Loss Method
SCS Curve Number
Used to estimate rainfall abstraction and direct runoff from the land-use and soil characteristics of each subbasin.
HEC-HMS · Technical workspace
Methods & Parameters
The Hare River HEC-HMS model was developed as an event-based rainfall-runoff model to estimate design-flood hydrographs at the Hare irrigation diversion site. The model combines GIS-derived watershed characteristics with the SCS Curve Number loss method, SCS Unit Hydrograph transformation and Muskingum channel routing.
Modelled watershed:
162.035 km² divided into five subbasins (S_1 to S_5), with runoff routed
through the watershed drainage network to the Hare Weir outlet.
1. Hydrologic Modelling Framework
HEC-HMS represents the rainfall-runoff process as a sequence of hydrologic components. For the Hare River watershed, precipitation falling over each subbasin is first reduced by infiltration and other abstractions. The resulting rainfall excess is transformed into a direct-runoff hydrograph, after which flows from the subbasins are combined and routed through the river reaches toward the Hare Weir.
Transform Method
SCS Unit Hydrograph
Used to convert rainfall excess into a runoff hydrograph using subbasin-specific lag times.
Baseflow
None
Baseflow was excluded from the design-event simulations so that the resulting hydrograph represents direct storm runoff generated by the adopted design rainfall.
Reach Routing
Muskingum Method
Applied to reaches R_1 and R_2 to account for translation and attenuation of the flood hydrograph before it reaches the Hare Weir.
2. SCS Curve Number Loss Method
The Soil Conservation Service Curve Number method was adopted to estimate direct runoff from design rainfall. The method relates runoff generation to land cover, hydrologic soil group and antecedent catchment conditions.
The Hare watershed was represented principally by Hydrologic Soil Group C. Curve Numbers were assigned to the mapped land-use classes and spatially combined within each subbasin to obtain a weighted Curve Number.
Land-use Curve Numbers adopted
| Land-use class | Hydrologic Soil Group | Adopted CN |
|---|---|---|
| Trees | C | 73 |
| Grass | C | 79 |
| Cropland | C | 85 |
| Shrub / Scrub | C | 70 |
| Built-up | C | 82 |
| Bare ground | C | 91 |
| Water | — | 100 |
Weighted Curve Numbers by subbasin
| Subbasin | Area (km²) | Weighted CN | Impervious (%) |
|---|---|---|---|
| S_1 | 77.065 | 78.26 | 0 |
| S_2 | 25.498 | 78.94 | 0 |
| S_3 | 25.502 | 75.63 | 0 |
| S_4 | 13.533 | 74.62 | 0 |
| S_5 | 20.437 | 78.60 | 0 |
| Total | 162.035 | — | — |
The Curve Number values therefore vary spatially according to the proportion of vegetation, cultivated land, settlement, bare land and other mapped land-cover classes within each subbasin.
SCS runoff equations
The potential maximum retention is calculated from:
S = (25,400 / CN) − 254
where S is the potential maximum retention in millimetres.
Initial abstraction was taken as:
Ia = 0.20 S
For precipitation greater than the initial abstraction, direct runoff is calculated as:
Q = (P − Ia)² / (P − Ia + S)
where P is storm rainfall and Q is direct runoff depth.
3. SCS Unit Hydrograph Transform
Rainfall excess from the Curve Number calculation was transformed into direct runoff using the SCS Unit Hydrograph method. This method requires a basin lag representing the delay between the centre of mass of rainfall excess and the peak of the resulting runoff hydrograph.
Lag values were derived from the physical characteristics of the delineated subbasins and subsequently reviewed for consistency with the watershed size, drainage paths and response time.
Final adopted subbasin lag times
| Subbasin | Area (km²) | HEC-HMS Transform | Lag Time (min) |
|---|---|---|---|
| S_1 | 77.065 | SCS Unit Hydrograph | 118 |
| S_2 | 25.498 | SCS Unit Hydrograph | 76 |
| S_3 | 25.502 | SCS Unit Hydrograph | 65 |
| S_4 | 13.533 | SCS Unit Hydrograph | 52 |
| S_5 | 20.437 | SCS Unit Hydrograph | 62 |
Important:
These are the revised lag values adopted in the final Hare HEC-HMS
design-event model. They replace the earlier preliminary lag estimates
used during initial model construction.
4. Baseflow Representation
The Hare HEC-HMS simulations were developed principally for design-storm flood estimation. Baseflow was therefore set to None for subbasins S_1 to S_5.
This allows the computed hydrograph to represent runoff generated directly by the design rainfall event without introducing uncalibrated groundwater recession parameters.
