The largest study of its kind in the world has revealed how our forests may be soaking up much more water in extreme rainfall than scientists thought.
The largest study of its kind in the world has revealed how our forests may be soaking up much more water in extreme rainfall than scientists thought.
The largest study of its kind in the world has revealed how our forests may soak up much more water in extreme rain than scientists first thought.
The new findings – showing how one forest north of Auckland responded to two of the year’s biggest deluges – come from a five-year, $13m project led by Scion.
The new data showed that, during the destructive Auckland Anniversary floods and ex-tropical Cyclone Gabrielle weeks later, Mahurangi Forest retained nearly 60 per cent of the 229mm of rain dumped on it.
Sensors revealed how, rather than flowing across the ground and into waterways, the rain was sponged up within the forest’s canopy, shallow groundwater and soil, all without soil near the surface saturating.
Scion scientist Dr Dean Meason said it’d been commonly assumed that forests reached saturation after a certain amount of rain, before the rest of the rain began flowing out of the catchment.
“These results suggest that planted forests like radiata pine can effectively retain far higher amounts of rainfall than previously thought.”
The MBIE-funded Forest Flows programme, led by Meason, has been monitoring 10 forests around New Zealand using 1717 sensors – and helping unlock some of the biggest mysteries about how water moves through catchments.
“Our network of field sensors, meteorological and hydrological equipment has been collecting data for about two years now,” he said.
“This type of analysis following extreme weather events hasn’t been done before, or at this scale.”
This year has already brought some of the most dramatic extreme rainfall in New Zealand’s history, giving the study team a wealth of data to explore.
The researchers were analysing measurements recorded at Tararua’s Titoki forest, which was also hit hard by Gabrielle’s high wind and rain.
Meason said five of the 10 forest sites were along a gradient that received between 800mm and 3000m rainfall each year – and all had had one-off rainfall when more than 100mm had dropped.
“The initial results suggest that there would be a high level of retention for most radiata pine forests, however, we are investigating what the amount of retention would be, and how long would it be stored.”
Scion scientist Dr Dean Meason is leading a study exploring how forests respond to extreme rain.
For a country with a land use profile like New Zealand’s - nearly 40 per cent of our area is blanketed with forestry – and with climate change worsening extreme rainfall, answering those questions couldn’t be more important.
In Gabrielle’s aftermath, for instance, scientists concluded that extreme deluges had become up to four times more common, unleashing up to 30 per cent more rain, in the hardest-hit East Coast regions.
Meason said it wasn’t yet clear how native forestry – covering 8 million hectares compared to the 2.1m ha under plantation forest – compared to radiata pine in its rainfall uptake.
But research in the 1950s to 1980s suggested its retention was high, and part of the programme was focused on native catchments in the Central North Island.
“From the evidence of previous research and the first results of the Forest Flows programme, it seems to suggest that native and exotic forests can retain similar amounts of rainfall.”
Ultimately, the programme – which also drew on remote sensing data from the University of Auckland and XERRA - aimed to provide accurate predictions of water storage and release for entire catchments, while also providing data on changes in water quality over time.
The research team also hoped the programme would help address some of the biggest rainfall-related problems facing the sector – including damaging forestry slash that’s created headaches for East Coast communities, and prompted a top-level inquiry.
“Understanding where the water goes will provide forest managers valuable information to mitigate any potential problems during extreme rainfall events - like woody debris.”
