GSSHA
Encyclopedia
Gridded Surface/Subsurface Hydrologic Analysis (GSSHA) Model
GSSHA is the U.S. Army Corps of Engineers, Engineer Research and Development Center two-dimensional, physically based, watershed model. GSSHA is a mathematical model
that simulates surface
and ground
water hydrology
, erosion
and sediment transport
. The GSSHA model is used for hydrologic engineering and research. GSSHA input is best prepared by the Watershed Modeling System interface, which effectively links the model with geographic information systems (GIS).
GSSHA uses a square-grid, constant grid-size representation of watershed topography and characteristics, similar to a digital elevation model
representation. Relevant model parameters are assigned to the model grids using index maps. Index maps are often derived from soils, landuse/land cover, vegetation, or other physiographic maps.
.
GSSHA uses a vector channel representation. This allows feature allows channels to flow in any direction and meander, independent from the grid resolution; this feature accurately preserves channel length and slope.
The GSSHA model was developed from the outset to be capable of 'long term' simulations consisting of multiple events. As such, required inputs include meteorological variables, and surface energy-balance parameters. Seasonality in evapotranspiration
parameters is included in the model.
Overland and channel flow hydraulics are based on explicit, finite-volume, diffusive wave schemes. The overland and channel flow routines use dynamic time stepping to improve model stability and decrease simulation times.
Surface and subsurface stores are linked though the vadose zone
using a number of different optional numerical methods. A two-dimensional finite-difference groundwater solver is coupled to streams through a stream bed conductance layer.
There are a number of optional methods to calculate erosion and sediment transport. The model can be used to simulate transport of sediments with specific gravity different from sand.
Current additions to the GSSHA model include source/sink/transport of nutrients and contaminants. For further information on the GSSHA model, see the GSSHA Wiki:GSSHA WIKI
GSSHA is the U.S. Army Corps of Engineers, Engineer Research and Development Center two-dimensional, physically based, watershed model. GSSHA is a mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...
that simulates surface
Surface water
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
and ground
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
water hydrology
Hydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
, erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
and sediment transport
Sediment transport
Sediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...
. The GSSHA model is used for hydrologic engineering and research. GSSHA input is best prepared by the Watershed Modeling System interface, which effectively links the model with geographic information systems (GIS).
GSSHA uses a square-grid, constant grid-size representation of watershed topography and characteristics, similar to a digital elevation model
Digital elevation model
A digital elevation model is a digital model or 3-D representation of a terrain's surface — commonly for a planet , moon, or asteroid — created from terrain elevation data....
representation. Relevant model parameters are assigned to the model grids using index maps. Index maps are often derived from soils, landuse/land cover, vegetation, or other physiographic maps.
History
The GSSHA model was derived from the CASC2D hydrologic model. GSSHA represents a significant improvement on CASC2D in terms of capabilities, options, and numerical procedures. GSSHA includes dynamic time-stepping depending on stability criteria, different time steps for different numerical processes, and the ability to run on multi-processor computers. Processes included in GSSHA include surface and ground water flow, channel hydraulics, evapotranspiration, erosion and sedimentation, storm drainage networks, tile drains, a variety of hydraulic structures, contaminant/nutrient fate and transport.Formulation
GSSHA uses a regular square grid computational discretization of the watershed. Elevation data are taken from a digital elevation modelDigital elevation model
A digital elevation model is a digital model or 3-D representation of a terrain's surface — commonly for a planet , moon, or asteroid — created from terrain elevation data....
.
GSSHA uses a vector channel representation. This allows feature allows channels to flow in any direction and meander, independent from the grid resolution; this feature accurately preserves channel length and slope.
The GSSHA model was developed from the outset to be capable of 'long term' simulations consisting of multiple events. As such, required inputs include meteorological variables, and surface energy-balance parameters. Seasonality in evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
parameters is included in the model.
Overland and channel flow hydraulics are based on explicit, finite-volume, diffusive wave schemes. The overland and channel flow routines use dynamic time stepping to improve model stability and decrease simulation times.
Surface and subsurface stores are linked though the vadose zone
Vadose zone
The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater is at atmospheric pressure . Hence the vadose zone extends from the top of the ground surface to the water table...
using a number of different optional numerical methods. A two-dimensional finite-difference groundwater solver is coupled to streams through a stream bed conductance layer.
There are a number of optional methods to calculate erosion and sediment transport. The model can be used to simulate transport of sediments with specific gravity different from sand.
Specific Process Simulation Options
- Rainfall input
- rain gages, inverse-distance squared or nearest neighbor interpolation
- radar-rainfall
- Evapo-transpiration using the Penman-Monteith equation
- InfiltrationInfiltration (hydrology)Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...
- Green & Ampt
- Multi-layer Green & Ampt
- Green & Ampt with redistribution
- Richards equationRichards equationThe Richards equation represents the movement of water in unsaturated soils, and was formulated by Lorenzo A. Richards in 1931 . It is a non-linear partial differential equation, which is often difficult to approximate since it does not have a closed-form analytical solution.Darcy's law was...
- Overland flow
- Explicit finite-volume diffusive wave
- Explicit finite-volume alternating-direction predictor-corrector diffusive wave
- Overland flow dikes, such as roadway embankments
- Channel flow using explicit finite-volume diffusive-wave
- Hydraulic structures
- Weirs
- Culverts
- Detention Basins
- Lakes
- Wetlands
- Rating Curves
- Rule Curves
- Scheduled releases
- Overland erosionErosionErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
and sedimentSedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
transport- Detachment limits
- Raindrop impact
- Deposition
- Arbitrary sediment size classes
- Arbitrary sediment specific gravity
- Sediment transport using three different optional equations
- Kilinc and Richardson
- Englund Hanson
- Stream powerStream powerStream power is the rate of energy dissipation against the bed and banks of a river or stream per unit downstream length. It is given by the equation:\Omega=\rho g Q S...
- Channel Sediment Transport
- Sand routing using stream power
- Fines routing using advection-diffusion
- Two-dimensional groundwaterGroundwaterGroundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
- Two-dimensional finite-difference scheme
- Wells
- Constant head and constant flux boundary conditions
- Stream/aquifer interaction
Current additions to the GSSHA model include source/sink/transport of nutrients and contaminants. For further information on the GSSHA model, see the GSSHA Wiki:GSSHA WIKI
Computational Specifics
GSSHA is programmed in C++, and runs on Windows or Linux computers. The model is command line driven and can be used in a batch mode. Parallel computing is enabled at present using the MPI or the OpenMP approach. Work is underway to port the code to run on massively parallel distributed memory architecture machines.Applications to Date
- Flash floodFlash floodA flash flood is a rapid flooding of geomorphic low-lying areas—washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over ice sheets or snowfields...
modeling. - Soil moisture predictions.
- Sediment loading to receiving water bodies.
- Tidal and hurricane storm surgeStorm surgeA storm surge is an offshore rise of water associated with a low pressure weather system, typically tropical cyclones and strong extratropical cyclones. Storm surges are caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea...
coastal flood forecastingFlood forecastingFlood forecasting is the use of real-time precipitation and streamflow data in rainfall-runoff and streamflow routing models to forecast flow rates and water levels for periods ranging from a few hours to days ahead, depending on the size of the watershed or river basin. Flood forecasting can also...
. - Engineering design.
- Hydrology education.
- Hydrologic research.