Interpretation:
The computed discharge is a design-event runoff hydrograph rather than a
continuous simulation of total river flow.
5. River Reach Routing
Flood hydrographs generated by the upstream subbasins do not arrive instantaneously at the Hare Weir. Travel through the river network causes both translation and attenuation of the flood wave. Reach routing was therefore represented using the Muskingum method.
GIS-derived reach characteristics
| Reach | Length | Average Slope | Routing Method |
|---|---|---|---|
| R_1 | 10.377 km | 0.11246 | Muskingum |
| R_2 | 4.745 km | 0.03330 | Muskingum |
Adopted Muskingum parameters
| Reach | K (hr) | X | Subreaches |
|---|---|---|---|
| R_1 | 2.40 | 0.20 | 10 |
| R_2 | 1.10 | 0.20 | 4 |
In the Muskingum method, K represents the approximate travel-time or storage constant of the reach, while X controls the relative weighting of inflow and outflow in the reach storage relationship.
Muskingum storage relationship
S = K [ X I + (1 − X) O ]
where:
- S = channel storage,
- I = reach inflow,
- O = reach outflow,
- K = storage/travel-time constant, and
- X = weighting coefficient.
Routing sequence
The upstream runoff is combined at the model junctions and transmitted through the river system as:
Upstream Subbasins → R_1 → Junction → R_2 → Hare Weir
The routing procedure reduces and delays the upstream hydrograph before the combined flood reaches the project outlet.
6. Computational Time Step
The design-event simulations were performed using a 15-minute computation interval. This interval is short relative to the adopted subbasin lag times and the Muskingum reach travel times, allowing the rising limb and peak of the flood hydrograph to be represented adequately.
Design Storm Duration
24 hours
Rainfall was distributed over a 24-hour synthetic design storm.
Computation Interval
15 minutes
All basin, transform and routing calculations used the same computational interval.
Simulation Window
72 hours
The extended simulation period allows the full hydrograph, including recession after the storm, to pass through the basin.
Outlet
Hare Weir
Peak discharge and design hydrographs are evaluated at the irrigation diversion site.
7. Parameter Sources
Model parameters were not entered as arbitrary values. Each group of parameters was derived from GIS analysis, watershed characteristics, hydrologic methodology or explicit modelling assumptions.
| Parameter | Basis / Source |
|---|---|
| Subbasin boundaries and areas | GIS watershed delineation using the project DEM and the Hare Weir as the model outlet. |
| Land-use distribution | GIS land-cover analysis for the Hare watershed. |
| Hydrologic Soil Group | Watershed soil classification; HSG C adopted for the Curve Number calculation. |
| Curve Numbers | SCS Curve Number method using land-use and hydrologic soil-group combinations. |
| Weighted CN | Area-weighted calculation within each HEC-HMS subbasin. |
| Subbasin lag | Derived from basin geometry and watershed response characteristics and entered in the SCS Unit Hydrograph transform. |
| Reach length and slope | Derived from the GIS drainage network and elevation data. |
| Muskingum K and X | Engineering routing parameters adopted for the Hare event-based design simulations. |
| Baseflow | Explicitly omitted for the design-storm simulations. |
| Computation interval | 15 minutes, selected to remain consistent with basin response and reach-routing time scales. |
8. Adopted Model Configuration
| Watershed Area | 162.035 km² |
|---|---|
| Number of Subbasins | 5 |
| Loss Method | SCS Curve Number |
| Initial Abstraction | Ia = 0.20S |
| Transform Method | SCS Unit Hydrograph |
| Baseflow | None |
| Routing Method | Muskingum |
| Number of Routed Reaches | 2 |
| Computation Interval | 15 minutes |
| Design Storm Duration | 24 hours |
| Simulation Duration | 72 hours |
| Model Outlet | Hare Weir |
No silent parameters.
The Hare model records the origin and purpose of the principal parameters
used in rainfall loss, runoff transformation and channel routing.
Parameters developed during preliminary testing were revised where
necessary before the final design-event simulations.
9. Role of the HEC-HMS Model in the Hare Study
The purpose of the HEC-HMS analysis is not simply to produce a single peak-discharge number. It provides the complete design hydrograph required to understand the magnitude, timing and duration of flood flow arriving at the Hare irrigation diversion.
The resulting hydrographs are subsequently used in evaluating the hydraulic capacity and safety of the diversion weir, under-sluice, headworks and associated river-control structures.
The HEC-HMS design-flood estimates will also be compared with the independent watershed simulation developed using SWAT, providing an additional hydrologic check for the reconstructed Hare Irrigation Project.