The New York DEP: A New Kind of Modeling Agency

by Katie Horner | November 2, 2010

Last Friday, Don Pierson of the NYC Department of Environmental Protection gave a lecture at the Columbia Water Center as part of the Fall Seminar Series. The talk, entitled “A Project to Evaluate the Effects of Climate Change on the NYC Drinking Water Supply,” detailed the process behind current DEP efforts to model the possible effects of climate change on New York’s water supply.

More specifically, Mr. Pierson explained, the Integrated Model Project aims to evaluate possible effects of climate change on water storage and systems operation, turbidity, and nutrient loading and eutrophication. The project grew out of discussions with the DEP Climate Change Task Force regarding the myriad ways in which Climate Change could affect the New York water system. This formidable task involves examining the complex interacting processes within the water system and the ways that these could be affected – both individually and collectively – by climate change. Indeed, Mr. Pierson made clear throughout his talk that the work he was presenting represented a collective effort to examine possible outcomes and did not predict future conditions.

Mr. Pierson described the project as having the following major tasks: to develop credible future climate scenarios to use in watershed and reservoir modeling; to develop watershed hydrology, biogeochemical, erosion, and sediment transport models that reflect the variability of climate mediated processes; to develop competency in forest modeling (a goal based on the fact that 90% of New York’s watershed is forested); and eventually, to apply models and data sets in order to make future forecasts regarding the New York City water supply.

To give a sense of how multi-faceted water quality modeling is, Mr. Pierson provided a comprehensive overview of initial climate change scenarios and their potential implications for New York’s water system. First, he explained that the DEP ran 8 General Circulation Models of Climate (GCM)/Emission scenarios and used the delta change method to determine how such scenarios might play out in New York.

The primary purpose of such modeling is to determine possible variations on average temperature and precipitation on a regional level. The secondary purpose, however, is to examine possible implications of various climate change scenarios for water supply systems. Indeed, it was the latter objective that constituted the meat of Mr. Pierson’s talk. He broke down his examination of the implications of climate change into three main areas: watershed hydrology, hydrothermal conditions, and nutrient loading.

In terms of watershed hydrology, Mr. Pierson explained that GCMs and delta change models (scaled-down GCMs) indicated that increases in air temperature and precipitation could produce an array of side-effects, including: increased evapotranspiration and downstream flow, decreased snowpack and increased snowmelt, and changes in seasonal streamflows. Consequently, such changes would carry tremendous implications for system operations and storage; namely, that greater winter streamflow would fill reservoirs much earlier and result in greater releases and spills in the spring. Such seasonal changes would also have impacts on reservoir turbidity. Overall, Mr. Pierson emphasized that effects of climate change on watershed hydrology would likely occur on various spatial and temporal scales and in complex, interrelated, and sometimes mutually amplifying ways.

Although less outwardly discernible, climate change would also affect hydrothermal conditions in reservoirs, with serious implications for the ecology of these water bodies. Hydrothermal effects of changing climate could include: increased water temperatures, reduced ice cover, prolonged seasonal thermal stratification, and changes in reservoir water levels and residence time. Because aquatic life is extremely sensitive to water temperature, thermal deviations could have sweeping effects on the composition of life in reservoirs, which, in turn, could affect water quality and quantity in unknown ways.

Finally, Mr. Pierson covered the potential impact of climate change on nutrient loading in reservoirs, which both affects and is affected by watershed hydrology and hydrothermal conditions. Nutrient loading is influenced by the frequency and magnitude of storm events, surface runoff, and watershed biogeochemistry; changes in a reservoir’s trophic status could subsequently affect phytoplankton growth and succession, among other things. Additionally, Mr. Pierson discussed the added consideration of seasonally specific changes (such as the possibility of nutrient loss due to increased winter spills produced by decreased snowpack and increased melt) to emphasize both the interconnectedness and complexity of attempting to model impacts of climate change on water quality.

In what was no small feat, Mr. Pierson wrapped up his comprehensive, information-packed lecture in a coherent and conclusive manner. He brought everything back to the beginning, reiterating that water quality monitoring is today merely a manner in which the DEP is examining possible scenarios and analyzing how watershed management might need to change in light of them. Mr. Pierson also noted that the multi-model approach employed by the DEP is allowing them to gain a holistic view of future watershed issues and management, which includes, but is not limited to, addressing impacts of climate change.

One of the most unique aspects of Mr. Pierson’s talk, however, was not the modeling data he presented (although that, too, was extremely interesting) but rather, his discussion of the modeling process itself. As part of his concluding remarks, Mr. Pierson discussed future challenges for the project. This involved a number of interesting and challenging questions: How to deal with the large number of derived future climate scenarios? How to account for uncertainty and extreme events within models? How to determine what processes will be most sensitive to climate change and how to reflect varied sensitivities when creating models? And finally, how to separate the effects of future changes in land use from those of climate change? In ending his talk with such questions, Mr. Pierson not only reinforced the importance of his team’s work at the DEP but also, implicitly indicated that even as we attempt to comprehend and adjust for the impacts of climate change, we must also focus on other sources of anthropogenic ecological degradation. Ultimately, Mr. Pierson’s presentation provided a reminder that a secure and clean future will necessitate efforts to mitigate and reverse not just the effects of climate change but also, of all human misuse of ecosystems and natural resources.