Rivers might not be as resilient to drought as once thought

2021-05-13

Rivers ravaged by a lengthy drought may not be able to recover, even after the rains return. Seven years after the Millennium drought baked southeastern Australia, a large fraction of the region’s rivers still show no signs of returning to their predrought water flow, researchers report in the May 14 Science.

There’s “an implicit assumption that no matter how big a disturbance is, the water will always come back — it’s just a matter of how long it takes,” says Tim Peterson, a hydrologist at Monash University in Melbourne, Australia. “I’ve never been satisfied with that.”

The years-long drought in southeastern Australia, which began sometime between 1997 and 2001 and lasted until 2010, offered a natural experiment to test this assumption, he says. “It wasn’t the most severe drought” the region has ever experienced, but it was the longest period of low rainfall in the region since about 1900.

Peterson and colleagues analyzed annual and seasonal streamflow rates in 161 river basins in the region from before, during and after the drought. By 2017, they found, 37 percent of those river basins still weren’t seeing the amount of water flow that they had predrought. Furthermore, of those low-flow rivers, the vast majority — 80 percent — also show no signs that they might recover in the future, the team found.

Many of southeastern Australia’s rivers had bounced back from previous droughts, including a severe but brief episode in 1983. But even heavy rains in 2010, marking the end of the Millennium drought, weren’t enough to return these basins to their earlier state. That suggests that there is, after all, a limit to rivers’ resilience.

Streamflow records going back to 1970 show that few of 161 selected river basins had low water flow levels relative to their normal flow (red line), even after a short drought in 1983 (left gray area). However, the lengthy Millennium drought, which lasted from around 1997 to 2010 (large gray area on the right), may have forced some of the rivers into a low-flow state — a new normal that many have stayed in even after the rains returned in 2010.

What’s changed in these river basins isn’t yet clear, Peterson says. The precipitation post drought was similar to predrought precipitation, and the water isn’t ending up in the streamflow, so it must be going somewhere else. The team examined various possibilities: The water infiltrated into the ground and was stored as groundwater, or it never made it to the ground at all — possibly intercepted by leaves, and then evaporating back to the air.

But none of these explanations were borne out by studies of these sites, the researchers report. The remaining, and most probable, possibility is that the environment has changed: Water is evaporating from soils and transpiring from plants more quickly than it did predrought.

Peterson has long suggested that under certain conditions rivers might not, in fact, recover — and this study confirms that theoretical work, says Peter Troch, a hydrologist at the University of Arizona in Tucson. Enhanced soil evaporation and plant transpiration are examples of such positive feedbacks, processes that can enhance the impacts of a drought. “Until his work, this lack of resilience was not anticipated, and all hydrological models did not account for such possibility,” Troch says.

“This study will definitely inspire other researchers to undertake such work,” he notes. “Hopefully we can gain more insight into the functioning of [river basins’] response to climate change.”

Indeed, the finding that rivers have “finite resilience” to drought is of particular concern as the planet warms and lengthier droughts become more likely, writes hydrologist Flavia Tauro in a commentary in the same issue of Science.

sciencenews.org, 13 May 2021
; https://www.sciencenews.org