AQUA TERRA Consultants

HSPF Support

The Hydrological Simulation Program FORTRAN (HSPF) is a cornerstone for many of the services that AQUA TERRA provides. HSPF is commonly recognized as the most complete and defensible process-based watershed model for quantifying runoff and addressing water quality impairments associated with combined point and nonpoint sources. Since its initial development nearly twenty years ago, the HSPF model has been applied throughout North America and numerous countries and climatic regimes around the world; it enjoys the joint sponsorship of both the U.S. Environmental Protection Agency and the U.S. Geological Survey, and continues to undergo refinement and enhancement of its component simulation capabilities along with user support and code maintenance activities. In addition to process-oriented enhancements, recent years have seen significant development efforts directed towards improved user interaction. Among these are:

» HSPEXP, an expert system for calibrating the hydrologic component of HSPF
» GenScn, a software product that provides the ability to change an HSPF input sequence interactively, run the model, and analyze results graphically
» WinHSPF, an independent interactive interface to HSPF that is also fully integrated with the BASINS 4.1 modeling system
» WDMUtil, a tool used to manage and create the watershed data management files (WDMs) that contain the meteorological data and other time series data used by HSPF
» HSPFParm, a tool for organizing a database of HSPF parameter values
» BASINS integration of HSPF which enables seamless use of the model in conjunction with national watershed data and geographic information systems (GIS) capabilities.
» HSPF Windows Help software, a hypertext version of the user's manual
» Continuing enhancement of the HSPF model

HSPF workshops of various formats and duration have been conducted by AQUA TERRA staff over the past 15 years, sometimes with assistance and computer support provided by client personnel.

To encourage the informed use of HSPF, we have collected, and periodically update, an HSPF bibliography that includes useful references related to both model development and applications.

AQUA TERRA Consultants is truly uniquely qualified to provide the full scope of support services related to the HSPF modeling system. In all, we have provided nearly $9 million of HSPF-related services to our clients. Highlights of our HSPF-related experience:

  • AQUA TERRA staff were principal scientists in the original development of HSPF.
  • As the EPA maintenance contractor, since the initial release of HSPF in 1980, all code changes and new releases have been performed by AQUA TERRA, in conjunction with the EPA Athens Laboratory and the USGS Office of Surface Water.
  • Since 1981, AQUA TERRA staff have been the principal authors of all versions of the HSPF User's Manual and the HSPF Application Guide.

Since 1981, AQUA TERRA personnel have supported or performed HSPF applications in 23 states, Saudi Arabia and Australia. We have used HSPF to model over 100 different watersheds, ranging in size from small test watersheds to the entire Chesapeake Bay Basin.

About HSPF


The Hydrological Simulation Program-FORTRAN, known as HSPF, is a mathematical model developed under EPA sponsorship for use on digital computers to simulate hydrologic and water quality processes in natural and man-made water systems. It is an analytical tool which has application in the planning, design, and operation of water resources systems. The model enables the use of probabilistic analysis in the fields of hydrology and water quality management. HSPF uses such information as the time history of rainfall, temperature, evaporation, and parameters related to land use patterns, soil characteristics, and agricultural practices to simulate the processes that occur in a watershed. The initial result of an HSPF simulation is a time history of the quantity and quality of water transported over the land surface and through various soil zones down to the groundwater aquifers. Runoff flow rate, sediment loads, nutrients, pesticides, toxic chemicals, and other quality constituent concentrations can be predicted. The model uses these results and stream channel information to simulate instream processes. From this HSPF produces a time history of water quantity and quality at any point in the watershed.


HSPF is an extension and improvement of three previously developed models: 1) The EPA Agricultural Runoff Management Model (ARM), 2) The EPA Nonpoint Source Runoff Model (NPS), and 3) The Hydrologic Simulation Program (HSP, including HSP Quality), a privately-developed proprietary program. EPA recognized 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 which contains the capabilities of these three models, plus many extensions.


HSPF is a valuable tool to water resource planners. Because it is more comprehensive than most systems, it permits effective planning. Benefits to the user include:

  • Flexibility in solving a wide range of water quantity and quality problems using a single model
  • Convenient data management features that save time and money
  • Modular program structure which facilitates program changes and additions for special applications

Application and Use

HSPF is currently the most comprehensive and flexible model of watershed hydrology and water quality available. It is the only available model that can simulate the continuous, dynamic event, or steady-state behavior of both hydrologic/hydraulic and water quality processes in a watershed. The model is also unusual in its ability to represent the hydrologic regimes of a wide variety of streams and rivers with reasonable accuracy. Thus, the potential applications and uses of the model are comparatively large including:

  • Flood control planning and operations
  • Hydropower studies
  • River basin and watershed planning
  • Storm drainage analyses
  • Water quality planning and management
  • Point and nonpoint source pollution analyses
  • Soil erosion and sediment transport studies
  • Evaluation of urban and agricultural best management practices
  • Fate, transport, exposure assessment, and control of pesticides, nutrients, and toxic substances
  • Time-series data storage, analysis, and display

HSPF is designed so that it can be applied to most watersheds using existing meteorologic and hydrologic data; soils and topographic information; and land use, drainage, and system (physical and man-made) characteristics. The inputs required by HSPF are not different than those needed by most other simpler models. The primary difference in data needs is that long, rather than short time-series records are preferred. Typical long time-series records include precipitation, waste discharges, and calibration data such as streamflow and constituent concentrations.

