Role of Models

Role of Models

Role of Models

Application of Models for Watershed Restoration

Using TMDLs

Keeping track of load allocations is a complicated process. There are a number of water-shed

models available, but few are designed specifically to deal with TMDL issues. Many

need extensive local calibration to provide realistic predictions of water quality. When

using models, it is important to keep in mind that the quality you get out of a model de-pends

on what you put into it. If data going in are not reliable then you’re not going to get

reliable information out of it.

BASINS: Better Assessment Science Integrating Point

and Nonpoint Sources

BASINS is a software package developed by the US Environmental Protection Agency that

combines standard water quality and watershed models with a Geographic Information

System (GIS) containing numerous national data layers. BASINS GIS allows the user to

examine watersheds from several perspectives (EPA, 1998) and to overlay landscape

features, pollution sources, and monitoring data layers (see Figure 11 below). BASINS

allows a user to fully visualize, explore, and query to bring a watershed to life (EPA, 1999).

FIGURE 11. BASINS GIS View Overlay with Reach Network (RF3), Point Sources (PCS), Monitoring

Locations, Elevations, and USGS HUC-8 Patuxent River Watershed Boundary.

The US EPA is proposing that BASINS be used as the standard approach for analyzing

baseline watershed conditions and loadings, for examining TMDL load allocation scenarios,

and in guiding monitoring strategies for in-depth watershed analyses and TMDL compli-ance

evaluations. EPA believes that this offers great promise for providing a uniform

approach to the entire TMDL program.

The BASINS software and data are available via the Internet (www.epa.gov/ost/basins) or as a

series of CD-ROM disks organized by EPA region. The simulation models run in a Windows

environment and require the standard GIS package ArcView, version 3.0a or 3.1.

BASINS contains four data types:

w Base cartographic data - atlas landmarks such as roads, towns, state, tribal and county

boundaries, etc

w Physical landscape features - the physical environment including rivers and streams,

elevations, land use, etc

w Environmental monitoring data - water quality data in statistical summaries and raw

monitoring data sets

w Point sources/loading data - National Pollution Discharge Elimination System (NPDES)

yearly loading values and other point source (Industrial Facilities Discharge (IFD), Toxic

Release Inventory (TRI), Superfund Sites (NPL), and Mineral Industry Locations (USBM

mine) (EPA, 1998)

A Nation’s Worth of Data

BASINS water quality was drawn from STORET - the legacy EPA database for water quality

data reaching as far back as the 1960s. BASINS Water Quality Station data tables are

statistical summaries of measured pollutant concentrations for each station over five year

increments. Summary statistics include the number of observations, the mean value, and

15 th , 25 th , 50 th , 75 th , and 85 th percentile values for each combination of station and pollut-ant,

over five-year periods.

A subset of the Water Quality Station data layer, called Water Quality Observation Stations,

contains raw data sets for pollutants with sufficient monitoring data sets to provide a starting

point for evaluating model simulation results. The water quality observation stations were

selected with the additional criteria that they provide a relatively uniform national coverage.

BASINS contains utilities to add, delete, or move water quality observation stations as well as to

append user-supplied data sets to the Water Quality Observation database table. Most important,

users can export water quality monitoring data sets – from programs such as EcoWatch for

Windows --to the BASINS Postprocessor for comparison against output from the NPSM, BA-SINS’

continuous-simulation watershed model (Fig. 2).

Figure 12. NPSM Postprocessor Comparison Plot of Modeled Stream Flow (red when

viewing a color copy) vs. Monitored USGS Gage Flow (blue when viewing a color copy).

The modeler can then iteratively adjust input values, re-run the model, and view the output

until the desired accuracy in calibration, and representation of the physical environment, is

achieved. Additionally, experienced NPSM/HSPF users can use water quality monitoring

time series, for parameters such as temperature and dissolved oxygen concentration, as

model input in lieu of modeling these parameters in separate modules. This both simplifies

and improves the accuracy of simulating complex processes such as nutrient cycling and/or

plankton dynamics.

The data products supplied by BASINS (data source, database acronym) include the following

(Battin, 1998):

Physical landscape features

- Watershed boundaries (USGS, HUC-8)

- River/stream networks at 1:500,000 (EPA, RF1) and 1:100,000 (EPA, RF3)

scales

- Land use/ land cover (USGS, GIRAS)

- Elevation (USGS, DEM)

- Dam locations (ACE & FEMA, NID)

- Soil Characteristics (USDA/NRCS, STATSGO)

Pollution sources

- Permitted dischargers (EPA, NPDES/PCS)

- Toxic Release Inventory (EPA, TRI)

- Industrial Facility Dischargers (EPA, IFD)

- Mineral Industry Locations (USBM, MAS/MILS)

- Superfund sites (EPA, NPL)

Environmental monitoring

- Water quality stations summaries (EPA, STORET)

- Bacteria station summaries (EPA, STORET)

- Water quality observation data (EPA, STORET)

- National Sediment Inventory (EPA, NSI)

- USGS gage stations and mean flow (EPA, STORET)

- Fish and wildlife advisories (EPA, LFWA)

- Shellfish contamination inventory (NOAA)

- Clean water needs survey (EPA, CWNS)

- Meteorological stations (NOAA, NCDC)

GIS Tools and Utilities

BASINS has a series of three customized GIS tools for performing environmental assessments on

water quality and permitted discharge data layers: TARGET, which summarizes data at the

watershed scale for areas covering multiple watersheds; ASSESS, which summarizes data at the

monitoring station or permitted discharge location level for single watersheds; and Data Mining,

which displays a set of dynamically linked tables for viewing all available data for a user-selected

set of monitoring stations or permitted dischargers.

