HIRDSv4 Usage

NIWA'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.

NIWA'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.

Go to the HIRDS tool

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 selected, site details and data coverage are displayed in a popup box. 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 requires authentication via Google authenticator. You will need to use an existing Google login or create a new one to use HIRDS. After signing in, users can generate rainfall estimates in three easy steps.

  1. Select a location by either:
    • searching using the Address Search
    • clicking anywhere on the NZ map
    • clicking one of the rain gauge locations
    • entering the latitude, longitude and name (WGS84 coordinate system).
  2. 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).
  3. Press ‘Generate Report’.

After a short delay the results will appear below the map in a series of tabs and as a downloadable spreadsheet.

Output description

The first tab contains information about the chosen site. The second ‘Historical Data’ tab contains the main HIRDS table of either rainfall depths or rainfall intensities for given 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.

The remaining tabs 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.

Read the HIRDSv4 Technical Report [6MB PDF].

The tool output can be downloaded as a spreadsheet via the download icon at the top right of the results section. This spreadsheet contains a copy of the results in all the displayed tabs. It 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.

Climate change projections

For this version of the HIRDS tool, climate change projection information is provided based on IPCC scenarios rather than for arbitrary temperature increases. This aligns with other information NIWA provides on climate change projections. In some cases the change in rainfall intensity due to a specific temperature increase may be required. This can be acheived 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.

Percentage change factors to project rainfall depths derived from the current climate to a future climate that is 1 degree warmer.
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.

GIS layers containing HIRDS derived rainfall depths for different durations and return periods are available via an ArcGis data service. Each surface contains rainfall depth (in mm) for a given average recurrence interval (in years) and event duration (in hours) on a 2km grid.

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] 


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