A

Aquifer – as defined in SWMM, it is the underground water bearing layer below a land surface, containing both an upper unsaturated zone and a lower saturated zone.

Areal Depletion - the process by which the land area covered by snow decreases as the total volume of snow decreases due to melting.

C

Capillary Suction Head - the soil water tension at the interface between a fully saturated and partly saturated soil.

Climate Data Online - an interactive web based data retrieval service operated by NOAA's National Climatologic Data Center for retrieving historical rainfall and climate data.

Cold Content - the difference between the heat content of a frozen snow pack and its base melt temperature.

Continuous Simulation - refers to a simulation run that extends over more than just a single rainfall event.

Curve Number - a factor, dependent on land cover, used to compute a soil's maximum moisture storage capacity.

Curve Number Method - a method that uses a soil's maximum moisture storage capacity as derived from its curve number to determine how cumulative infiltration changes with cumulative rainfall during a rainfall event. Not to be confused with the NRCS (formerly SCS) Curve Number runoff method as embodied in TR-55.

D

Darcy's Law - states that flow velocity of water through a porous media equals the hydraulic conductivity of the media times the gradient of the hydraulic head it experiences.

Depression Storage – the volume over a surface that must be filled prior to the occurrence of runoff. It represents such initial abstractions as surface ponding, interception by flat roofs and vegetation, and surface wetting.

Design Storm - a rainfall hyetograph of a specific duration whose total depth corresponds to a particular return period (or recurrence interval), usually chosen from an IDF curve.

Directly Connected Impervious Area - impervious area whose runoff flows directly into the collection system without the opportunity to run onto pervious areas such as lawns.

**Dividing Temperature - t**he temperature below which precipitation falls in the form of snow.

F

Field Capacity - the amount of water a well-drained soil holds after free water has drained off, or the maximum soil moisture held against gravity. Usually defined as the moisture content at a tension of 1/3 atmospheres.

G

Global Historical Climatology Network - a data base administered by NOAA's National Climatic Data Center that archives daily climate observations from approximately 30 different sources for about 30,000 stations across the globe.

Green-Ampt Method - a method for computing infiltration of rainfall into soil that is based on Darcy's Law and assumes there is a sharp wetting front that moves downward from the surface, separating saturated soil above from drier soil below.

H

Hargreaves Method - an empirical formula for estimating daily evaporation that depends on air temperature and solar radiation.

Horton Curve - an empirical curve that describes the exponential decrease in infiltration rate with time during a rainfall event.

Horton Method - a method for computing infiltration of rainfall into soil that uses the Horton Curve to relate infiltration rate to time, with modifications made to consider times where the rainfall rate is less than the curve's infiltration rate.

Hydraulic Conductivity - the rate of water movement through soil under a unit gradient of hydraulic head. Its value increases with increasing soil moisture, up to a maximum for a completely saturated soil (known as the saturated hydraulic conductivity or K~sat~).

Hydrograph - a plot that shows how runoff flow varies with time.

Hydrologic Soil Group - a classification that indicates a soil's ability to infiltrate water.

Hyetograph - a plot that shows how rainfall rate varies with time.

I

IDF Curves – a series of curves that determine the average rainfall intensity (I) for a given duration of storm (D) that occurs at a specific annual frequency (F), e.g., the intensity of a 6-hour storm that occurs once every 10 years.

Impervious Surface – a surface that does not allow infiltration of rain water, such as a roof, roadway or parking lot.

Infiltration – the process by which rainfall penetrates the ground surface and fills the pores of the underlying soil.

Infiltrometer - a device used to measure the rate of water infiltration into soil or other porous media.

Initial Abstraction – precipitation that is captured on vegetative cover or within surface depressions that is not available to become runoff and is removed by either infiltration or evaporation.

L

LID Control – a low impact development practice that provides detention storage, enhanced infiltration and evapotranspiration of runoff from localized surrounding areas. Examples include rain gardens, rain barrels, green roofs, vegetative swales, and bio-retention cells.

Link – a connection between two nodes of a SWMM conveyance network that transports water. Channels, pipes, pumps, and regulators (weirs and orifices) are all represented as links in a SWMM model.

M

Manning Equation – the equation that relates flow rate to the slope of the hydraulic grade line for gravity flow in open channels.

