Hydrology methods are used to estimate runoff volumes, peak flow rates, and the manner by which rainfall interacts with land surfaces. These methods help engineers design effective drainage and flood control systems, and stormwater infrastructure.
GeoSTORM includes several methods for modeling physical runoff processes. The user can select the most suitable hydrology method based on data availability (e.g., Rainfall data, soil properties, land use/land cover data, etc.) and project requirements. Selecting the right hydrology method ensures accurate modeling, reduces flood risks, and supports sustainable stormwater management.
The following hydrology methods are supported:
- DeKalb Rational
- EPA SWMM
- Modified Rational
- Rational Method
- SCS TR-20/TR-55
Follow the steps below to select a hydrology method in GeoSTORM:
- From the Input ribbon menu, click the Scenario Manager dropdown menu, and select the Scenario Manager command.
- The Scenario Manager dialog box will be displayed, as shown below.
- From the Hydrology analysis engine dropdown combo box, select the hydrology method to use.
Refer to this article in our knowledge base to learn more about the Scenario Manager command.
The following sections describe each hydrology method available in GeoSTORM.
DeKalb Rational Method
The DeKalb Rational method was developed by DeKalb County Public Works Department located in Decatur, Georgia. This method computes runoff ordinates by scaling the peak discharge by a scale factor. One of the deficiencies of the Rational method is that it produces only a peak rate of runoff and not a complete flood hydrograph. The DeKalb County Dimensionless Hydrograph method was developed to generate a flood hydrograph based on the Rational Formula and the unit hydrograph theory. This hydrology method can be used with either the hydrodynamic or kinematic wave hydraulic routing methods for computing flows through a stormwater network.
The application of this method includes designing detention ponds and outlet drainage structures. This method assumes that the land uses in the subbasin are homogeneously distributed. If a large portion of a residential subbasin contains commercial, woodland, or other non-residential land use, the subbasin should be further subdivided into separate smaller homogeneous subbasins when using the hydrology method. This hydrology method can be used for calculating runoff hydrographs for subbasins of less than 10 acres.
Limitations
- Limited to small subbasin areas (less than 10 acres).
- Assumes uniform land use within the subbasin.
- Not suitable for large or complex watersheds.
EPA SWMM Method
The EPA SWMM hydrology method is used to simulate how rainfall turns into surface and subsurface runoff for both single storm and long-term (continuous) events. It is primarily applied in urban and suburban areas and operates on a collection of subcatchments within a subbasin. These subcatchments receive precipitation and generate runoff after simulation of evaporation and infiltration losses from the subcatchments.
In EPA SWMM, each subcatchment is conceptualized as a nonlinear reservoir to simulate the rainfall-runoff process. Unlike traditional hydrology methods that rely on simplified empirical equations, EPA SWMM dynamically models runoff by continuously solving a water balance equation.
In the nonlinear reservoir model, inflow components include precipitation and any designated upstream subcatchments. Outflow components include infiltration, evaporation, and surface runoff. The reservoir capacity is defined by maximum depression storage, which represents the surface storage due to ponding, interception, and surface wetting. Surface runoff occurs when the water level in the reservoir exceeds the maximum depression storage, with the outflow calculated using Manning’s equation. The depth of water over the subcatchment is continuously updated with time by solving numerically a water balance equation.
The image below illustrates how rainfall runoff is modeled using a nonlinear reservoir model.
The EPA SWMM method calculates the rainfall runoff value using the following equation:
Where:
𝑄 = Flow rate (cfs or m³/s)
𝑛 = Manning’s roughness coefficient
𝐴 = Flow area (ft² or m²)
𝑅 = Hydraulic radius (ft or m)
𝑆 = Slope of the subcatchment
Since the flow area and hydraulic radius depend on the evolving depth of water in the subcatchment, EPA SWMM solves this equation iteratively over time to update runoff conditions dynamically.
Limitations
- Requires accurate parameter estimation (e.g., roughness, depression storage).
- Not optimized for real-time monitoring or operational control.
