About HSPF
The Hydrological Simulation Program - FORTRAN (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:
- The EPA Agricultural Runoff Management Model (ARM),
- The EPA Nonpoint Source Runoff Model (NPS), and
- 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
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.
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.
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 phosphorus 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
