet season rains and then releasing it slowly during the dry season. It’s a nice theory, but the hydrological reality is a lot more complex.
A few years ago I undertook a detailed review of the literature (focussing on Thailand but also looking at international studies) on the relationship between forests and water. Here is a copy of the full paper [walker-2002-forests-and-water.pdf]. The key conclusions are:
Analysis of rainfall data suggests that there is no clear evidence of a long-term regional decline in rainfall, despite significant reductions in forest cover over this period.
Numerous international hydrological studies, and some studies undertaken in Thailand, show that forest clearing has the effect of increasing annual stream flow, given that clearing forest lowers the percentage of rainfall that is lost to the atmosphere in the form of ET [evapotranspiration].
A limited number of regional studies suggest that forests have higher rates of infiltration than cultivated land covers (though the actively cultivated portions of the cultivated landscape may have rates of infiltration similar to that of forests). Infiltration in cultivated landscapes is reduced by soil compaction and by the presence of hard surfaces such as roads, pathways and settlements. However, relatively accessible areas of forest (which are also probably the areas most likely to be cleared) also appear to be subject to a degree of compaction and reduced infiltration.
Hydrological studies and modelling exercises suggest that the higher rates of run-off in cultivated landscapes tend to alter the seasonal pattern of stream flow with a greater percentage of annual stream flow occurring in the wet season.
Hydrological studies and modelling exercises suggest that while clearing of forest for agriculture may change the pattern of stream flow, the absolute level of dry season flow does not necessarily decline and it may increase. This arises from the fact that the level of annual flow is higher given the reduction in ET. The positive impact on dry season flow of reduced ET will be outweighed by the negative impact of reduced infiltration only when runoff reaches relatively high levels.
These may seem like rather technical hydrological arguments. But the politics of hydrology is important. All too often upland farmers are blamed for destroying lowland water supplies. The reality is much more complex and those complaining about the loss of the forest “sponge” tend to ignore the fact that many lowland farmers are using much more dry season irrigation water than they ever did before.
19 responses so far ↓
1 Resource Management in Asia Pacific » Forests and water // Apr 11, 2007 at 9:46 pm
[...] readers may be interested in my recent post on New Mandala about the supposed relationship between forest loss and dry season water shortage in [...]
2 anon // Apr 12, 2007 at 9:07 am
Andrew, so you’re saying the King’s monkey cheeks idea doesn’t work?
3 Andrew Walker // Apr 12, 2007 at 9:21 am
The royal family has certainly contributed a lot to popular views about the relationship between forests and water. Here is just one example (from the paper I mention above). It is a quote from a brochure put out by one of the queen’s projects in northern Thailand:
But I am not sure that the “monkey cheeks” idea is really about the hydrological role of forests. Here is a brief description:
I don’t have any particular knowledge or expertise on this specific proposal.
4 Bystander // Apr 12, 2007 at 10:31 am
The monkey cheek idea was put forward when bangkok was flooded back in the 90s and has nothing to do with the forest.
That said, it’s not clear that this kind of “royal wisdom” on very technical matter are on the whole a good thing. A thorough and rigorous study may prove otherwise, and nobody really can say they know for sure without having all the data. However, with everyone accepting this kind of wisdom from on high, it’s not going to be easy to get things done right.
I heard a while back that some big shots in Thailand is trying to find someone in a food science department in reputable first world university to conduct a study ‘that proves that Thai food is good for health’. Well, if you already ‘know’ the answer you want…
5 nganadeeleg // Apr 12, 2007 at 10:45 am
Well, if you already ‘know’ the answer you want…
Lets cut down forests – it will improve the dry season flow.
6 John Francis Lee // Apr 12, 2007 at 12:07 pm
This article on forests seemed alredy to know the answer it wanted : forests are not effective buffers of rainfall.
It concludes
and its hypothesizes
Hydrological studies and modelling exercises suggest that while clearing of forest for agriculture may change the pattern of stream flow, the absolute level of dry season flow does not necessarily decline and it may increase.
The first is a statement of the obvious. If you cut down all the trees the water they used to transpire will then flood out into the water courses. This will necesarily take place when it rains, that is in the rainy season when there is “too much” rain. Chiang Mai especially seems to be illustrating that point over the past few rainy seasons.
The second point is pure speculation. The bit about dry season runoff (”it may increase”) seems to be the result of imagining that ET will be reduced in a linear fashion year round after the loss of the forest. I look at the forests in the dry season, noting that many trees have lost their leaves and that others’ leaves are dusty, inefficient transpiration agents, and wonder if forest transpiration doesn’t shut down in the tropical dry season as it does in the temperate cold season.
Too, the bit refuting the “mi pa, mi fon”, seems especially mean-spirited, as so much of the criticism of Thailand and of the Thai does on this site. The very high rates of ET during the rainy season seem sure to increase the relative humidity in the areas adjoining forests and the resultant fogs and secondary rainfall may well be that to which the local wisdom refers. The rains that are transpired are not lost to outer space.
I admit that I love forests, that I love orchids, that I love wood, that I love shady streams, that I am prejudiced in a direction diametrically opposed to the author’s and to his survey of the literature for proof of his point.
7 John Francis Lee // Apr 12, 2007 at 12:10 pm
and its hypothesizes
Apologies again for misformatting, there’s no way to preview a post on this site, is there? The above is a quote from the author and not mine.
8 Andrew Walker // Apr 12, 2007 at 12:21 pm
John – it’s not “pure speculation” but the informed conclusion from many respected hydrological studies. Read the literature. There are lots of good reasons for loving forests, but the “catchment sponge” is not one of them.
9 Jon Fernquest // Apr 12, 2007 at 6:47 pm
>All too often upland farmers are blamed for destroying lowland >water supplies.
Thanks for injecting a bit of rationality into an area that needs it.
Lieberman in Strange parallels covers the debate over climate factors for much earlier periods. I think your study clearly demonstrates the need for hard science to counterbalance the “discourse” issues that have come to dominate intellectual bandwidth in social science / historical research recently.
10 John Francis Lee // Apr 12, 2007 at 11:42 pm
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.
11 Erik Davis // Apr 13, 2007 at 3:54 am
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.
12 Deathpower in Cambodia » Blog Archive » Interested in Water and Forests? // Apr 13, 2007 at 4:36 am
[...] read Andrew Walker’s post “Wishful Thinking About Forests and Water” over at the excellent New Mandala [...]
13 Paul Sidwell // Apr 13, 2007 at 8:45 am
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.
14 Andrew Walker // Apr 13, 2007 at 9:20 am
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.
15 Bystander // Apr 13, 2007 at 12:10 pm
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.
16 New Mandala » Roadside hydrology // Apr 13, 2007 at 3:40 pm
[...] 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 [...]
17 Cambodia » Blog Archive » Ko Chang journal: Cambodia to Ko Chang // Apr 13, 2007 at 6:43 pm
[...] 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 [...]
18 Royal hydrology // Aug 12, 2007 at 9:27 pm
[...] 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 [...]
19 buddh•ism ad•junkt › Urbanism, Sprawl, and Water // Aug 14, 2007 at 5:07 am
[...] 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 [...]
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