                                                           File: ABSTRACT.TXT


                       QUAL2EU Model System Abstract

               Center for Exposure Assessment Modeling (CEAM)
     National Exposure Research Laboratory - Ecosystems Research Division
                  Office of Research and Development (ORD)
               U.S. Environmental Protection Agency (U.S. EPA)
                          960 College Station Road
                         Athens, Georgia 30605-2700

                                706/546-3549



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                                  Summary

The Enhanced Stream Water Quality Model QUAL2E and QUAL2E-UNCAS (1) permits
simulation of several water quality constituents in a branching stream system
using a finite difference solution to the one-dimensional
advective-dispersive mass transport and reaction equation.  The conceptual
representation of a stream used in the QUAL2E formulation is a stream reach
that has been divided into a number of subreaches or computational elements
equivalent to finite differences.  For each computational element, a
hydrologic balance in terms of flow (Q), a heat balance in terms of
temperature (T), and a materials balance in terms of concentration (C) is
written.  Both advective and dispersive transport are considered in the
materials balance.  Mass can be gained or lost from the element by transport
processes, external sources and sinks (e.g., waste discharges or withdrawals)
or by internal sources and sinks (e.g., benthic sources or biological
transformations).  The equation is solved for the steady flow, steady state
condition in a classical implicit backward difference method.  The specific
equations and solution technique are described in detail in the QUAL2E
computer program documentation.

                             Data Requirements

In its present state, 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 uncertainty analysis procedures incorporated in the computer program
serve both to guide the user in the calibration process as well as to provide
information about the uncertainty associated with the calibrated model.

Prototype representation in QUAL2E consists of dividing a stream in a network
of "Headwaters", "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 properties (model input parameters
or coefficients) are constant along 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.  The forcing function used for estimating transport is the
streamflow rate, which, as mentioned above, is assumed to be constant.
Stream velocity, cross-sectional area, and depth are computed from
streamflow.

                                   Output

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 computer program includes the major interactions
of the nutrient cycles, algal production, benthic and carbonaceous oxygen
demand, atmospheric reaeration, and their effect on the dissolved oxygen
balance.  In addition, the computer program includes a heat balance for the
computation of temperature and mass balances for conservative minerals,
coliform bacteria, and nonconservative constituents such as radioactive
substances.  Chlorophyll a is modeled as the indicator of planktonic algae
biomass in QUAL2E.

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 (BOD) is
modeled as a first-order degradation process in QUAL2E, which also takes into
account removal by settling and does not affect the oxygen balance.  QUAL2E
will convert the traditional 5-day BOD values to ultimate BOD for internal
calculations.  The processes discussed above represent the primary internal
sinks of dissolved oxygen in the QUAL2E computer program.  The major source
of dissolved oxygen, in addition to that supplied from algal photosynthesis,
is atmospheric reaeration.

A major problem faced by the user when working with a complex model such as
QUAL2E is model calibration and determination of the most efficient plan for
collection of calibration data.  This problem can be addressed by application
of principles of uncertainty analysis.  These strategies have been applied to
QUAL2E and the resulting computer program is named QUAL2E-UNCAS.  Three
uncertainty analysis techniques are employed in QUAL2E-UNCAS: sensitivity
analysis, first order error analysis, and monte carlo simulation.  The
computer program uses pre- and post-processing algorithms to select the input
variables and/or parameters to be altered without the user having to manually
restructure the input data set and to store and manipulate only the output of
interest.  The modeler is free to select the important variables and
locations in the stream network where uncertainty effects are desired.

                        Assumptions and Limitations

There seems to be some confusion about QUAL2E's transport capabilities as
QUAL2E is sometimes called a "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 proper
term for this capability is "diel," indicating variation over a 24 hour
period.

                            Application History

The QUAL series of computer programs has a long history in water quality
management.  The foundation upon which the series is built was laid by the
Texas Water Development Board (TWDB) in the late 1960s.  In the early 1970s,
EPA began a program to provide water quality models for major river basins
and specified that QUAL-I be used as the basis for developing new, more
advanced, basin-specific models.  Many versions of the QUAL-II model emerged
from this effort.  One version was further improved in the mid-1970s for the
Southeast Michigan Council of Governments (SEMCOG), the areawide wastewater
planning agency for the Detroit metropolitan area.  Because of its
flexibility and thorough documentation, the SEMCOG version of QUAL-II, known
as QUAL-II/SEMCOG received widespread use, especially in wasteload allocation
studies.

In the late 1970's, the National Council of the Paper Industry for Air and
Stream Improvement (NCASI) undertook a thorough review, testing and
documentation project covering a variety of water quality models, including
QUAL-II/SEMCOG.  Changes resulting from this review were incorporated in the
program and the model was renamed QUAL-II/NCASI.  NCASI and other groups such
as the U.S. Geological Survey (USGS) tested the revised program on several
intensively sampled rivers across the United States. Recently, the model has
seen several European applications in England, Greece, Belgium and Spain.
Other applications of QUAL-II range from South America to the Far East,
including South Korea, the People's Republic of China, and Thailand.  A
number of applications and ongoing work with the QUAL2E model are described
by Barnwell et al. (2).

                                 References

1.  Brown, L. C., and T. O. Barnwell.  1987.  The Enhanced Stream Water
Quality Model QUAL2E and QUAL2E-UNCAS: Documentation and User Manual.
EPA-600/3-87/007, U.S. EPA, Athens, GA, 30605.

2.  Barnwell, T. O., L. C. Brown, and R. C. Whittemore.  1987.  QUAL2E:  A
Case Study in Water Quality Modeling Software.  In:  Systems Analysis in
Water Quality Management.  Pergamon Press, Oxford.

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