Tree Rings Might Predict Monsoon Rainfall

Oct. 30, 2001
A new University of Arizona study of tree rings could tell scientists enough about monsoon patterns to accurately predict summer rains and help Tucson make water-use decisions.

Determining past monsoon levels from tree-ring data allows scientists to look at rainfall patterns over a much longer period of time than they could before.

"The ultimate goal is . . . a better ability to understand what's driving the monsoon," said UA research specialist David Meko.

A pilot study by Meko and Christopher Baisan, both research specialists with the UA's Laboratory of Tree-Ring Research, took data from five sites in the San Pedro basin of southeastern Arizona. They found monsoon rainfall levels revealed by Douglas fir tree-ring data matched instrumental records.

Instrumental records date back to 1868, and the tree-ring data go back to 1791. Meko said that with more data, they could easily trace summer rainfall levels back to the 1500s.

"It is not well understood what are the predictors of a good or bad monsoon," Meko said.

Understanding the longer-term patterns of monsoon levels will allow scientists to determine whether trends operate in clusters of several years or decades of good or poor monsoons, he said.

Seeing the summer rainfall variance over a longer period of time helps scientists gain a better perspective of the highs and lows of summer rainfall and evaluate what's happening in the present, Baisan said.

"One of the issues on the current radar is climate change," he said. "Summer precipitation is an important component of natural ecosystems and plant distribution."

Evaluating the monsoon data from tree rings allows scientists to discern whether current weather patterns differ from the historical norm.

Understanding the trends of summer precipitation over a longer period is also "a helpful planning tool for water supply or engineering issues," Baisan said. "If we have a better idea of the range of variance, we can make better decisions."

Tucson Water spokesman Mitch Basefsky said any new data that could provide a strong method of predicting monsoon levels would be considered for long-range planning.

"If there's some additional evidence, it would be very valuable to us," Basefsky said.

Having another tool to predict monsoon levels would help officials determine the necessary water supply and whether conservation will be mandated. Additionally, better evidence of monsoon patterns could affect the water company's revenue picture, which affects rate structures and capital improvement plans.

Tree rings have been used successfully for a long time as climate indicators, Meko said, including for reconstruction of annual precipitation. But the focus of this study is to isolate parts of the tree ring that grow in response to stored winter water from those that result from summer rains. Meko said the late-growth part of the ring correlates with the summer monsoon rain.

The pilot study, published in the spring, proved the methods could work, at least in the Southwestern United States, where certain conifers thrive. In addition to the Douglas fir, ponderosa pines show a distinction between early and late growth in the tree rings.

The current study extends the work by collecting data from 31 sites over a much greater area, including parts of Arizona, Colorado, New Mexico, Texas, California, Oklahoma, Arkansas, Missouri, Baja California and mainland Mexico.

"There seems to be evidence that good monsoons (in the Southwest) are associated with dry conditions in the Great Plains," Meko said. "We want to see if that type of relationship holds up over a longer period of time."

Source: Arizona Daily Star

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