"One expected implication of climate change in the tropics is a transition to less frequent but more intense rainstorms," Scott Jasechko of the University of Calgary, Canada, told environmentalresearchweb. "Understanding how the intensification of tropical precipitation will impact groundwater supplies is important in order to develop plans that can improve water and food security."

Jasechko and Richard Taylor from University College London, UK, examined stable isotope ratio data for oxygen and hydrogen in rainwater and groundwater. They looked at 15 tropical sites that spanned the globe and had different geologies and amounts of rainfall.

In the tropics, the ratios of 18O/16O and 2H/1H depend strongly on precipitation intensity (unlike in other regions, where they’re linked to surface air temperature). Because of the way clouds develop, low-intensity rainfall tends to contain more heavy isotopes. So comparing the isotope composition of rainfall over the long-term with that in groundwater can reveal the intensity of the rainfall that recharged the groundwater.

The team found that groundwater contained less heavy oxygen than overall rainfall did at all the test sites, except Manila in the Phillipines. In Manila, intensive rainfall contained only a small amount less heavy oxygen than the long-term mean.

"We show that intensive rainfall in the tropics is critical for groundwater replenishment," said Jasechko. "This means that further intensification of precipitation due to climate warming could increase groundwater recharge in the tropics."

Atmospheric moisture content is currently increasing at around 7% per degree of temperature rise, whilst the frequency of extreme precipitation has increased at a global average of roughly 6% per degree of temperature change, according to the team writing in Environmental Research Letters (ERL).

"An earlier study by co-author Richard Taylor in Tanzania showed that groundwater recharge is greatest during extreme rainstorms," added Jasechko. "However, as of 2015, the ubiquity of this observed bias of groundwater recharge to intensive rain had not been established."

Scientists believe that less intense precipitation in the tropics tends not to recharge groundwater because of the large amounts of evapotranspiration.

Jasechko and Taylor also worked out the intensity of the rainfall needed to recharge the aquifers. At all the sites except Manila and Alice Springs in Australia, the groundwater isotope ratios matched those of monthly rainfalls in at least the top 30% of intensity. For humid conditions, this corresponds to around 100–300 mm of rain per month, the pair say. At Alice Springs, the only arid site, the threshold rain intensity for recharge was around 30 mm.

"It is important to note that the results simply indicate a tendency towards increased groundwater recharge from extreme rainfall," said Taylor. "Other influences on groundwater storage including excessive pumpage, substantial changes in total precipitation, and land-use change can undermine and overwhelm this resilience of groundwater resources in the tropics to climate change."

Now the scientists plan to use their findings to improve existing forecasts of groundwater supplies in the tropics, where it’s a "life-sustaining" resource. "Groundwater resources in the tropics may be resilient to ongoing and anticipated climate change," said Jasechko.

Jasechko and Taylor reported their results in Environmental Research Letters (ERL).

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