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Posts Tagged Ecohydrology

Land use hydrology paradox in Central Texas Daniel Collins Apr 13

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ResearchBlogging.orgWhen it comes to conversion of grassland to shrubs or trees, the typical story goes like this. More rainfall is caught be the foliage and evaporated straight back into the air. Higher rates of transpiration deplete soil moisture faster, and deeper roots inhibit drainage of water from soil to aquifer. This story is typical because it is observed time and time again [1], but it is not the whole story. Nor is it always true.

A recent paper in Geophysical Research Letters, by Bradford Wilcox and Yun Huang, tells a different story [2]. They document annual streamflow for several rivers in the Edwards Plateau, central Texas. While the region has experienced an expansion of shrubs and trees – AKA, “woody encroachment” – spring-fed river flow has paradoxically increased.

The reason, they propose, is that while woody plants have expanded, grazing has reduced. Livestock trample and compact the soil, and eat the herbaceous plants, both of which reduce infiltration of water into the soil, and ultimately reducing recharge of the aquifers that feed the springs. But as grazing declined, spring-flow seems to have increased. It thus seems that grazing in the Edwards Plateay has a greater impact of river flow than woody encroachment.

While Wilcox and Huang have started to tell a different story, the story is far from over. They have eliminated historical rainfall as the source of the change, but they have not eliminated historical evaporative demand. And while spring-fed river flow has increased, it seems that flow from other sources has also paradoxically increased. If spring-flow increased due to herbaceous plants favouring infiltration, you’d think surface runoff would drop accordingly, but apparently not.

Whatever the answer, the researchers are on the case, and it is fair to say that another exception to the rule of land use hydrology will be found.

[1] Farley, K., Jobbagy, E., & Jackson, R. (2005). Effects of afforestation on water yield: a global synthesis with implications for policy Global Change Biology, 11 (10), 1565-1576 DOI: 10.1111/j.1365-2486.2005.01011.x

[2] Wilcox, B., & Huang, Y. (2010). Woody plant encroachment paradox: Rivers rebound as degraded grasslands convert to woodlands Geophysical Research Letters, 37 (7) DOI: 10.1029/2009GL041929

Euro geoscience conference enters the blogosphere Daniel Collins Apr 08

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I’ll be heading to Vienna at the end of the month for the European Geoscience Union conference, to give a talk on large-scale ecohydrology.

Wondering whether science bloggers have started talking about a meet-up, a google search found the official EGU 2010 conference blog. It’s just got off the ground, offering tips for pending attendees, but I expect it’ll be there live-blogging Kuhn’s the scientific revolution.

The sound of a tree singing the blues Daniel Collins Mar 24

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When plants are water-stressed, water in their xylem (their water-conducting veins) can be under so much tension that the air dissolved in the water expands explosively to fill the encasing xylem unit. This is known as cavitation. And with the right equipment, or the right trees and conditions, you can hear it.

BoingBoing has a delightful recoding of a one tree cavitating. Just ignore the explanation – it’s wrong.

Bioacoustician Bernie Krause, source of the above recording, is also seen here describing how he came upon the sounds (but again, ignore the explanation):

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PS. This has nothing to do with bluegrass, which is pretty much devoid of percussion instruments.

System to monitor grape vine water status wins water start-up prize Daniel Collins Mar 09

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Fruition Sciences, based in the US and France, has won the top prize in Imagine H2O’s competition “to help innovators and entrepreneurs turn great ideas into real-world solutions that ensure available clean water and sanitation.” Fruition showed particular business potential by making significant water savings for nine California grape growers.

“Fruition co-founder Sebastien Payen said he saw a real challenge in the wine industry because there were “absolutely no plant-based sensors to optimize water management.”

He combined his expertise in sensor and information technology with co-founder Thibaut Scholasch’s research on vine water status to create the Web application.”

The system seems to work as follows.

Sap flow sensors at the base of the vines monitor how much water is flowing through the plants, and hence how much is transpiring. Weather data and knowledge of the site are used to estimate the evaporative demand at any particular time. This is the rate the soil and plants would lose water by evapotranspiration if they had all the water they could use. These data are sent off for analysis by “proprietary algorithms”, to compute how much irrigation should be applied, and when, in order to optimise fruit composition.

