Droughts may permanently change watersheds and ecosystems

After a watershed experiences drought, many scientists have reasoned that a resumption of regular precipitation would add moisture back to the ecosystem and that river flow would return to its predrought state. That thinking has been challenged in some recent research. For instance, since 2009 Tim Peterson of Monash University in Victoria, Australia, and colleagues have been theoretically exploring how water catchment areas recover from droughts. They learned that watersheds may exist in multiple stable states .

During a drought, a watershed may, for example, evolve from a state with high stream runoff to one with low stream runoff because of a biophysical adaptation in which vegetation transpires more precipitation through leaves and back to the atmosphere. That positive feedback means that the same amount of rainfall produces more streamflow before a drought than it does after. In other words, even after rains return, the watershed persists in a drought state.

Now Peterson and another group of collaborators have found some observational evidence in support of multiple stable watershed states. They analyzed runoff from 161 watersheds in southeastern Australia before, during, and after the Millennium Drought, which lasted from about 1997 to 2010. The data show the limits of the resiliency of ecosystems to sustained changes in climate.

The analysis revealed that seven years after the drought ended, 37% of the watersheds remained in a drought-like state with low-runoff conditions. Many of those watersheds showed no signs of returning to a high-runoff state. The statistical results indicated that runoff decreased by an average of 92 mm a year, and the persistently low runoff even after the drought ended was not because of changes in land cover or variability in precipitation.

In figuring out where 92 mm of lost water went each year, the researchers found that compared with vegetation in recovered watersheds, the vegetation in nonrecovered watersheds transpired a greater percentage of rainfall after the drought than before it. Peterson hopes to figure out why by teaming up with plant biologists. Another critical question also remains unanswered: What mechanism, if any, could make a watershed recover to its predrought stable state? (T. J. Peterson et al., Science 372, 745, 2021 .)