Groundbreaking Study Probes Water Cycle

Tucked away in a remote corner of Rowland Hall, UC Irvine Earth system science Professor James Famiglietti sat down in his immaculate office, leaned back in his chair, arms crossed behind his head, and started to talk.

Famiglietti and a team of scientists and researchers from the Department of Applied Geology, Indian School of Mines; College of Marine Science, University of South Florida; Jet Propulsion Laboratory/California Institute of Technology and Remote Sensing Systems, Santa Rosa, CA published a first-of-its-kind study titled “Satellite-based global-ocean mass balance estimates of interannual variability and emerging trends in continental freshwater discharge” in the journal “Proceedings of the National Academy of Sciences.”

“The study focused on discharge,” Famiglietti said. “[Discharge] is an important part of the water cycle for a couple of reasons. One: it’s directly related to water availability. Two: it is an indicator of climate and global change, so if there are significant global changes, they should show up in this discharge factor.”

The lack of observation-based studies on the water cycle has forced scientists to rely on computer models to predict future weather patterns. As a result, scientists have been unable to make accurate predictions as to what global weather changes, if any, are coming in the future.

“It’s impossible to assemble a global-scale data set on discharge,” Famiglietti said. “So if you want to do it, you have to get creative. What most people do is use computer models. As a model developer, I know that the models have several shortcomings that keep them from accurately predicting stream flow because … we build them, we tweak them, we come up with new ideas and we’re just not ready to do that global scale prediction. So you need to get creative … let’s just imagine that rather than think of river flow as an output from the continent, we thought of it as input to the ocean. [This] data actually had less uncertainty, and that allowed us to do these estimates with more certainty, so we get a clearer picture of what’s going on.”

Although the study did not measure the stream flow from specific rivers such as the Amazon or the Congo, it provides data that can serve as an integrated global measure of freshwater discharge into the world’s oceans. This means that scientists can get a clearer picture of how the world’s climate is changing.

The study measured the freshwater discharge from 1994-2006, and found that 18 percent more freshwater flowed into the oceans across the globe in 2006 than in 1994 with an annual average increase of 1.5 percent. Although the annual percentage increase seems small, it indicates that the water cycle is accelerating.

“When thinking about climate change, one of the most talked about aspects is its impact on the water cycle,” Famiglietti said. “As a group, the changes are called ‘water cycle acceleration’ or ‘intensification of the water cycle.’ What that means specifically are two things. It means increasing precipitation [and] increasing evaporation. The other [aspect] of water cycle acceleration is how that increasing precipitation is spread over the globe. It’s not like a uniform increase everywhere and everything is going to be great, we’d have more water. The dry areas basically get dryer, and the wet areas, tropics, get wetter and the arctic is getting wetter. There’s a redistribution of the rainfall, from the mid-latitudes, to the high and the low latitudes. The third part of the acceleration is intensification; we expect that rainfall or lack of rainfall will be delivered with more intensity, so more frequent flooding events, and longer, more persistent droughts.”

Famiglietti and his team faced many problems as they set out to conduct their groundbreaking study. Stream flow discharge seems like a simple component of the water cycle to measure. However, the number of stream gauges across the world is declining. Further complicating the data gathering process is different countries’ unwillingness to share the data they do have because of different economic, social, political and security implications included by default in the data.

“Some of those resources are shared,” Famiglietti said. “If you’re upstream and you’re using maybe more than you’re supposed to be using, you don’t necessarily want to be sharing your data with the people downstream.”
The team was able to use data gathered from NASA satellites on sea level rise around the world. As a result, they could only use data going back to 1994, when satellite technology that was able to accurately measure sea levels was put into use. After collecting the sea level rise data, Famiglietti and his team were able to gather some of the inflow and outflow data. They then solved for the only remaining unknown – discharge.

“It’s been very difficult to prove that it’s happening,” Famiglietti said. “This is one of the first observation-based studies that I think suggests that water cycle acceleration is already happening. Before this study, we hadn’t been able to actually observe and estimate [the effects].”