Manning Roughness – a coefficient that accounts for friction losses in the Manning flow equation.

Modified Horton Method – a modified form of the Horton infiltration method that tracks cumulative infiltration volume instead of time along the Horton curve to determine how infiltration rate changes with time during a rainfall event.

Moisture Deficit – the difference between a soil's current moisture content and its moisture content at saturation.

N

Newton-Raphson Method – a commonly used iterative numerical method for solving nonlinear equations that makes use of the derivative of the equation with respect to the unknown variable.

Node – a point in a runoff conveyance system that receives runoff and other inflows, that connects conveyance links together, or that discharges water out of the system. Nodes can be simple junctions, flow dividers, storage units, or outfalls. Every conveyance system link is attached to both an upstream and downstream node.

Nonlinear Reservoir Model – a simple conceptual model of a storage reservoir where the change in volume with respect to time equals the difference between a known inflow rate and an outflow rate that is a nonlinear function of the current stored volume.

O

Overland Flow Path – the path that runoff follows as it flows over the surface of a catchment area until it reaches a collection channel or storm drain.

P

Pervious Surface – a surface that allows water to infiltrate into the soil below it, such as a natural undeveloped area, a lawn or a gravel roadway.

Pollutograph – a plot of the concentration of a pollutant in runoff versus time.

Porosity - the fraction of void (or air) space in a volume of soil.

R

Rainfall File – an external text file that contains rainfall data for a single rain gage in one of the several different formats that SWMM can recognize.

Rainfall Interface File – a binary file generated by SWMM that contains the rainfall time series used in a simulation for all of the rain gages in the project. This file can be used to input rainfall in subsequent simulation runs.

Rain Gage – a SWMM object that provides precipitation data, either as an internal time series or through an external data file, to one or more subcatchment areas in a SWMM model.

RDII – rainfall dependent inflow and infiltration are stormwater flows that enter sanitary or combined sewers due to "inflow" from direct connections of downspouts, sump pumps, foundation drains, etc. as well as "infiltration" of subsurface water through cracked pipes, leaky joints, poor manhole connections, etc.

Richards Equation – the nonlinear partial differential equation that describes the physics of water flow in unsaturated soil as a function of moisture content and moisture tension.

Routing Interface File – a text file that contains the time history of external flow and water quality inflow to different locations of the conveyance network of a SWMM model. It can be generated from a previous SWMM run or can serve as a replacement for SWMM's runoff calculations.

RTK Unit Hydrograph – a triangular unit hydrograph that represents the time pattern of rainfall entering a sewer system as RDII. R is the fraction of total rainfall entering the system (i.e., the area under the hydrograph), T is the time at the hydrograph peak, and K is the ratio of the length of the receding limb of the hydrograph to the time to peak.

Runge-Kutta Method – a numerical method for solving systems of ordinary differential equations over a series of sequential time steps.

Runoff Coefficient – the ratio of total runoff to total rainfall over a study area.

S

Shape Factor – the ratio of a watershed's area to the length of its main drainage channel squared. It is used to estimate the runoff width of a catchment area.

Snow Catch Factor – a multiplier used to correct for inaccurate snowfall measurements due to wind blowing snow away from the precipitation gage.

Snow Pack – the accumulation of snow cover that blankets an area. Snow pack depth increases as new snow falls and decreases as snow melts.

Subcatchment – a sub-area of a larger catchment area whose runoff flows into a single drainage pipe or channel (or onto another subcatchment).

Subcatchment Discretization – the process of dividing a study area into subcatchments that properly characterize the spatial variability in overland drainage pathways, surface properties and connections into drainage pipes and channels.

T

Two-Zone Groundwater Model – a conceptual model that represents the subsurface region beneath a subcatchment as consisting of an unsaturated upper zone that lies above a lower saturated zone. The extent of each zone and the moisture content of the upper zone can change in response to variations in surface infiltration, evapotranspiration, and groundwater outflow.

U

Unit Hydrograph – represents the unit response of a watershed (in terms of runoff volume and timing) to a unit input of rainfall. Unit hydrographs are specific to particular catchments and typically have either a triangular or bell curve shape.

W

Wilting Point - the soil moisture content at which plants can no longer extract moisture to meet their transpiration requirements. Usually defined as the moisture content at a tension of 15 atmospheres.