The New York City Operations Support Tool (OST): Managing Water for Millions of People in an Era of Changing Climate and Extreme Hydrological Events

Abstract

With an average daily delivery of 1.1 billion gallons ([Formula: see text]) of drinking water to approximately nine million people in New York City (NYC) and four upstate counties, the NYC Water Supply is among the world’s largest unfiltered systems. In addition to reliably supplying water in terms of quantity and quality, the city has to fulfill other flow objectives to serve downstream communities. At times, such as during extreme hydrological events, water quality issues may restrict water usage from parts of the system; the city is proactively implementing a number of programs to monitor and minimize the impact. To help guide operations and planning, NYC has developed the Operations Support Tool (OST), a decision support system that utilizes ensemble forecasts provided by the National Weather Service (NWS) Hydrologic Ensemble Forecast Service (HEFS). This paper provides an overview of OST and shows two operations case studies to illustrate how OST is used to support risk-based water supply management. As the modeling uncertainty is strongly impacted by the forecast skill, we also discuss how changes in patterns of hydrological extreme events elevate the challenge faced by water supply managers and the role of the scientific community to integrate non-stationarity approaches in hydrologic forecasting.