Facing the difference between how much water plants need, and how much they’ll get

We hear a lot about how climate change will affect rainfall in California, but climate scientists are increasingly looking at a new indicator: water deficit.

“Climatic water deficit” relates to how much water plants need to survive. “It’s the difference between what a plant would use if it had the water and what is actually available,” Alan Flint, research hydrologist with the US Geological Survey, explained on Wednesday at the North America Congress for Conservation Biology.

The value combines temperature, rainfall, the soil’s capacity to hold water and how plants use water. In agriculture, farmers irrigate their crops to make up the water deficit, but plants in the natural world aren’t so lucky.

Using modeling, Flint and his colleagues are finding that the Bay Area’s water deficit is likely to increase, even under a wetter climate future. “We see that excess water go to runoff in the wintertime. But if you’re a plant species trying to live in March and April and growing leaves, you only have the water that’s in the ground,” Flint said.

Warmer air temperature increases evapotranspiration – the water lost through evaporation from soil and from plants themselves. That increases a plant’s demand for water.

“Water deficit is going to be a big issue. It’s what will control the health of forests in the Sierra Nevada. It will control landscape changes in the Bay Area,” said Flint.

That could include oaks, which are often found on north-facing slopes in the Bay Area, “where the water deficit is low. On the south slopes, we don’t see them there,” said Flint. In a warmer future, “if those areas where oaks live have the same water deficits as the south facing slopes, the oaks may not be able to regenerate there. They may survive, but they may not be able to regenerate.”

Water deficits could also have a large impact in the Sierra Nevada, which Flint calls a potential “worst case.” Water deficits are associated with fires and with forest die-offs. “It will really stress the plants and allow bark beetles or other things to come in and kill them. And it doesn’t have to change by a lot,” said Flint.