Enhanced Stream Water Quality Model (QUAL2EU)
-- Description
MS-DOS/Windows
v3.22
May 1996
Yes (for Windows version)
USEPA

	
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Introduction

  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

Overview of QUAL2E

  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.

Prototype Presentation

  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.

Uncertainty Analysis

  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.

Data Requirements

  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.

Output File

  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.

QUAL2E Implementation in Windows

  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:

  1. Dissolved oxygen (DO)
  2. Biochemical oxygen demand (BOD)
  3. Temperature
  4. Algae as chlorophyll a
  5. Phosphorus cycle (organic and dissolved)
  6. Nitrogen cycle (organic, ammonia (NH3), nitrite (NO3), nitrite (NO2))
  7. Coliforms
  8. Arbitrary nonconservative constituent
  9. 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.

Input Screen Sequence in QUAL2E Windows Interface

 
Data	     Description
											Input 
Component    Input Data   Content					         FIle	Screen
----------------------------------------------------------------------------------------------
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
-----------------------------------------------------------------------------------------------

Minimum System Requirements

  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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