sc/EPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-82-059 August 1982
Project Summary
Maintenance and Testing of
Hydrological Simulation
Program—FORTRAN (HSPF)
R. C. Johanson and D. Kliewer
The Hydrological Simulation Program
FORTRAN (HSPF) is a mathematical
model that simulates hydrology and
water quality in natural and man-made
water systems. This report describes
the work involved in maintaining and
testing HSPF over a period of one year
following its initial development. An
account is given of the chronology of
major events during the maintenance
work. The testing included work with
hypothetical data and checks against
outputs produced by three predecessor
models, the ARM, NPS, and HSP-
QUALITY models. Through this process,
it was determined that the HSPF
model functioned as designed.
This Project Summary was developed
by EPA's Environmental Research
Laboratory. Athens. GA, to-announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The purpose of the work described in
this report was to maintain and test the
Hydrological Simulation Program—
FORTRAN (HSPF) developed under a U.S.
Environmental Protection Agency (EPA)
contract.
HSPF is a mathematical model for
simulating the hydrologic and water
quality processes in and under the land
surfaces of a watershed and in the
associated streams and lakes. The roots
of HSPF go back to the Stanford
Watershed Model, which was one of the
first rainfall-runoff computer models
and was developed under National
Science Foundation sponsorship. Many
newer models have been developed
from it; among the best known is the
Hydrocomp Simulation Program, which
incorporated a sophisticated time series
management system. Hydrocomp also
developed a water quality model that
simulates the accumulation of constit-
uents on a watershed surface, their
washoff into streams and lakes and the
biochemical transformations that occur
in such water bodies.
The "Lands" section of the Stanford
Watershed Model was also used as the
basis for the Agricultural Runoff Man-
agement (ARM) Model, which was also
developed under EPA contract. The
ARM model simulates sediment produc
tion, as well as the behavior of pesticides
and nutrients, on agricultural lands.
The EPA also sponsored the development
of the Non-Point Source (NPS) Model,
which simulates the washoff of consti-
tuents from land surfaces by relations
with washed-off sediment.
Although all the above models origi-
nated from the Stanford Watershed
Model, they have each undergone
development in their own specialized
directions. Each is a powerful tool for
use in its area of specialization, but it is
not easy to use them together in
situations where their combined strength
is required. With the goal of overcoming
this problem, EPA sponsored the devel-
opment of a "comprehensive package
for the simulation of watershed hydro-
logy and water quality," which later
became known as HSPF. The objective
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was to incorporate the capabilities of all
of the above models in a single,
consistently designed set of well-
documented software, written as far as
possible in ANSI FORTRAN (1966
version). This was part of EPA's progra m
to develop engineering tools to help
pollution control officials achieve water
quality goals through watershed man-
agement.
Objective
The objective of this project was to
provide a comprehensive initial test of
HSPF and correct any errors or short-
comings found in the code. A new
release of the program was to be
prepared at the end of the project. In
addition, utility modules that would
enhance the power of the system were
to be added to the system. These
modules included an interface with a
digital plotter, a neatly formatted
summary output table, a statistical
analysis package, and a module to
perform time series calculations.
Approach
The purpose of the testing program
was to check that HSPF correctly
implemented the modeling algorithms
outlined in the User's Manual. These
algorithms are, forthe most part, similar
to those embodied in the predecessor
models, but the manner in which they
are included are very different since
HSPF has a radically different structure.
Thus, most of the testing consisted of
comparing HSPF output against similar
runs of ARM, NPS. and HSP-QUALITY. If
the results were similar, it would be
reasonable to expect that the algorithms
were correctly implemented. Differences
would have to be investigated to
determine whether HSPF was in error.
This was a different approach than us-
ually taken where model output is com-
pared against observed data but it was
reasoned that the predecessor models
had already been checked this way.
Tests were conducted in three phases.
First, very simplistic hypothetical data
sets were constructed and simulation
output was compared against manual
calculations. Then the NPS and HSP-
QUALITY simulations performed for the
Northern Virginia Planning District
Commission (NVPDC) were reproduced
using HSPF. Finally, the agricultural
runoff module of HSP was compared to
ARM simulations of a Michigan State
University test watershed. This site also
allowed testing of the snowmelt algorithms.
