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MS-DOS/Windows |
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v3.22 |
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May 1996 |
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Yes (for Windows version) |
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USEPA |
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The Enhanced Stream Water Quality Model (QUAL2E)
is a comprehensive and versatile stream water
quality model. It can simulate up to 15 water
quality constituents in any combination desired
by the user.
The model is applicable to dendritic streams that
are well mixed. It uses a finite-difference
solution of the advective-dispersive mass
transport and reaction equations. The model is
intended for use as a water quality planning tool.
QUAL2E-UNCAS is an enhancement to QUAL2E that
allows the user to perform uncertainty analysis.
Three uncertainty options are employed in
QUAL2E-UNCAS: sensitivity analysis, first order
error analysis, and Monte Carlo simulation
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QUAL-I was initially developed by the Texas
Water Development Board in the 1960s. Several
improved versions of the model were developed
by EPA as part of this effort, and after
extensive review and testing the QUAL-II
series became widely used. Present support
for the model is provided by the Environmental
Protection Agency's Center for Exposure
Assessment Modeling (CEAM).
QUAL2E simulates up to 15 water quality
constituents in branching stream systems.
The model uses a finite-difference solution
of the advective-dispersive mass transport
and reaction equations.
A stream reach is divided into a number of
computational elements, and for each computational
element, a hydrologic balance in terms of
stream flow (e.g., m3/sec),
a heat balance in terms of temperature
(e.g., ºC), and a material balance
in terms of concentration (e.g., mg/l) are
written.
Both advective and dispersive transport
processes are considered in the material
balance. Mass is gained or lost from the
computational element by transport processes,
wastewater discharges, and withdrawals.
Mass can also be gained or lost by internal
processes such as release of mass from
benthic sources or biological transformations.
The program simulates changes in flow conditions
along the stream by computing a series of steady-state
water surface profiles. The calculated stream-flow
rate, velocity, cross-sectional area, and water depth
serve as a basis for determining the heat and mass
fluxes into and out of each computational element due
to flow. Mass balance determines the concentrations
of conservative minerals, coliform bacteria, and
nonconservative constituents at each computational
element.
In addition to material fluxes, major processes
included in mass balance are transformation of
nutrients, algal production, benthic and carbonaceous
demand, atmospheric reaeration, and the effect of
these processes on the dissolved oxygen balance.
QUAL2E uses chlorophyll a as the indicator of
planktonic algae biomass. The nitrogen cycle is
divided into four compartments: organic nitrogen,
ammonia nitrogen, nitrite nitrogen, and nitrate
nitrogen. In a similar manner, the phosphorus
cycle is modeled by using two compartments. The
primary internal sink of dissolved oxygen in the
model is biochemical oxygen demand (BOD). The major
sources of dissolved oxygen are algal photosynthesis
and atmospheric reaeration.
The model is applicable to dendritic streams that
are well mixed. It assumes that the major transport
mechanisms, advection and dispersion, are significant
only along the main direction of flow (the longitudinal
axis of the stream or canal). It allows for
multiple waste discharges, withdrawals, tributary
flows, and incremental inflow and outflow. It also
has the capability to compute required dilution flows
for flow augmentation to meet any pre-specified
dissolved oxygen level.
Hydraulically, QUAL2E is limited to the simulation
of time periods during which both the stream flow in
river basins and input waste loads are essentially
constant. QUAL2E can operate as either a steady-state
or a quasi-dynamic model, making it a very helpful
water quality planning tool. When operated as a
steady-state model, it can be used to study the
impact of waste loads (magnitude, quality, and location)
on instream water quality.
By operating the model dynamically, the user
can study the effects of diurnal variations in
meteorological data on water quality (primarily
dissolved oxygen and temperature) and also
can study diurnal dissolved oxygen variations
due to algal growth and respiration. However,
the effects of dynamic forcing functions, such
as headwater flows or point loads, cannot be modeled in QUAL2E.
