ESNZ's High Intensity Rainfall Design System (HIRDS) offers planners and engineers more certainty about the frequency of high-intensity rainfall events, enabling them to better design stormwater drainage, flood defence systems and other vital structures.
How to use the HIRDS tool
The HIRDS tool provides a map-based interface to enable rainfall estimates to be provided at any location in New Zealand. The map also displays the locations of all rain gauges used in the HIRDSv4 analysis. When a gauge is clicked on, site details and data coverage are displayed in a popup box and this location can be selected using the "Copy details" link. Whether a location is selected by clicking on a gauge, clicking anywhere on the map or using the input boxes makes no difference to the way the HIRDS tables are generated.
The HIRDS tool is hosted by ESNZ's Datahub. After signing in, users can generate rainfall estimates after providing a location and output format.
- Select a location by:
- clicking anywhere on the NZ map
- clicking one of the rain gauge locations and selecting “Copy details”
- entering the latitude, longitude and name (WGS84 coordinate system).
- Select output format:
- depth-duration-frequency: returns the amount of rain fallen during the event (in mm).
- intensity-duration-frequency: returns the average rate of rainfall during the event (in mm/hr).
For issues related to DataHub see the DataHub FAQ.
Output description
The tool output is downloadable as a spreadsheet which includes information about the chosen site followed by a series of tables. The first contains the main HIRDS output of either rainfall depths or rainfall intensities for a range of storm durations and recurrence intervals (ARI). The table also provides the annual exceedance probability (aep) which is the probability of a given rainfall being exceeded in any one year. Below this is a second table containing the standard error of each estimate.
This table provides a measure of the uncertainty in each HIRDS rainfall estimate where adding and subtracting this value from the estimate will give a confidence interval of approximately 68%.
The remaining tables contain high intensity rainfall estimates for two future time periods (2031-2050 and 2081-2100) for four representative concentration pathways (RCPs). More information about these scenarios can be found in the HIRDSv4 Technical Report.
The spreadsheet also includes a simple calculator on rows 8 to 10 which can be used to generate rainfall depths for specific durations or return periods that are intermediate to those displayed in the tables. Simply enter the required duration and return period in columns C and D and the result will appear in column G.
Read the HIRDSv4 Technical Report [6MB PDF].
Climate change projections
For this version of the HIRDS tool, climate change projection information is provided based on IPCC CMIP5 scenarios rather than for arbitrary temperature increases. This aligns with other information ESNZ provides on CMIP5 climate change projections. In some cases the change in rainfall intensity due to a specific temperature increase may be required. This can be achieved using percentage change factors provided in the table below taken from the HIRDSv4 Technical Report. The appropriate factor should be multiplied by the required temperature increase and applied to the historical HIRDS rainfall estimate.
For example, if the historic estimate for a 1-hour, 10-year is 35mm, then the projected value given a 2.1°C temperature increase would be calculated as follows. First the percentage increase per degree of warming for this duration and event frequency is selected from the table and multiplied by 2.1, i.e. 2.1 × 13.1% = 27.5%. The projected rainfall amount assuming a 2.1°C warming is then 35mm × 1.275 = 45mm.
The percentage change factors provided for storm durations of 1 hour should also be used for durations shorter than one hour.
| Duration/ARI | 2 yr | 5 yr | 10 yr | 20 yr | 30 yr | 40 yr | 50 yr | 60 yr | 80 yr | 100 yr |
| 1 hour | 12.2 | 12.8 | 13.1 | 13.3 | 13.4 | 13.4 | 13.5 | 13.5 | 13.6 | 13.6 |
| 2 hours | 11.7 | 12.3 | 12.6 | 12.8 | 12.9 | 12.9 | 13.0 | 13.0 | 13.1 | 13.1 |
| 6 hours | 9.8 | 10.5 | 10.8 | 11.1 | 11.2 | 11.3 | 11.3 | 11.4 | 11.4 | 11.5 |
| 12 hours | 8.5 | 9.2 | 9.5 | 9.7 | 9.8 | 9.9 | 9.9 | 10.0 | 10.0 | 10.1 |
| 24 hours | 7.2 | 7.8 | 8.1 | 8.2 | 8.3 | 8.4 | 8.4 | 8.5 | 8.5 | 8.6 |
| 48 hours | 6.1 | 6.7 | 7.0 | 7.2 | 7.3 | 7.3 | 7.4 | 7.4 | 7.5 | 7.5 |
| 72 hours | 5.5 | 6.2 | 6.5 | 6.6 | 6.7 | 6.8 | 6.8 | 6.9 | 6.9 | 6.9 |
| 96 hours | 5.1 | 5.7 | 6.0 | 6.2 | 6.3 | 6.3 | 6.4 | 6.4 | 6.4 | 6.5 |
| 120 hours | 4.8 | 5.4 | 5.7 | 5.8 | 5.9 | 6.0 | 6.0 | 6.0 | 6.1 | 6.1 |
Additional resouces
The HIRDSv4 Technical Report also provides areal reduction factors (section 5) and temporal storm design profiles (section 6) specific to New Zealand. These can be used when applying results from the HIRDS tool to flood and inundation modelling.
A full list of the rain gauges used in the HIRDSv4 analysis along with the data availability for various duration categories are available in spreadsheet form.
View the complete list of gauges used [300KB CSV]