Results
The manual calculations were very
useful. They were easy to set up and
permitted many aspects of the model to
be checked quickly.
The NVPDC comparison showed that
it is sometimes difficult to produce
identical results with HSPF because of
its radically different structure. This
produced a cumbersome HSPF input
sequence and resulted in lower execution
efficiency than if the sub-basins had
been segmented in a manner more
appropriate for HSPF. The results
pointed out some subtle differences
between HSPF and predecessor models
but, for the most part, comparable
results could be produced by reasonable
adjustments to accommodate the dif-
ferences in implementation. A significant
difference was found in the phytoplank-
ton simulations due to a different
definition of "water body depth" used in
the light extinction equations.
The Michigan State University simu-
lations produced similar results. Minor
discrepancies in the snowmelt simulations
were attributable to slight differences in
implementation of the algorithms. In
many of these cases, it was believed
that the HSPF code more closely
represents natural processes. Compari-
sons of the water balance routines
resulted in a correction to the manner in
which HSPF handled snowmelt. Good
agreement was found in the sediment
simulations. Pesticide and nutrient
simulations showed a difference that
was attributed to differences in com-
putation sequencing in the two models.
HSPF was modified to more closely
coincide with the ARM model where it
was deemed appropriate. The testing
program did raise questions about the
adequacy of the way the near-surface
region of the soil is simulated in both
ARM and HSPF.
Conclusions and
Recommendations
The care that went into the design,
coding, and documentation of HSPF
was deemed to be worthwhile. It is
relatively easy for a well-trained person
to use the system, and bugs have been
easy to locate and fix. New modules
were easy to add. It was also found,
however, that the model was more
costly to operate than its predecessor
models, largely because of the flexibility
built into the design.
The testing program found that the
algorithms described in the User's
Manual were correctly implemented.
When checked against predecessoi
models, HSPF produced similar output
with these notable exceptions:
1. Simulation of nutrient behavior in
pervious land segments (PERLND
module) did not agree with the
results produced by the ARM
model. This is attributable to:
a. Intermittent calculation of reac-
tion fluxes in ARM Model runs
b. Problems inherent in having a
thin surface layer, with moisture
storage dependent only on
overland flow depth (a feature
of both ARM and HSPF)
2. Simulation of phytoplankton in
streams and reservoirs. Because
HSPF and HSP-QUALITY use dif-
ferent definitions of "water body
depth" in the light extinction equ-
ation, they produce radically dif-
ferent light-limiting phytoplankton
growth rates.
Recommendations for future develop-
ment of the model include:
1. Elimination of half-word integers,
to make it easier to install on a
variety of machines.
2. Development of a special version
for large-memory installations,
designed to minimize disc input/-
output and associated costs.
3. Improved trapping of user's errors,
by the Run Interpreter.
4. Study of the feasibility of having a
dynamically varying internal time
step for certain processes - fine
when there is rapid variation,
coarse when there is not. This
would save computer time.
Two problems in the agricultural
chemical simulation system of the
PERLND module need to be solved:
1. The MSTLAY section needs to be
reformulated, so that the system
will give results that are not a
direct function of the time step,
thus freeing this part of HSPF from
the 5 and 15 minute time steps to
which it (and ARM) is limited.
2. The problems posed by the use of a
thin surface layer, with moisture
storage totally dependent on over-
land flow depth, must be over-
come.
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R. C. Johanson and D. Kliewer are with Hydrocomp, Inc., Mountain View, CA
94040.
T. O. Barnwell is the EPA Project Officer (see below).
The complete report, entitled "Maintenance and Testing of Hydrological
Simulation Program—FORTRAN (HSPF)." (Order No. PB 82-237 033; Cost:
$10.50 subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U. S. Environmental Protection Agency
Athens, GA 30613
* US GOVERNMENT PRINTING OFFICE. 19(2-559-017/0790
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