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Glossary of Terms

Alum – Hydrated aluminum sulfate, a coagulant added to untreated drinking water to cause aggregation of suspended solids and the conversion of dissolved natural organic matter to a particulate form.

Analysis of covariance – Form of linear regression used to explore relative effects of both categorical (integer) and continuous variables.

Bias correction – Adjusts projected raw daily global climate model (GCM) output using the differences in the mean and variability between the GCM results and observations in a reference period.

Calibration – As used in modeling, the process of assigning values to adjustable model parameters to best match model simulations to known conditions (data).

CE-QUAL-W2 – Two-dimensional (longitudinal and vertical), deterministic, hydrodynamic and water quality (WQ) modeling software platform.

Climate – Usually defined as average weather. More rigorously, it is the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years.

Climate model – Numerical representation of the climate system based on the physical, chemical, and biological properties of its components and

their interactions and feedback processes and accounting for some of its known properties. Climate models are applied as a research tool to study and simulate the climate and for operational purposes, including monthly, seasonal, interannual, and, increasingly, long-term climate predictions.

Conditional probability distribution – Probability distribution of a random variable conditional on the value of another random variable.

Diversion – As used by New York City Department of Environmental Protection (NYC DEP), controlled withdrawal of water from a reservoir to a water conveyance structure (tunnel, aqueduct, pipeline) for transfer to another reservoir or to meet demands of users (NYC and other communities).

Downscaling – Taking climate information known at large scales to make predictions at local scales. Dynamical downscaling requires running high-resolution climate models on a regional subdomain, using observational data or lower-resolution climate model output as a boundary condition. Statistical downscaling is a two-step process consisting of the development of statistical relationships between local climate variables (e.g., surface air temperature and precipitation) and large-scale predictors (e.g., pressure fields) and the application of such relationships to GCM output to simulate local climate characteristics. See Appendix B.

ECDF (Empirical CDF) – A cumulative distribution function derived from actual observations of the variable of interest.

Ensemble forecast – Set of forecasts (e.g., for streamflow, temperature, precipitation, or wind) to provide a wider range of possible future outcomes than a single-point forecast.

Flocculation – Physical process in which fine particles suspended in water aggregate prior to removal by processes such as flotation, sedimentation, and filtration.

Forecast – Projection or estimate of future conditions, such as for precipitation, temperature, and streamflow.

Forecast processor – Module within the Operations Support Tool that modifies time-series forecasts of future conditions, typically streamflows, precipitation, and temperature, for site-specific application for the NYC DEP watersheds.

Gate – Engineered water control structure that can be opened and closed to regulate the flow of water.

GCMs – Global climate models, also referred to as general circulation models.

Marginal probability distribution – Probability distribution of a random variable when the probabilities of other related random variables are not known.

NYC DEP – New York City Department of Environmental Protection, used in this report to represent the Bureau of Water Supply, which operates New York City’s water supply system.

OASIS – Water quantity-based mass balance model software platform developed by HydroLogics and used by NYC DEP.

OST – Operations Support Tool that combines OASIS and CE-QUAL-W2 and produces results that are used to inform NYC DEP’s water management decisions.

Position analysis (PA) mode – Use of the Operations Support Tool to simulate future operations from the current date using an ensemble of multiple time series of input data.

Release – As used by NYC DEP, a planned outflow of water from a reservoir into a stream or channel downstream of the reservoir.

Reservoir – Manmade body of water used for storage.

Risk – Used in this report to signify the probability of an occurrence (e.g., high-flow event) multiplied by the potential consequences of the event.

Rule curve – Tool for water management, typically in the form of an x-y line graph showing a target storage level in a reservoir on the vertical axis and the day of the year on the horizontal axis.

Shaft – In water management, a vertical opening typically used to divert water from a reservoir to a water conveyance structure.

Simulation model – Mathematical model intended to represent a real-world process, enabling the user to examine the possible influence of changing model inputs on simulated outcomes.

Simulation (Sim) mode – Use of the Operations Support Tool to simulate operations using a single time series of input data.

Spill – Usually an uncontrolled reservoir release, typically over a broad-crested weir or spillway, resulting from reservoir inflows that cause the reservoir elevation to exceed the elevation of the spillway.

Stop shutter – Device used in water control that affects the flow of water through an aqueduct or other conveyance structure, either by blocking flow completely or by causing flow to pass over the stop shutter, thus impacting the depth of water in an open channel conduit.

Streamflow – Flow of water through streams, rivers, or other channels.

Turbidity – Cloudiness of water (or any fluid) caused by the scattering of visible light by very small suspended particles (e.g., from eroded soils or sediments).

Validation – Process of assessing a model’s ability to simulate actual outcomes that occurred in the past, using historical data that were not used in the process of model calibration.

Weir – Low dam across a river, stream, or other waterbody that enables control of water level or downstream flow.

W2 – Shortened acronym for the CE-QUAL-W2 water quality and hydrodynamics model.