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Prototype representation in QUAL2E consists
of dividing a stream into a network consisting of
"Headwater," "Reaches," and "Junctions." The
fundamental reason for subdividing sections of
a stream into "reaches" is that QUAL2E assumes
that some 26 physical, chemical, and biological
parameters (model input parameters or
coefficients) are constant along a "reach."
For example, different values for Manning's
roughness coefficient, sediment oxygen demand,
and algal settling rate can be specified by the
user for different reaches, but each of these
values remains constant over a particular reach.
However, the state variables change within a
reach; e.g., DO is calculated at each computational
element and thus can vary within a reach. The
question that must be addressed in order to
define a "reach" is what constitutes "significant"
change in these model inputs "significant" in the
sense of their impact on simulation results,
not necessarily in the sense of change in the
inputs themselves.
Mass transport in the QUAL2E computer program is handled in a relatively
simple manner. There seems to be some confusion about QUAL2E's transport
capabilities because it is sometimes called a "quasi-dynamic" model.
However, in all of the computer programs in the QUAL series, there is an
explicit assumption of steady flow; the only time-varying forcing
functions are the climatologic variables that primarily affect
temperature and algal growth. A more appropriate term for this
capability is "diel," indicating variation over a 24-hour period. The
forcing function used for estimating transport is the stream flow rate,
which, as mentioned above, is assumed to be constant. Stream velocity,
cross-sectional area, and depth are computed from stream flow.
One of the most important considerations in determining the assimilative
capacity of a stream is its ability to maintain an adequate dissolved
oxygen concentration. The QUAL2E program performs dissolved oxygen
balance by including major source and sink terms in the mass balance
equation. The nitrogen cycle is composed of four
compartments: organic nitrogen, ammonia nitrogen, nitrite nitrogen, and
nitrate nitrogen. The phosphorus cycle is similar to, but simpler than,
the nitrogen cycle, having only two compartments. Ultimate carbonaceous
biochemical oxygen demand (CBOD) is modeled as a first-order degradation
process in QUAL2E.
If the modeler uses BOD5 as an input, QUAL2E converts
5-day BOD to ultimate BOD for internal calculations. Oxidation processes
involved in CBOD decay and in the nutrient cycles represent the primary
internal sinks of dissolved oxygen in the QUAL2E program. The major
source of dissolved oxygen, in addition to that supplied from algal
photosynthesis, is atmospheric reaeration.
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Uncertainty analysis for model simulations is assuming a growing
importance in the field of water quality management. QUAL2E allows the
modeler to perform uncertainty analysis on steady-state water quality
simulations. Three uncertainty analysis techniques are employed in
QUAL2E-UNCAS: sensitivity analysis, first-order error analysis, and
Monte Carlo simulation.
With this capability, the user can assess the
effect of model sensitivities and of uncertain input data on model
forecasts. Quantifications of the uncertainty in model forecasts will
allow assessment of the risk (probability) of a water quality variable
being above or below an acceptable level. The user can select the
important input variables to be perturbed and locations on the stream
where the uncertainty analysis is to be applied. |
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QUAL2E requires some degree of modeling sophistication and expertise on
the part of a user. The user must supply more than 100 individual
inputs, some of which require considerable judgment to estimate. The
input data in QUAL2E can be grouped into three categories: a
stream/river system, global variables, and forcing functions.
Additionally, there are three data groups for simulation control and
uncertainty analysis.
The first step in preparing the QUAL2E inputs is to describe a complete
stream/river system by applying the rules that are defined by the model.
The stream system should be divided into reaches, which are stretches of
stream that have uniform hydraulic characteristics. Each reach is then
subdivided into computational elements of equal length. Thus, all
reaches must consist of an integer number of compu- tational elements.
Functionally each computational element belongs to one of seven types
(described later). River reaches are the basis of most input data.
The global variables include simulation variables, such as units and
simulation type, water quality con- stituents, and some physical
characteristics of the basin. Up to 15 water quality constituents can be
modeled by QUAL2E.
