United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA-600/S3-82-046 Sept. 1982
Project Summary
Executive Summary for the
Hydrological Simulation
Program FORTRAN (HSPF)
G. P. Grimsrud, D. D. Franz, R. C. Johanson, and N. H. Crawford
This executive summary introduces
water resource managers, engineers,
and programmers to the Hydrological
Simulation Program —FORTRAN
(HSPF) and provides them with
information that can help them in
deciding whether HSPF would be
useful and practical for them to use.
HSPF uses digital computers to
simulate hydrology and water quality
in natural and man-made systems.
Although data requirements are
extensive and running costs are
significant, HSPF is thought to be the
most accurate and appropriate
management tool presently available
for the continuous simulation of
hydrology and water quality in
watersheds.
The executive summary begins with
a general overview of the model, how
it compares with other models, and
how it can be applied. For those more
interested in technical details, the
executive summary presents detailed
information on the model's input
requirements, capabilities, innovative
features and design criteria. This latter
information should be of particular
interest to those acquainted with
simulation models, who can apprec-
iate the advanced programming
techniques used to develop HSPF.
This Project Summary was devel-
oped by EPA's Environmental Re-
search 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 Hydrological Simulation
Program-FORTRAN (HSPF) is a
mathematical model designed for use
on digital computers to simulate
hydrologic and water quality processes
in natural and man-made water
systems. It is an analytical tool that has
application in the planning, design, and
operation of water resources systems.
The model enables the use of probabil-
istic analysis in the fields of hydrology
and water quality management. To
simulate the processes that occur in a
watershed, HSPF uses such
information as the time history of
rainfall, temperature, and solar
intensity and the parameters related to
land use patterns, soil characteristics,
and agricultural practices. The initial
result of an HSPF simulation is a time
history of the quantity and quality of
water transported through various soil
zones down to the groundwater
aquifers. Runoff flow rate, sediment
loads, nutrients, pesticides, toxic
chemicals and other quality constit-
uent concentrations can be predicted.
The model then takes these results and
information about the receiving water
channels in the watershed and
simulates the processes that occur in
these channels This part of the
simulation produces a time history of
water quantity and quality at any point
in the watershed.
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HSPF is an extension and
improvement of three previously
developed models: (1) The Agricultural
Runoff Management (ARM) Model, a
non-proprietary program written in
FORTRAN; (2) The Non-Point Source
Runoff (NFS) Model; also a non-
proprietary program written in
FORTRAN; and (3) The Hydrologic
Simulation Program (HSP, including
HSP Quality), a Hydrocomp proprietary
program written in PL1. The EPA
recognized several years ago that the
continuous simulation approach
contained in these models would be
valuable in solving many complex water
resource problems. Thus, a fairly large
investment was devoted to developing a
highly flexible non-proprietary
FORTRAN program that contains the
capabilities of these three models, plus
many extensions. The result of this
investment is HSPF. It is geared to the
serious user; one who understands
continuous simulation modeling, and
can make use of the flexibility that it
provides.
HSPF Philosophy
Management of water resources
involves the commitment of other
resources to protect and develop our
water. This commitment may range
from construction of facilities to the
non-use of undeveloped land. Such
commitments can only be justified
when one knows the benefits or
effectiveness of the protection to be
achieved. A fundamental question in
benefit evaluation is whether an
undesirable water quality event occurs
once a month, once a year, or once a
decade.
Information on hydrologic or water
quality events cannot be used
effectively for water resources planning
unless the probability of occurrence of
the events is known. The occurrence of
a water quality event is strongly
conditioned by antecedent events. As
an example, the current population of
phytoplankton in a lake is a function of
the phytoplankton, zooplankton, and
nutrient population yesterday. Likewise,
the population of phytoplankton five
days ago has a small but finite effect
upon the current population of
phytoplankton.
Although it is reasonable to assume
the physical, chemical, and biological
laws that govern interactions in a body
of water are invariant, specific reaction
rates are determined by what is in the
water body a nd by the stresses i mposed
upon the water body by inflows. These
inflows can be man-caused flows such
as wastewater discharges and
irrigation return flow, or natural flows
such as runoff from rainfall and snow-
melt. Natural flows are governed by the
climate, soil type, vegetal cover, and
other physical features of the
watershed. Since vegetal cover (forests,
grassland) and human activity (land
development, channelization, and
reservoirs) can, and often do, change
rapidly over short periods of time, they
too must be considered variables in
water resources planning. Thus, the
only constants in water resources
planning are the physical, chemical, and
biological laws of nature, and the soil
types and other fixed physical
characteristics of the watershed.
Planners need a method to determine
the frequency characteristics of the
hydrology and water quality of a
watershed under alternative plans for
future development. This need led to
the development of continuous
dynamic simulation, the simulation of
extended time periods with various
flows and meteorologic conditions.
From five to more than one hundred
years of data may be required to
adequately determine the statistics for
streamflow and water quality
constituents. Simulation of "critical
period" cannot provide the information
necessary to determine the probability
of water quality events. In fact, the
critical period itself might change from
one wastewater management scheme
to the next. Hence, HSPF simulation
deals with the total process, hydrologic
and bio-chemical, as a continuous
process in time.
Why are continuous models like
HSPF an improvement over dynamic
event models? The long-term
probabilistic character of hydrology and
water quality, as discussed earlier, is
only available from continuous models.
The magnitudes and recurrence
intervals of certain harmful events
cannot be predicted using event
models. One common mistake is that
rainfall events of given recurrence
intervals are assumed to result in
streamflow and flood conditions of the
same recurrence interval (i.e., the 25-
year rainfall event leads to a 25 year
flood). This assumption is erroneous
due to factors such as snowmelt and
soil water storage, that modify the
statistics of rainfall as the water flows
on the land and river channels. The use
of this invalid assumption in using event
models commonly leads to inappropriate
planning conclusions.
G. P. Grimsrud, D. D. Franz, R. C. Johanson, and N. H. Crawford are with
Hydrocomp, Inc., Mountain View, CA 94040.
T. O. Barnwell is the EPA Project Officer (see below).
The complete report, entitled "Executive Summary for the Hydrological Simula-
tion Program FORTRAN (HSPF)," (Order No. PB 82-231 846; 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
College Station Road
Athens, GA 30613
> US.OOVERNMENTPRINT1NOOfFICe:19tt-559-Ol7/08Z5
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