Putting the weight of the world to work
Any model that aims to inform real-world decisions needs to be accurate. But how do you verify accuracy when there are few “on-the-ground” measurements to check against? For Canada1Water, one option is to let gravity do the talking. John Crowley of the Canadian Geodetic Survey explains how time-variable gravity, measured from space, will help validate C1W results.
Q: Can you briefly describe your work at the Canadian Geodetic Survey?
JOHN CROWLEY: I’m the Gravity and Height Systems team leader. We do a lot of things, but relevant to Canada1Water, we’re part of an ongoing project using GRACE[1] satellite gravity data from NASA and the German Aerospace Centre. GRACE stands for “Gravity Recovery and Climate Experiment”. It makes monthly maps of the Earth’s gravity, and we use those maps to track changes in water storage all over the planet.
Q: How do you map Earth gravity — and what role does water play in that?
JOHN CROWLEY: All matter has gravitational pull. The bigger the object, the stronger the pull. GRACE has a pair of satellites orbiting the Earth, and the orbits they follow are determined by gravity. Any change in the orbits indicates a change in gravitational pull. On Earth, there are two dominant signals of gravitational change: one is plate tectonics and continental motion; and the other is due to changes in water, in both liquid and ice form. The tectonic contribution is well modelled and understood, so when we remove it from the measurements, we assume any residual change is caused by water.
Q: How are those gravity maps contributing to Canada1Water?
JOHN CROWLEY: Canada1Water covers parts of the country where there’s not necessarily geological information or observations to test and validate against. GRACE has a record of water-related gravitational changes from the past two decades, and we know from comparing those to places with verified on-the-ground data, like the Great Lakes Basin, that they match up well. So we can provide broad validation for water storage across all of Canada and help fill gaps for Canada1Water where there are few terrestrial measurements.
Q: What are you getting from Canada1Water in return?
JOHN CROWLEY: Canada1Water has much higher data resolution than what we get from GRACE. We can see there’s a total change in water on or near the surface of the Earth but can’t say where it is exactly, or at what depth. Canada1Water will help us calculate more detailed changes in gravity — and confirm our assumption that when you take out the tectonic contributions, water is responsible for the rest of the large-scale changes. Basically, we’re helping validate and calibrate the large-scale Canada1Water models, and they’re helping us refine our gravitational observations.
“Basically, we’re helping validate and calibrate the large-scale Canada1Water models, and they’re helping us refine our detailed gravitational observations.”
John Crowley is the Team Leader for the Gravity and Height Systems at the Canadian Geodetic Survey, Natural Resources Canada.
HOW GRACE MEASURES GRAVITATIONAL CHANGE
Liquid water and snow have mass that contributes to Earth’s gravitational pull on the paired GRACE satellites. Any change in mass produces a change in gravity that can be measured.
Q: What will those more detailed insights allow you to do?
JOHN CROWLEY: It ties in with what I was just saying. GRACE’s resolution is limited to about 300 kilometres due to certain spatial limitations and data errors. We’re constantly trying to improve our methods to resolve those errors. One way to do that is to test new ways of detecting water changes with GRACE. But there are very few places in the world where we know exactly how water storage is changing. Canada1Water fills in some of that picture for us so we can refine our spatial analysis methods. The added spatial resolution of the Canada1Water models can be used to test spatial limits and processing methods for GRACE. Like most science, it’s an iterative process.
Q: How do you feel about being part of the Canada1Water project?
JOHN CROWLEY: I was really happy to be asked to contribute. The results will be important on their own, but the multidisciplinary aspect of it is also a great achievement and I think is one of the big successes. It’s bringing a lot of people together in really interesting ways to take a broad Earth systems approach to questions about water and climate.
[1] The original GRACE ran from 2002 to 2017. The current mission, GRACE-FO (“Follow On”), began in 2018.