Forests play key role to preserve clean water
University of California - Santa Barbara
It's as much fact as sentiment. Trees really do link the ground to the sky by exchanging energy and matter between the soil and the atmosphere.
Researchers believe that
understanding this connection could provide both a wealth of scientific insight
into ecosystems and practical applications that address challenges such as
water resource conservation and management.
A recent study led by UC Santa Barbara's Marc Mayes investigates how patterns in tree water loss to the atmosphere, tracked with satellite imagery, relates to groundwater supplies. The results validate at landscape-wide scales ideas that scientists have proposed based on decades of research in labs and greenhouses.
What's more, the
techniques lend themselves to an accurate, efficient way of monitoring
groundwater resources over large areas. The findings appear in the
journal Hydrological Processes.
For all their diversity, most plants have a very simple game plan. Using energy from sunlight, they combine water from the ground with carbon dioxide from the air to produce sugars and oxygen. During photosynthesis, plants open small pores in their leaves to take in CO2, which also allows water to escape.
This process of water loss is called
evapotranspiration -- short for soil evaporation and plant transpiration -- and
it's essentially a transaction cost of transporting the ingredients for
photosynthesis to the leaves where the process occurs.
Just like evaporating sweat cools
down our own bodies, the evapotranspiration from the trees cools down the
forest. With the proper understanding and technology, scientists can use
thermal image data from satellites as well as manned and unmanned aircraft to
understand the relationship between plants and groundwater: cooler temperatures
correlate with more evapotranspiration.
"The core hypothesis of this
paper is that you can use relationships between plant water use [as] measured
by [satellite] image data, and climate data including air temperature and
rainfall, to gauge the availability of, and changes in, groundwater
resources," said Mayes, an Earth scientist and remote sensing expert based
at the university's Earth Research Institute (ERI).
Mayes and his colleagues focused on
the flora of dryland rivers -- those in deserts and Mediterranean climates.
Throughout these regions, many plants have evolved adaptations that minimize
water loss, like slow growth, water retention or boom-bust lifecycles. However,
plants that dominate river channels -- species like sycamore, cottonwood and
willows -- evolved to take advantage of the surplus groundwater the habitat
offers relative to the surrounding landscape.
"Rather than slowing down its
water use when water becomes scarce, this vegetation will basically drink
itself to death," Mayes said. This makes it a good window into conditions
below the surface.
The team used satellite-based thermal imaging to look at temperatures across the San Pedro River corridor in southern Arizona. On cloud-free days the satellites can gather data on surface temperatures at high resolution over large areas of land.
By comparing the
temperatures along the river to those in nearby, more sparsely vegetated areas,
the researchers were able to determine the extent of evapotranspiration along
different parts of the river at different times. They found that it correlated
with air temperature in water-rich environments and with rainfall in
water-scarce environments.
The findings support recent advances
in our understanding of plant water use. The hotter and drier the air, the
stronger it pulls water from the leaves, and the more water the plant uses.
Consequently, Mayes and his colleagues expected to see evapotranspiration vary
with air temperature as long as the stream has abundant groundwater for the
plants to draw on.
On the other hand, where groundwater
is scarce, plants will close the openings on their leaves to avoid water loss;
it's more important to avoid drying out than to take advantage of the extra
sunshine on a warm day. As a result, evapotranspiration will correlate much
more strongly with rainfall and streamflow, which increases the supply of water
to trees through their roots.
Scientists had demonstrated the
predictable effect of evapotranspiration in lowering surface temperatures in
lab and small field experiments. However, this is the first study to
demonstrate its impact over large areas. The technology that made this possible
has matured only within the past five years.
"This remote sensing method
shows great promise for identifying the relevant climatic versus other controls
on tree growth and health, even within narrow bands of vegetation along
rivers," said coauthor Michael Singer, a researcher at ERI and lead
investigator on the project that funded Mayes' work.
In fact, these ecosystems are
vitally important to the southwestern U.S. "Despite taking up about 2% of
the landscape, over 90% of the biodiversity in the Southwest relies on these
ecosystems," said coauthor Pamela Nagler, a research scientist at the U.S.
Geological Survey's Southwest Biological Science Center.
The same techniques used in the paper could be applied to the perennial challenge of groundwater monitoring. In fact, this idea helped motivate the study in the first place.
"It's very
hard to monitor groundwater availability and change[s] in groundwater resources
at the really local scales that matter," Mayes said. "We're talking
about farmers' fields or river corridors downstream of new housing
developments."
Monitoring wells are effective, but
provide information only for one point on the map. What's more, they are
expensive to drill and maintain. Flux towers can measure the exchange of gasses
between the surface and the atmosphere, including water vapor. But they have
similar drawbacks to wells in terms of cost and scale. Scientists and
stakeholders want reliable, cost-effective methods to monitor aquifers that
provide wide coverage at the same time as high resolution. It's a tall order.
While it may not be quite as precise
as a well, remote thermal imaging from aircraft and satellites can check off
all of these boxes. It offers wide coverage and high resolution using existing
infrastructure. And although it works only along stream corridors, "an
inordinate amount of agricultural land and human settlements in dry places ends
up being where the water is, along stream paths," Mayes said.
The idea is to look for shifts in
the relationships of evapotranspiration to climate variables over time. These
changes will signal a switch between water-rich and water-poor conditions.
"Detecting that signal over large areas could be a valuable early warning
sign of depleting groundwater resources," Mayes said. The technique could
inform monitoring and pragmatic decision-making on groundwater use.
This study is part of a larger Department of Defense (DOD) project aimed at understanding how vulnerable riverine habitats are to droughts on DOD bases in dryland regions of the U.S.
"We are using multiple methods to understand when and why these plants
become stressed due to lack of water," said Singer, the project's lead
scientist. "[We hope] this new knowledge can support the management of
these sensitive ecological biomes, particularly on military bases in dryland
regions, where these pristine habitats support numerous threatened and
endangered species."
Mayes added, "What's coming
down the pipe is a whole ensemble of work looking at ecosystem responses to
water scarcity and water stress across space and time that informs ways we both
understand ecosystem response and also improve the monitoring."
