"Our results show that freshwater discharge into the Arctic Ocean is greater than that estimated by traditional steam gauges," lead author Hassan Syed of the University of California, Irvine told environmentalresearchweb. This is because the satellite method accounts for all flows, some of which can bypass stream gauges – for example in braided rivers and floodplains and submarine groundwater discharge. The technique also accounts for streamflow from ungauged portions, which can make up as much as 30% of the drainage region. Finally, it allows near-real time, monthly estimates of discharge from the entire Pan-Arctic region, something that was not possible until now.
Arctic discharge is a critical component of the Arctic Ocean's freshwater budget. Increasing freshwater flows may slow the rates of North Atlantic Deep Water formation (water mass that forms in the North Atlantic Ocean) and heat transport by the thermohaline circulation. This is the part of the ocean circulation that is driven by density differences in sea and ocean water.
The new method for calculating Arctic discharge is based on the principle of mass conservation, explains principal investigator Jay Famiglietti. Changes in water storage within a topographically defined drainage region and the atmosphere above it must equal the inflows and outflows of water though that region. One of these outflows is the stream flow discharge.
"We can then write a 'water balance equation' for the region that includes the storage changes and all the inflows and outflows," says Famiglietti. "In our case, we know everything in the equation except discharge, so we solve for it." The storage change part of the water balance equation was unknown before the GRACE mission, so the researchers were unable to apply their method before now.
"Our results may also indicate that Pan-Arctic discharge rates are increasing, which agree with observations of acceleration of glacial melting," says Famiglietti.
The technique is a great improvement on traditional gauge measurements, which are mostly based on in-situ observations of river height ("stage") at a particular point in the river channel. The volume of water flowing though that point is inferred from a previously established stage-discharge relationship. "Not only are these gauges located long distances from the mouth of the river (the point where the river meets the ocean), there are many logistic difficulties in maintaining these gauges, particularly in the extreme nature of the Arctic climate," explains Syed. "Freezing rivers also produce additional constraints on the gauge-based estimates of Arctic river discharge."
The researchers have already applied their technique to other drainage regions around the globe, including the Atlantic, Pacific and Indian Oceans. These results have been submitted to the Journal of Hydrometeorology. "We will also be extending all of our discharge data from 2002 to the present, as more GRACE data becomes available," adds Syed.
The current study was published in Geophysical Research Letters.