The water saving most probably comes from irrigating only if it would increase transpiration. Above a certain level of soil moisture, called the stress point, transpiration levels out. At this point, more water won’t mean more transpiration, but would mean more evaporation. And if you go even higher, to the soil’s field capacity, you’d start losing water below the roots from gravity drainage. For the plants, evaporation and drainage are a waste of water.

The proprietary algorithms no doubt estimate the actual transpiration rate of the plants and the potential rate imposed by the microclimate, and then assess whether the plant is stressed, how stressed, and whether it should be irrigated.

Sap flow sensors are are a step up from measuring soil moisture status alone, which is also monitored to guide irrigation applications, but is further removed from what is actually going on in the plant and hence typically less informative.

Kenya forest dwellers evicted: video Daniel Collins Nov 23

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Reuters has a video documenting the eviction of occupants of the Mau Forest in south-west Kenya. Again in the coverage, the journalist cites water supply concerns as a driving force behind the evictions. I don’t buy it.

Trickle down carbon sequestration Daniel Collins Nov 20

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Nick Smith has suggested to farmers that they start offsetting their greenhouse gas emissions by planting trees. A risk with this is that planting trees may compromise water supply.

Compared with pasture, trees tend to reduce the amount of rainfall that reaches rivers and aquifers. A larger canopy traps more rain as it falls, so it evaporates directly from the leaves before even seeing the soil. Deeper rooted trees are also able to tap deeper soil water and groundwater stores, supplying more water for plant transpiration. On the other hand, a row of shelter trees slows the drying of pastures during strong, warm winds, such as during Canterbury nor’westers.

If there is ample water to begin with, the effect may be inconsequential. If the region already experiences seasonal water shortages, planting trees may be a risky proposition.

A rule of thumb I use to delineate at-risk regions is a threshold of 600-700 mm of annual rainfall. Any less, and planting forests where they were not previously may translate to drier streams and lower water tables. These numbers are rough, and would need more attention for any given region, but the message is simple.

The effect on water supply would increase with the area of forest planted. So would the amount of carbon sequestered. This leads to a trade-off between carbon sequestration and water supply. Farmers should consider carefully where they plant their trees so as not too compromise their irrigation needs.

Kenya to evict forest dwellers to increase water supply Daniel Collins Nov 16

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The NYT has a tragic story about a hunter-gatherer group in Kenya that may lose its ancestral forest home.

The Kenyan government is gearing up to evict tens of thousands of settlers, illegal or not, from the Mau Forest, the Ogiek’s ancestral home and a critical water source for this entire country. The question is: Will the few thousand remaining Ogiek be given a reprieve or given the boot?

My question is: Will this eviction actually improve the water resource situation?

I have serious doubts.

The NYT continues:

No doubt the Mau Forest is crucial. It is — or more accurately, used to be — a thick, staggeringly beautiful forest in western Kenya, capturing the rains and the mist and, in turn, feeding more than a dozen lakes and rivers across the region, even contributing to the flow of the Nile.

But in the past 15 years, because of ill-planned settlement schemes (the government essentially handed out chunks of forest to cronies), 25 percent of the trees have been wiped out. Much of the forest is now simply meadow. The Ogiek say there are fewer antelope and bees. They constantly use the Kiswahili word “haribika,” which means spoiled. Scientists say the environmental destruction has led to flash floods, micro-climate change, soil erosion and dried up lakes.

There is a lot of mythology wrapped up in forests. For over a century we – many scientists and non-scientists alike – have believed that trees are good for water resources. That trees make rain. I even ran into this impression while in Uganda last year, near the Kenyan border. Unfortunately, in most circumstances, it is the opposite that is true: forests reduce the quantity of available water compared with other land cover types.

On the one hand, it’s nice to see your pet research interest getting air time in the NYT. On the other hand, it may be being misunderstood or misused by the Kenyan government to the detriment of a large group of people.

I have asked the reporter about the scientific backdrop to his article (while also suggesting that it my not be as cut-and-dry as he was led to believe), and will root around myself for the low-down.