Geospatial data is increasingly abundant in the earth science community and many hydrologic modeling parameters can be estimated from such data. An Expression Calculator is included in HEC-HMS to facilitate parameter estimation from GIS data. The Expression Calculator can be launched from select global basin editors. Global editors that include the Expression Calculator option are listed below:
- Loss: Deficit and Constant
- Loss: Green and Ampt
- Transform: Mod Clark
- Transform: Clark
- Transform: S-Graph
- Baseflow: Linear Reservoir
The Expression Calculator is launched from the Calculate... button a global editor.
The Calculate option in the global editor requires the basin model to include spatial features (georeferenced subbasin elements).
In the Expression Calculator, grids and characteristics and can be used as variables. Grids are available from the Grids tab. When a grid is included in the expression, a zonal average value of the feature on the grid is used in the calculation.
In the image below, a percolation rate grid has been included the expression.
Characteristics are available on the Stats tab. When a characteristic is used in the expression, the characteristic value for a given feature (subbasin element) is used in the calculation. In the image below, the time of concentration has been included in the expression.
The number of significant figures reported by the expression calculator can be controlled in Tools | Program Settings on the Compute tab. The program setting is called Expression calculator precision. The valid range for the setting is 1 to 5 significant figures. The program uses a default value of 3 unless modified by the user.
