New York City’s Operations Support Tool: Utilizing Hydrologic Forecasts for Water Supply Management

The New York City Department of Environmental Protection (DEP) supplies over one billion gallons per day (BGD) of water to more than nine million people in the New York City metropolitan area, making it one of the largest suppliers of surface water in the United States. DEP’s water supply system is as complex as it is large; it draws water from three distinct watersheds and features a number of interconnections and redundancies allowing for a large number of potential operating conditions. The system has a wide range of objectives – from supplying clean, reliable water for municipal demand to meeting environmental flow requirements for downstream stakeholders. Combined with the existing system complexity, these disparate objectives can make operational decision making a challenge.

In 2013, DEP launched the Operations Support Tool (OST) – a state-of-the-art model built to assist the utility in water supply operation decisions. OST consists of a system model (OASIS) to simulate water supply operation decisions and a linked hydrodynamic two-dimensional water quality model (CE-QUAL-W2). The model is initialized using current system conditions (e.g., reservoir elevations, water quality conditions) and is driven forward in time using ensemble hydrologic forecasts. This setup gives DEP the ability to simulate a wide variety of operational strategies in near real-time, allowing for objective alternative analysis prior to making operational decisions. Ensemble hydrologic forecasts are a critical part of the success of this approach, as they enable DEP to evaluate decisions probabilistically by explicitly considering hydrologic uncertainty.

This chapter provides an overview of the New York City water supply system, details the hydrologic forecasts used in OST, and reviews a handful of real operational applications of OST and the hydrologic forecast system.