Rational Method
The Rational method is a simplified approach used to estimate peak flow rates for stormwater design when hydrograph information is not required. It is widely used in urban drainage design, particularly for sizing storm sewers. For example, this method is commonly used when designing catch basin inlets because it provides a peak flow value that helps determine captured flow, bypass flow, and gutter spread.
The Rational method estimates the peak flow rate using the following equation:
Q = CIA
Where:
Q = Peak flow rate (acre-inch/hr)
C = Dimensionless runoff coefficient used to adjust for the rainfall abstraction, where C has been adjusted with the storm frequency factor
I = Rainfall intensity for a duration equal to the time of concentration of the individual basin (inch/hr)
A = Basin area (acres or hectares)
Limitations
- Limited to small drainage areas (not recommended for areas larger than 100 – 200 acres).
- Not recommended for storage volume design, like detention pond sizing.
- Less accurate for complex basins with varied land uses or topography changes.
Modified Rational Method
The Modified Rational method extends the Rational method by generating a runoff hydrograph, which provides flow variation over time instead of just a peak flow. It is commonly used for detention pond sizing.
This method first applies the following equation to estimate the peak flow rate:
Q = CIA
Then, the hydrograph is computed based on the following assumptions:
- Time of Concentration (Tc) = Time to Peak (Tp) = Time to Recede (Tr)
- The length of the Critical Duration Storm (De) is from 0 minutes until the time of selected duration.
- The flow rate is 0 at time 0 minutes, which increases linearly with time until the peak flow rate is reached at time Tp.
- The peak flow rate is maintained from time Tp until the duration of the Critical Duration Storm (De). The flow rate then decreases to 0 at time De plus Tr.
- The peak flow rate is based on the average rainfall intensity (I) for the given storm duration.
Limitations
- Same limitations as the Rational method, so it is not good for large or complex areas.
- Assumes uniform rainfall intensity, which may not match real storm patterns.
- Better suited for urban areas with well-defined drainage systems.
SCS TR-20/TR-55 Method
The Soil Conservation Service (SCS) TR-20 and TR-55 methods are widely used hydrologic models developed by the USDA Natural Resources Conservation Service (NRCS, formerly SCS). These methods estimate stormwater runoff based on rainfall, land use, and watershed characteristics, making them essential for designing drainage systems, flood control structures, and stormwater management plans.
SCS TR-20
SCS TR-20 is a complex hydrologic model designed for the simulation of rainfall runoff occurring from a single storm event in watersheds of any size. It provides a detailed flood hydrograph from rainfall runoff and routes the flow through stream channels and reservoirs. Hydrographs, peak discharges, and peak elevations can be obtained at any cross section along the stream at the outlet of a structure. The application of this method includes designing detention ponds and reservoirs, floodplain analysis, FEMA flood mapping, etc.
SCS TR-55
SCS TR-55 is a simplified hydrologic model designed for estimating peak discharge and runoff volume in small watersheds (typically less than 25 square miles). The application of this method includes designing small urban and rural watersheds (≤10 mi² or 6,400 acres).
Limitations
- Less accurate for small storm events.
- Assumes uniform soil and land use, which may not reflect groundwater variations.
- Uses predefined design storms that may not match actual rainfall events.
- Not suitable for detailed urban drainage systems or engineered infrastructure.
Hydrology Method Comparisons
| Hydrology Method | Primary Use | Strengths | Limitations |
| DeKalb Rational Method | Hydrograph generation from Rational Method peak flow | Converts Rational Method peak flow into a hydrograph | Assumes uniform land use and is limited to small subbasins (<10 acres) |
| EPA SWMM Method | Continuous runoff modeling | Dynamic, realistic representation of urban hydrology | Requires detailed parameter inputs |
| Modified Rational Method | Runoff hydrograph estimation | Extends Rational Method with time-based hydrograph | Assumptions needed for hydrograph shape |
| Rational Method | Peak flow estimation | Simple, widely used for storm sewer sizing | Only provides peak flow, no hydrograph |
| SCS TR-20/TR-55 Method | Watershed runoff hydrograph modeling | Well-suited for larger watersheds | Requires curve number (CN) estimation |