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Welcome to CivilGEO Knowledge Base
HEC-HMS Model Components
Model Components
Model components are used to simulate the hydrologic response in a watershed. The primary GeoHECHMS model components are basin models, meteorologic models, and control specifications. A simulation calculates the precipitation-runoff response in the basin model given input from the meteorologic model. The control specifications define the time and time step of the simulation run.
In basin and meteorologic models, the input data components such as time-series data, paired data, and gridded data are often required as parameters or boundary conditions.
Basin Model Component
Basin models are one of the major components of a project. Their main purpose is to convert atmospheric conditions into streamflow at a specific location in the watershed. The hydrologic elements of a basin model such as subbasin, reach, junction, etc., are used to break the watershed into manageable entities. These elements are connected in a dendritic network to form a representation of the stream system. A background map can also be used to help place the elements in a spatial context.
Contributing Elements
Hydrologic contributing elements are the basic building blocks of a basin model. An element represents a physical process such as a watershed catchment, stream reach, or confluence. Each element contributes to the total response of the watershed to atmospheric forcing.
The different contributing elements available in GeoHECHMS are as follows:
- Subbasin: The subbasin element is used to represent the physical watershed. Given precipitation, outflow from the subbasin element is calculated by subtracting precipitation losses, transforming excess precipitation to stream flow at the subbasin outlet, and adding baseflow.
- Reach: The reach element is used to convey stream flow downstream in the basin model. Inflow into the reach element can come from one or many upstream hydrologic elements. Outflow from the reach is calculated by accounting for translation and attenuation of the inflow hydrograph.
- Junction: The junction element is used to combine stream flow from hydrologic elements located upstream of the junction element. Inflow into the junction element can come from one or many upstream elements. Outflow is simply calculated by summing all the inflows and assuming no storage at the junction.
- Source: The source element is used to introduce flow into the basin model. The source element has no inflow and its outflow is defined by the user.
- Sink: The sink element is used to represent the outlet of the physical watershed. There is no outflow from the sink element. Inflow into the sink element can come from one or many upstream hydrologic elements.
- Storage Areas: The storage area element is used to model the detention and attenuation of a hydrograph caused by a reservoir or detention pond. Inflow into the storage element can come from one or many upstream hydrologic elements. Outflow from the storage element can be calculated in three ways. In a first way, the user can enter the relationship among the storage outflow, elevation storage outflow, or elevation area outflow. In a second way, the user can enter the elevation storage or elevation area relationship and define one or more outlet structures. Finally, the user can specify a time-series of outflow.
- Diversion: The diversion element is used for modeling stream flow leaving the main channel. Inflow into the diversion element can come from one or many upstream hydrologic elements. Outflow from the diversion element consists of diverted flow and non-diverted flow. Diverted flow is calculated using input from the user. Both diverted and non-diverted flows can be connected to hydrologic elements downstream of the diversion element.
In GeoHECHMS, the Contributing Subbasins and Contributing Reaches commands are present, where the user can right-click on any node or reach and ask the software to show the contributing drainage area or reaches.
Network Connectivity
The flow network is the skeleton that connects hydrologic elements into a representation of the stream system in the watershed. Each link in the network is a one-way connector that takes outflow from an element and connects it as an inflow to a downstream element. The connection information of the flow network along with the drainage area at each element is used to sort the elements in hydrologic order.
Reach elements are often connected between two junctions or possibly between other element types. Reach elements must be connected to the upstream and downstream elements.
In GeoHECHMS, the Network Connectivity command allows the user to check the network connectivity in the model, which ensures that there are not any disconnected (i.e., disjointed) network segments present in the model. To use this command, select any node or reach from the Map View and then right-click and select the Network Connectivity command from the displayed context menu. Now all the subnetworks will get highlighted, which provides better visualization and helps to identify any discontinuity in the network.
Routing Methods
In developing hydrologic models, there are two essential requirements – a runoff-generation component and a routing component. Routing is an essential component of any hydrology modeling project for the derivation of time series of flows into the oceans and studies of climate/land use change on water resources.
The different routing methods available in GeoHECHMS are as follows:
- Kinematic Wave
- Lag Time
- Lag Time & Attenuation
- Modified Puls
- Muskingum
- Muskingum Cunge
- Normal Depth
- Straddle Stagger
Refer to this article in our knowledge base to learn more about HEC-HMS routing methods.
Hydrology Methods
The hydrology methods work together to estimate the total runoff from a subbasin during the precipitation event. Total runoff from a subbasin is computed by subtracting losses, transforming excess precipitation, and adding baseflow.
The different hydrology methods available in GeoHECHMS are as follows:
- Infiltration (loss)
- Runoff (transform)
- Baseflow
- Surface storage
- Canopy
Refer to this article in our knowledge base to learn more about HEC-HMS hydrology methods.
Meteorologic Model Component
The meteorologic model is responsible for preparing the boundary conditions that act on the watershed during a simulation. The meteorologic model is prepared to use with one or more basin models. If the basin model contains subbasin elements, then the meteorologic model must specify how precipitation will be generated for each subbasin. Evapotranspiration should be included in the meteorologic model when the basin model is configured for a continuous simulation.
The different precipitation and evapotranspiration types available in GeoHECHMS are as follows:
- Precipitation Types
- Frequency Storm
- HMR52 Storm
- Inverse Distance Weighted
- Rain Gage
- Rainfall Distribution
- SCS Storm
- Specified Hyetograph
- Standard Project Storm
- Evapotranspiration Types
- Annual Evapotranspiration
- Monthly Average
- User-Defined
Refer to this article in our knowledge base to learn more about meteorology methods.
Control Specifications Component
Control specifications are one of the main components of the hydrology project. Their major purpose is to control when simulations start and stop, and what time interval is to be used in the HEC-HMS hydrology simulation. The control specifications also include the time interval that will be used to perform computations during a simulation. The time interval defined in the control specification will also be used in displaying time-series results from the simulation.
Refer to this article in our knowledge base to learn more about control specifications.