Forcing functions are user-specified inputs that drive the system being
modeled. These inputs are speci- fied in terms of flow, water quality
characteristics, and local climatology. QUAL2E accommodates four types
of hydraulic and mass-load-forcing functions in addition to local
climatological factors: headwater - inputs, point sources or
withdrawals, incremental inflow/outflow along a reach, and the
downstream boundary concentration (optional).
Local climatological data are required for the simulation of algae and
temperature. The temperature simulation uses a heat balance across the
air-water interface and thus requires values of wet and dry bulb air
temperatures, atmospheric pressure, wind velocity, and cloud cover. The
algal simulation requires values of net solar radiation. For dynamic
simulations, these climatological data must be input at regular time
intervals over the course of the simulation and are applied uniformly
over the entire river basin. For modeling steady-state temperature and
algae, average daily local climatological data are required and may vary
spatially over the basin by reach.
The uncertainty analysis procedures incorporated into the computer
program guide the user in the calibration process, in addition to
providing information about the uncertainty associated with the
calibrated model.
To create QUAL2E input files, the user has to follow data type sequences
within one particular input file. There are five different input files
for which certain combinations must be created before running the model.
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QUAL2E produces three types of tables hydraulics, reaction coefficient,
and water quality in the output file. The hydraulics summary table
contains flows, velocities, travel time, depths, and cross-sectional
areas along each reach. The reaction coefficient table lists the
reaction coefficients for simulated constituents. The water quality
table reports constituent concentrations along a reach. A summary of
temperature calculations may also be included.
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QUAL2E Implementation in Windows
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The QUAL2E Windows interface is designed to be as user-friendly as
possible. The interface consists of 24 screens that cover all the data
required by QUAL2E and QUAL2E-UNCAS. The first 20 screens represent the
data for QUAL2E, and the last four screens are for QUAL2E-UNCAS. The
screen input sequence for QUAL2E is given in Table 3.1. In general, the
interface is divided into six data components: QUAL2E simulation
control, a stream system, global variables, functional data, climatology
data, and uncertainty analysis. The QUAL2E simulation control describes
simulation control variables and number of reaches in the reach system.
A complete stream system is described by the reach connection, element
type, and a computational length. River reaches, which are aggregates of
computational elements, are the basis of most data input. The global
variables include number of constituents to be simulated, geographical
and clima- tological information, option for plotting DO/BOD, and
kinetics and temperature correction factors. The functional data provide
flow data, reaction coefficients, and forcing functions. Initial
conditions, boundary conditions, and point source loads are input as
forcing functions. The global climatology data are required only for
diurnal DO simulations. The uncertainty analysis (optional) data consist
of types of uncertainty analyses, input and output conditions, and input
variables with perturbations.
Of 24 screens, the first 3 screens where a complete stream system is
entered are most important because the majority of the data on the
following screens are dependent upon the information given by Screens
1-3. The stream system can be described by reach name, beginning and
ending reach in terms of river miles or kilometers, and an indication of
the headwater. The sequence of the reaches given on Screen 2 is used by
the interface to display the reach connections. Each reach is then
subdivided into computational elements of equal length, which are also
displayed on the reach graphics screen. Once this information has been
pro- vided, the interface will automatically link all reaches to a
stream system and assign the element types as headwaters, junctions,
standards, or a downstream boundary on Screen 3.
There are seven different types of computational elements: headwater
element, standard element, upstream element from a junction, junction
element, downstream element, point source, and withdrawal element. A
headwater element begins every tributary as well as the main river
system, and therefore must always be the first element in a headwater
reach. A standard element is one that does not qualify as one of the
remaining six element types. An upstream element from a junction is used
to designate an element on the mainstream that is just upstream of a
junction. A junction element has a simulated tributary entering it. A
downstream element is defined as the last element in a stream system.
Point sources and withdrawals represent elements that have inputs (waste
loads and unsimulated tributaries) and water withdrawals, respectively.
Table 3.2 lists seven element types allowed in the QUAL2E input
(represented below as numbers) and eight in the QUAL2E interface
(indicated by capital letters).