BASINS also includes six utilities:

w Watershed delineation - allows the user to manually digitize their own watersheds based on

river/reach segment locations, elevation data, and the user-selected pour point

w DEM reclassification - redraws the elevation data layer to change viewing resolution or to

adjust the viewing resolution to emphasize hilltop or valley zones

w Import GIS data - user can import delineated watersheds, local land use data, and BASINS’

DEM and high resolution reach data (RF3)

w Land use reclassification - these tools allow the user to redefine land use classifications for

individual land use polygons as well as entire land use categories; used for modeling land-use

change scenarios and general land use data management

w Water quality observation data management - allows the user to append the observation

data table and export data for select stations and pollutants

w Lookup tables - provides lists of water quality criteria, Standard Industrial Classification

(SIC) codes, agency codes; useful for interpreting water quality and discharge data sets

BASINS also generates a series of watershed characterization reports, each one a combination of

maps, charts, and/or tables detailing watershed specific information on 1) point sources, 2) water

quality monitoring statistics, 3) toxic air emissions, 4) land use distribution, 5) soil characteris-tics,

and 6) watershed topography. BASINS’s customized reports enable the TMDL analyst to

rapidly synthesize relevant environmental data for developing an understanding of point/nonpoint

source, landscape, and water quality impact relationships as well as to guide further analysis,

monitoring, or pollutant modeling studies.

Models Range from Simple to Complex

BASINS contains three environmental fate and transport models for water quality and watershed

simulations. Each model extracts some input values from the BASINS GIS and includes a

Windows interface to help enter the remaining input parameters. The three models, in order of

increasing complexity, are:

· TOXIROUTE - a steady state water quality model that simulates dilution and first order pollut-ant

decay for point sources at given flow rates. Reach flow, reach network, and point source data

are extracted to populate the model for the user-selected watershed;

· QUAL2E - a steady state and quasi-dynamic water quality model for simulating point source

impact on water quality including nitrogen and phosphorus cycling, dissolved oxygen and

BOD, algae, fecal coliform, other conservative and non-conservative substances, and

temperature for given flow patterns. Reach flow, reach network, and point source data are

extracted for user selected reach segments;

· NPSM/HSPF - a process-based, continuous-simulation watershed model which enables

accurate model calibration against monitored flow and pollutant concentration data. Its

modular structure accommodates a wide range of pollutants and model complexities.

NPSM extracts reach characteristics, reach network, land use distribution, and point source

data sets from the BA-SINS

GIS, and provides a

Windows interface to the

legacy watershed model

Hydrologic Simulation

Program - Fortran (HSPF).

Model results, for

TOXIROUTE and

QUAL2E, can be viewed in

the GIS. NPSM contains a

postprocessor for viewing

time series output of flow,

pollutant concentration,

and loading time series as

well as for model calibra-tion

and validation against

gauge station and water

quality monitoring time

series data. The

postprocessor can view

time series graphically at

daily, monthly, and yearly

time steps and in text

reports for hourly, as well

as daily, monthly, and yearly time steps.

While TOXIROUTE can be set up and run in a matter of minutes to hours and requires an

understanding of simple water quality modeling concepts, QUAL2E requires minutes to

hours for the simplest water quality simulations, but weeks to months to accurately calibrate

complex simulations. Compared to the effort required to manually generate a valid HSPF

input file, the introduction of the NPSM has dramatically reduced the entry-level require-ments

for an initial HSPF watershed model simulation. Accurate NPSM simulations, how-ever,

still require a substantial time investment by modelers experienced with complex

hydrology and pollutant fate and transport modeling concepts.

An alternative, less parameter-intensive watershed model, the Soil and Water Assessment Tool

(SWAT), is set to be released in the BASINS version 2.1 upgrade, due out in the year 2000. The

addition of the SWAT model is intended to appeal to users with experience with agricultural

watershed simulation models.

Limitations of Models

Technical Support is Available

Efforts to further demystify the NPSM/HSPF watershed model are

being addressed with technical support, a BASINS list server, and a

BASINS Technical Notes series (see the BASINS web page at

www.epa.gov/ost/basins for details, or join the BASINS listserv.

To join this listserv, send an e-mail to

listserver@unixmail.rtpnc.epa.gov and in the body of the e-mail

text simply write "subscribe BASINSinfo" (do not include the

quotes). Then, on the same line as subscribe BASINSinfo write

your first name & your last name (e.g., John Doe or Mary Smith)

and then click to send the e-mail to the listserv. The purpose of the

BASINS listserv is to provide a vehicle for technical support to

BASINS users, as well as to facilitate the development of a com-munity

of BASINS users to develop and assist one another. The

listserv lets individual users become aware of the multitude of

locations and situations in which BASINS is being used as well as

provide the critical mass of technical expertise to stimulate coop-erative

growth. More information on use of the listserv is available

at: http://www.epa.gov/ost/basins/listserv.htm.

Models are excellent tools for organizing data, integrating processes, and gaining a broad,

dynamic view of watershed processes. However, all watershed models are simplifications of

complex natural systems. The quality of their output depends on how well the model is adapted

to the specific watershed conditions it is trying to simulate, both in terms of the actual

parameter values, and the mathematical representation of processes occurring in the water-shed.

To the extent practical, the parameters entered into the model should be derived from

the watershed under study, or from a watershed very closely related to the watershed of

interest. "Global" parameters rarely give useful results.

Make sure you understand the assumptions behind the model. Simple models often are very

useful because one can understand the relationships and basic assumptions behind the modeling.

Complex models can be very confusing and sometimes of little use where one wants to actually

understand what is happening in a system. Output from a model should never be accepted

uncritically.

Output from the model needs to be compared with field-collected data as often as is possible to

determine how accurately the model predicts reality. Remember - if high-quality field data don’t

agree with the model, the data are right and the model is wrong, in its calibration or its represen-tation

of that process.