Hydrology quantifies how rainfall becomes runoff across a catchment: the foundation for stormwater, flood, and water supply design. Catchment analysis in Australia is underpinned by Australian Rainfall and Runoff (ARR 2019).
Catchment Delineation
A catchment (drainage basin) is the area draining to a single outlet point, delineated from contours/DEM data such that all runoff within its boundary flows to that outlet.
| Input Data | Purpose |
|---|---|
| Digital Elevation Model (DEM) / LiDAR | Automated catchment/flow path delineation |
| Contour survey | Manual delineation, ridge-line tracing |
| Land use / land cover mapping | Runoff coefficient and loss parameter estimation |
| Soil type mapping | Infiltration loss parameters |
Rainfall Data
Intensity-Frequency-Duration (IFD)
IFD data (via the Bureau of Meteorology) provides design rainfall intensity for a given AEP and duration, used directly in the Rational Method or as input to design storm temporal patterns for hydrograph methods.
Design Rainfall Temporal Patterns
For hydrograph-based methods, a total design rainfall depth is distributed over time using ARR's regional temporal patterns, reflecting how real storms typically build and recede.
Rainfall Losses
Not all rainfall becomes runoff: losses account for infiltration, depression storage, and interception:
| Loss Model | Description |
|---|---|
| Initial loss – continuing loss (IL-CL) | Fixed initial loss, then constant continuing loss rate: ARR's standard approach |
| Curve Number (CN) | US-derived, less commonly used in current Australian practice |
Hydrograph Methods
For larger or more complex catchments, a full rainfall-runoff model produces a hydrograph (flow vs time), rather than just a single peak flow value as in the Rational Method.
| Method/Model | Application |
|---|---|
| Unit Hydrograph methods | Convert unit rainfall excess into a runoff hydrograph |
| RORB, WBNM, URBS | Common Australian catchment routing/runoff models |
| RAFTS | Urban catchment modelling (still used in some jurisdictions) |
Time of Concentration and Lag
$$ t_c = \text{time for runoff to travel from the hydraulically most distant point to the outlet} $$Longer $t_c$ generally produces lower peak flow but greater runoff volume for the same rainfall depth, since intensity decreases with duration.
Baseflow and Losses in Larger Catchments
For larger catchments and longer-duration events, baseflow (groundwater contribution to streamflow) becomes a non-negligible component of total flow, particularly relevant to flood studies and environmental flow assessments.
Climate Change Considerations
Current Australian guidance requires sensitivity testing of rainfall intensity uplift (reflecting projected increases in short-duration rainfall intensity under climate change) for flood and drainage studies with long design lives.
Catchment Analysis Outputs
| Output | Used For |
|---|---|
| Peak flow (single value) | Pipe/culvert sizing (Rational Method) |
| Full hydrograph | Detention basin sizing, flood modelling, dam spillway design |
| Runoff volume | Retention/detention storage sizing, water balance |
Practical Notes
- Catchment delineation from automated DEM tools should always be sanity-checked against site knowledge and survey: urban drainage infrastructure (pipes, kerbs) frequently redirects flow paths in ways a bare-earth DEM won't capture.
- The Rational Method is appropriate for small, relatively uniform catchments; larger or highly variable catchments need a hydrograph method to capture storage and timing effects properly.
- Climate change rainfall uplift sensitivity testing is increasingly an explicit requirement, not an optional extra, for infrastructure with a multi-decade design life.