Model Capabilities

HSPF contains three application modules and five utility modules. The three application modules simulate the hydrologic/hydraulic and water quality components of the watershed. The utility modules are used to manipulate and analyze time-series data. A brief description of each of the modules follows.

Application Modules

The three application modules are:

  • PERLND - Simulates runoff and water quality constituents from pervious land areas in the watershed.
  • IMPLND - Simulates impervious land area runoff and water quality.
  • RCHRES - Simulates the movement of runoff water and its associated water quality constituents in stream channels and mixed reservoirs.

As PERLND simulates the water quality and quantity processes that occur on pervious land areas, it is the most frequently used part of HSPF. To simulate these processes, PERLND models the movement of water along three paths: overland flow, interflow, and groundwater flow. Each of these three paths experiences differences in time delay and differences in interactions between water and its various dissolved constituents. A variety of storage zones are used to represent the processes that occur on the land surface and in the soil horizons. Snow accumulation and melt are also included in the PERLND module so that the complete range of physical processes affecting the generation of water and associated water quality constituents can be represented. Some of the many capabilities available in the PERLND module include the simulation of:

  • Water budget
  • Snow accumulation and melt
  • Sediment production and removal
  • Nitrogen and phosphorous behavior
  • Pesticide behavior
  • Movement of a tracer chemical

IMPLND is used in urban areas where little or no infiltration occurs. However, some land processes do occur, and water, solids, and various pollutants are removed from the land surface by moving laterally downslope to a pervious area, stream channel, or reservoir. IMPLND includes all of the pollutant washoff capabilities of the commonly used urban runoff models, such as the STORM, SWMM, and NPS models.

RCHRES is used to route runoff and water quality constituents simulated by PERLND and IMPLND through stream channel networks and reservoirs. A number of processes can be modeled, including:

  • Hydraulic behavior
  • Water temperature
  • Inorganic sediment deposition, scour, and transport by particle size
  • Chemical partitioning, hydrolysis, volatilization, oxidation, biodegradation, and radionuclide decay
  • DO and BOD balances
  • Inorganic nitrogen and phosphorous balances
  • Plankton populations
  • pH, carbon dioxide, total inorganic carbon, and alkalinity

Utility Modules

The five utility modules are used to access, manipulate, and analyze time series information stored by the user in HSPF's TSS (Time Series Store) and WDM (Watershed Data Management) files. These time series, such as hourly precipitation, daily evaporation, daily streamflow, are used by the application modules and are often a valuable resource in the analysis of a watershed's characteristics. The five utility modules are:

  • COPY - copy data in the TSS to another file
  • PLTGEN - generates a plot file for data display on a plotter
  • DISPLY - creates data display tables
  • DURANL - performs frequency, duration, and excursion analyses; computes statistics; and performs toxicity/lethality analysis
  • GENER - permits the transformation of a time series to produce a second, different time series

Program Structure and Design

HSPF consists of a set of modules arranged in an organized structure, which permit the continuous simulation of a comprehensive range of hydrologic and water quality processes. Experience with sophisticated models indicates that much of the human effort in model application is associated with data management. This fact, often overlooked by model builders, means that a successful comprehensive model must include a sound data management system. Otherwise, the user may spend so much time on data manipulation that actual progress on the simulation work itself is drastically retarded. For this reason, the HSPF program is planned around a time-series management system operating on direct access principles. The simulation modules read input data from the TSS and WDM files, and are capable of writing output to them. Because these transfers require very few instructions from the user, problems with data handling are minimized.

HSPF's design incorporates a hierarchy of program subroutines, each of which performs a major task during the program's execution. The subroutines are grouped into different levels of operations in a hierarchical structure.

Typical of these levels of operations is the Run Interpreter, which is a group of subprograms that reads and interprets the User Control Input. It provides instructions to HSPF's modules specifying the sequence of operations to be performed. It stores the initial conditions and the parameters for each operation in the appropriate file on disk, and creates an instruction file that will ensure that time-series data are correctly passed between operations, where necessary.

Other operations include TSS management modules used to create, modify, or remove data sets from the Time Series Store; operations supervision modules which use information supplied by the Run Interpreter and turn on appropriate application and/or utility modules as needed; and the actual application and utility modules which perform the hydrologic and water quality related simulation calculations.

The importance of this program structure lies in its modular design. This allows for the addition and/or replacement of individual modules and allows HSPF to be easily adapted for special applications designed by the user.