Using Tree Rings to Understand and Protect New York’s Water

In 1957, French geographer Jean Gottman used the term “megalopolis” to describe a huge and growing urban archipelago that stretched from Boston to Washington D.C. and included New York City, Baltimore and Philadelphia. With a population of nearly 50 million –one of the largest conglomerations of people in the world–this megalopolis faces a unique set of challenges in the coming era of climate change, including sea level rise, public health challenges and water resources management.

In 2010, as part of its Regional Integrated Sciences and Assessment (RISA) program, the National Ocean and Atmospheric Administration awarded six new grants to regional institutions across the country to improve the nation’s capacity to adapt to climate variability and change. The Consortium on Climate Risk in the Urban Northeast (CCRUN) addresses climate-related challenges affecting the urban corridor of Boston, New York and Philadelphia. The team is comprised of experts from five universities, including Columbia.

Drought and New York City’s Water

Unlike many other parts of the country, residents of the urban northeast are used to abundant and consistent precipitation and water supply; New York City’s urban drinking water supplies are one of the purest in the nation as well. However, given the sheer number of people who depend on this supply, understanding the potential impacts of climate change and variability on the watersheds that feed it is of critical importance.

The upper Delaware River Basin System is one of the largest water supply systems for the city of New York. With a cumulative storage capacity of 1.5 billion cubic meters from five major reservoirs, the system supplies the city with about 3 million cubic meters per day for consumptive water use. The Delaware River Basin Commission and the New York City Department of Environmental Protection are primarily responsible for managing the releases from the major reservoirs to meet the daily water demand of the city of New York and to maintain the downstream ecosystem.

Today our understanding and management of these reservoir systems (for example, estimation of drought frequency or rules that guide how much water to hold or release in reservoirs) is based on the short historical records of data. Given that the drought of record in the basin was in the 1960s, it is not clear that these records provide guidance for effective drought preparation and system operation.

In other words, to really understand the prognosis for the Delaware River it’s not enough to look at the short record of precipitation and stream flow; scientists will need to find a way to look further into the past.

But how? It turns out that one of the answers lies in tree rings.

As many people know, trees usually add one ring of growth to their trunks every year. Since the early 20th century, scientists have counted these rings as a way to provide an accurate estimate of a tree’s age as well as a surrogate for regional climatic conditions of a given period. It turns out that in wet years, trees tend to have a lot of growth, and hence, thicker rings. During drought years, rings are thinner.

Last Friday, Naresh Devineni, a post-doctoral fellow at the Columbia Water Center, presented preliminary results from the Consortium on Climate Risk’s Delaware Streamflow Reconstruction project to Water Center colleagues. The project uses tree ring data collected from a number of Delaware River watersheds to assess the risk of drought in a given period using Hierarchical Bayesian Regression techniques.

Early results presented some surprising findings. For example, the New York Region’s devastating 1960s drought has long been considered the benchmark of the kind of extreme event that the water managers of the Delaware River need to be aware of. That drought, which lasted from 1961 to around 1967 was so extreme that in 1965, New York City stopped releasing water from its Delaware River reservoirs, creating a risk that low water levels would cause salt water to intrude into Philadelphia’s water supply. The event prompted President Lyndon Johnson to convene a special meeting of governors and mayors to create emergency measures for managing the Delaware.

However, Devineni points out that looking at a longer record (1801-1999) reconstructed from tree rings suggests that while a drought as severe as the one in the 60s is possible, it is not the most likely risk. On the other hand, there is a much greater chance that the area could experience recurring droughts of shorter duration.

“Given that these are small reservoirs,” that are the major water supply source for NYC , he says, “even a drought of short or moderate duration is a concern.” Understanding the likelihood of such droughts, along with the associated climate variables that could trigger a drought, could have important implications for reservoir management.

Devineni says that Columbia Water Center is working on applying this framework for understanding droughts under conditions where the probability of an event is always changing, to evaluate reservoir water allocation and release policies. In addition the team is also working on ideas that could lead to the reformulation of the Delaware River Compact across multiple states and users, through flexible and adaptive flow policies; such a methodology could then be extended to other large river, multi-reservoir, multi-state water allocation processes.

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