What has happened is that they have winkled out a protocol that gives usreliable ground water data that can be zeroed with available ground data. This folks will be welcome informationeverywhere and perhaps someone will work to create a layer linked directly togoogle earth for all land surfaces on Earth.
This returns me to my concept of water shed management programs thatestablish a cooperative system for optimizing the biological productivity ofthe watershed. This is a natural toolthat easily supports such a program.
In the meantime, we now know it can be done.
Satellite Data Provide A New Way ToMonitor Groundwater In Agricultural Regions
In farmland of the
,a sea of low-quality InSAR data, indicated by blue, is punctuated by spots ofhigh-quality data, indicated by yellow and orange. Byoverlaying a Google Earth image of the same location on the InSAR data, Reevesdetermined that the high-quality data occur in the areas between center-pivotirrigation circles. Credit: InSAR images courtesy of Jessica Reeves. San Luis Valley
When you dive into that salad full of lettuce grown in the American West,there's a good chance you are enjoying the product of irrigation from anunderground water source. These hidden groundwater systems are preciousresources that need careful management, but regulatory groups have a hard timemonitoring them, owing to a lack of accurate data.
Now, scientists atStanford have found a way to cheaply and effectively monitor aquifer levels inagricultural regions using data from satellites that are already in orbitmapping the shape of Earth's surface with millimeter precision.
The amount of water ina groundwater system typically grows and shrinks seasonally. Rainfall andmelted snow seep down into the system in the cooler months, and farmers pull water out to irrigate their crops in thewarmer, drier months.
In agriculturalregions, groundwater regulators have to monitor aquifer levels carefully toavoid drought. They make do with direct measurements from wells drilled intothe aquifers, but wells are generally few and far between compared to the vastsize of most groundwater systems.
"Groundwaterregulators are working with very little data and they are trying to managethese huge water systems based on that," said Jessica Reeves, a geophysics doctoral student. But now, Reeves hasshown how to get more data into the hands of regulators, with satellite-basedstudies of the ground above an aquifer.
Reeves presented herresults on Monday, Dec. 13, at the American Geophysical Union annual meeting in
. San Francisco
As the amount of waterin an aquifer goes up and down, specialized satellites can detect the movementsof the land above the water system and hydrologists can use that information toinfer how much water lies below.
Previously, accurateelevation data could only be acquired on barren lands such as deserts. Plants -especially growing crops, whose heights change almost daily - create"noise" in data collected over time, reducing their quality.
Now, a team of scientistsled by Reeves has found a way around this "growing" problem.
The study began as acollaboration between Reeves' faculty advisers, Rosemary Knight, a geophysicistwho studies groundwater systems, and Howard Zebker, a geophysicist andelectrical engineer who uses satellite-based remote sensing techniques to studythe Earth's surface.
Knight and Zebkerhoped that the combination of their expertise, and the efforts of theirgraduate student, would lead to new ways of using satellite data forgroundwater management.
Reeves analyzed adecade's worth of surface elevation data collected by satellites over the
San Luis Valley in .Although the valley is rich with growing crops, Reeves and her advisers hopedthat recent advances in data-processing techniques would allow her to gain anunderstanding of the aquifer that lay below. Colorado
As part of heranalysis, Reeves produced maps of satellite measurements in the valley and sawa regular pattern of brightly colored high-quality data in a sea of dark,low-quality data. After overlaying the maps with a Google Earth image of thefarmland, the team realized that the points of high-quality data were in thedry, plant-free gaps between circles of lush crops on the farms.
, the majority of irrigation isdone by center-pivot irrigation systems. Like a hand on a clock, a line ofsprinklers powered by a motor moves around, producing the familiar circles seenby airline passengers. San Luis Valley
The circles don'toverlap, leaving small patches of arid ground that don't receive any water andso don't have any plants growing on them.
Reeves confirmed thatthese unvegetated data points were trustworthy by comparing the satellite datato data collected from wells in the area - exactly the kind of proof that wouldbe important to hydrologists studying aquifers.
The satellites useinterferometric synthetic aperture radar, known as InSAR. It is a radartechnique that measures the shape of the surface of Earth and can be used totrack shape changes over time. Earth scientists often use InSAR to measure howmuch the ground has shifted after an earthquake.
While continuouslyorbiting, a satellite sends an electromagnetic wave down to the surface. Thewave then bounces back up and is detected by the satellite. The properties ofthe wave tell scientists how far the wave traveled before it was reflectedback. This distance is directly related to the position of the ground.
After the satellitecompletes a circle around the globe, it returns to the same location to senddown another radar wave and take another measurement. Measurements are takenevery 35 days and data collection can go on for years.
Compared to drillingwells for monitoring groundwater aquifers, using InSAR data would be muchcheaper and provide many more data points within a given area.
Traditional methodsrely on wells that were not built with scientific data sampling in mind andtheir results can be inconsistent. Moreover, the number of wells drilled intoany particular aquifer is much too small to be able to cover the entiregroundwater system.
Hydrologists andregulatory bodies looking for more data to better understand their groundwatersystem could one day set policies requiring farmers to leave a patch of landclear for InSAR data collection. Furthermore, the technique could be used inagricultural regions anywhere in the world, even those that lack modern infrastructuresuch as wells.
"I think itreally has potential to change the way we collect data to manage ourgroundwater," said Reeves.