Certain element types on Screen 3 are grayed out, such as headwater
elements and junction elements. This means those types or fields cannot
be changed. The only element types or fields that can be changed are the
standard elements where the Ss are located. The standard elements could
be further defined as point sources, withdrawals, or dams. The user
should indicate the locations of point sources, withdrawals, or dams if
they are applied. River reaches and computational elements are the basis
of most data input. Screen 4 is used to identify water quality
parameters to be simulated. As mentioned previously, QUAL2E can simulate
up to 15 water quality constituents in any combination desired by the
user. Constituents that can be modeled are:
- Dissolved oxygen (DO)
- Biochemical oxygen demand (BOD)
- Temperature
- Algae as chlorophyll a
- Phosphorus cycle (organic and dissolved)
- Nitrogen cycle (organic, ammonia (NH3), nitrite (NO3), nitrite (NO2))
- Coliforms
- Arbitrary nonconservative constituent
- Three conservative constituents
Water quality constituents can be simulated under either steady-state or
quasi-dynamic conditions. If either the phosphorus cycle or the nitrogen
cycle is not being simulated, the model presumes they will not limit
algal growth. Note that QUAL2E can simulate either ultimate BOD or 5-day
BOD (BOD5).
The model simulates ultimate BOD in the general case. If the user wishes
to use 5-day BOD for input and output, the program will internally make
the conversion to ultimate BOD. On Screen 4, if only BOD is chosen, the
ultimate BOD will be simulated; if both BOD and BOD5 are selected, the
5-day BOD input/output option is applied.
Geographical and climatological data are entered on Screen 5.
Climatological data can be varied with reaches or constant throughout
reaches depending on the simulation type. Temperature correction factors
could be defaults by the model or user-specified. Also, if the user has
observed DO data that are stored in a .DO file, that could be specified
under Observed Dissolved Oxygen file on Screen 5. The observed data are
stored on Screen 7.
Functional data are input on Screens 10 through 19. Flow characteristics
of the reach system can be described by dispersion coefficients,
discharge coefficients or a geographical representation (i.e.,
trapezoidal channels), and Manning's n. Flow augmentation may be applied
when the DO concentration drops below some required target level.
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Input Screen Sequence in QUAL2E Windows Interface
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Data Description
Input
Component Input Data Content FIle Screen
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1 QUAL2E Simulation (1) Title, simulation type, unit, time-step
control (2) Uncertainty analysis, flow augmentation, *.RUN 1
trapezoidal channels, no. of reaches
2 Stream system (1) Reach ID and river miles/km, headwater, length *.RUN 2
(2) Element type for each reach *.RUN 3
3 Global variables (1) Water quality (no. of constituents) *.RUN 4
(2) Geographical & climatological data *.RUN 5
(Lat., long., dust., elev., evap.)
(3) Plot DO/BOD (List reach numbers to be plotted) *.RUN 6
(4) Observed DO file *.DO 7
(5) Global kinetics, temp. correct. factor *.RUN 8,9
4 Functional data (1) Flow
- Flow augmentation *.RUN 10
- Hydraulic data/local climatology *.RUN 11
(2) BOD/DO, algae, N, P, reaction coefficient *.RUN 12,13
(3) Forcing function
- Initial conditions *.RUN 14
- Incremental inflow *.RUN 15
- Headwater *.RUN 16
- Point loads/withdrawals *.RUN 17
- Dams *.RUN 18
- Downstream boundary *.RUN 19
5 Climatological data (1) Global climatological data file *.CLI 20
6 Uncertainty Analysis(1) Sensitivity analysis, first order error *.UNS 21
analysis, Monte Carlo simulation
(2) Input conditions, output *.UNS
(3) Input variables for sensitivity analysis *.UNS 22
(4) Input variables for first order and *.VAR 23
Monte Carlo analyses
(5) Reach (element) numbers to be printed *.UNS 24
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Minimum System Requirements
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The system runs under Microsoft Windows.
The minimum system requirements are provided below:
- Windows Version 3.1 or higher
- 80386 processor
- 4 megabytes RAM
- 10 megabytes hard disk space
NOTE: A math coprocessor is recommended but not required.
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