Rainfall and Runoff Guidelines

There is widespread support within the 3 waters sector for developing national rainfall and runoff guidelines. For over seven years, groups such as the Engineering NZ/Water NZ Rivers Group and the Water NZ Modelling and Stormwater Special Interest Groups have consistently identified this as a priority sector need.

The scale of the project and the lack of a clear ‘owner’ for the work has been a challenge to getting work underway and securing funding. A Steering Group has now been established to drive the work forward – more information on the Steering Group is provided below.

It is important to note that this project is not a Water NZ led project. The Steering Group is providing overall leadership as there are multiple interests in this initiative. Water NZ is providing project management support.

Current situation

In NZ, a limited number of locally specific guidelines have been developed based on rainfall and flow data available at the time (for example, TP108 developed for Auckland in 1999). These are frequently used outside the regions where they were developed, often inappropriately. In addition, where methods do exist for an area, they can be applied inconsistently giving different results from the same input data. The scale of error can be as high as +/- 50-100%. In some locations there can also be gaps in data and/or data quality issues that can skew statistical estimation of patterns that are used in modelling future scenarios. Gathering robust data and developing locally specific guidelines is costly. This is a key reason why only some places have done so.

These issues present a high risk that poor or sub-optimal investment decisions are being made. Communities are bearing the cost of these decisions through higher rates and housing costs, or through physical damage as a result of infrastructure failures (e.g. flooding).

Most other countries that NZ would consider peers have national rainfall and runoff guidelines or something equivalent. The well-known Australian Rainfall and Runoff (ARR) guidelines have recently been updated and are now being rolled out with training. There is opportunity to leverage off some of the recent ARR work.

The business case

National guidelines would support improved decision-making and cost efficiencies on matters such as natural hazard risk assessment (especially for floods) and right-sizing infrastructure investment such as flood protection schemes, stormwater systems, wastewater systems and transport infrastructure.

Climate change is bringing changes to weather patterns (especially more extreme rain events and droughts) and inundation in coastal areas. It will be important to factor in knowledge about how the climate is changing into decisions about infrastructure investments that are being made now for the long term.

Rainfall and runoff analyses are also critical inputs to river flow and contaminant models that are needed for water management decisions. National guidelines would therefore support better decision-making about how to best improve water quality or how to address water quantity pressures via allocation policies.

Many areas of NZ that are exposed to flood risk lack knowledge, skills, or the money to access flood forecasting techniques or to invest in the capital works to fully mitigate risks. Development of national rainfall and runoff guidelines could be of considerable benefit to these communities by providing better information to prioritise limited resources and manage residual risks via emergency planning.

There would also be some cost-efficiencies in terms of Resource Management Act processes such as consenting and planning. Resolving methodological issues in the development of national guidelines will narrow the range of disputes that take precious time and money to resolve, which could be better spent on the infrastructure itself. A more certain planning environment gives developers greater confidence in investing in communities by providing a clear understanding of costs early in the process.

Steering Group

A Steering Group was established in late 2017 to guide the work - Click here for its terms of reference.


Mark PenningtonTonkin + Taylor
Helen ShawEnvironment Canterbury
Nick BrownAuckland Council
Graham LevyBeca

Engineering NZ/Water NZ Rivers Group

Zeb WorthCKL Surveys Limited

Water NZ Stormwater SIG

Nadia NitscheAuckland Council

Water NZ Modelling SIG

Assad ShamseldinUniversity of Auckland
Roddy HendersonNIWA
Martin DoyleTasman District Council
Sarah BooneMinistry for the Environment

Overall Objectives for the guidelines

  • Reduce variability of rainfall and runoff analyses and advice by developing a standard approach on how data and numbers should be derived (not ‘a standard’).
  • Improve understanding about the reliability and accuracy of data and models, the potential error of information provided, and how to use this information appropriately in decision-making.
  • Improve accessibility of available resources (ideally map based).
  • Improve knowledge about the risk profile of infrastructure.


There are no current funding ‘pots’ suitable for developing the guidelines in one go. The work is too applied to suit most MBIE funds, except the Partnerships Fund and co-funding requirements are prohibitive (at least $600,000 per annum). The work as a whole is too large to be suitable for Envirolink, a fund for regional councils which funds applied work. The science challenges are possible option that will be pursued as the challenge funding is reviewed in 2018-19. By way of comparison, the ARR has cost AU$34 million to date.It is anticipated that the NZ guidelines could be done for MUCH less, perhaps a few million, although the gap analysis work (outlined below) will help quantify the size of the task.

The current approach is therefore to break the work into smaller chunks and prioritise effort on some priority sub-projects. Funding will be sought where suitable options can be identified.In the meantime, however, work is being progressed voluntarily.

We have established a technical team to progress the work – this is basically a list of interested technical experts who have put up their hands to help voluntarily until such time as funding can be secured. If you want to be involved in the technical work, please get in touch with us.


We are conducting a two-stage gap analysis as follows:

  1. SHORT screening survey about relative priorities.
  2. More detailed gap analysis of specific topic areas identified in Stage 1 as being the most important and urgent.

The first tranche of the survey was completed during September to October 2017. The results (see below) are guiding work now underway. Due to ongoing interest we have left the survey open and any further responses received will inform the next phase of work - click here to download the survey - It is only five questions so won’t take much time.

The gap analysis will explore questions such as:

  • What existing guidelines and methods are being used across the different geographic areas of NZ and for what kinds of decisions?
  • How up-to-date are they and/or are they meeting the needs of the sector?
  • What work is needed to get national consistency in each topic area?

We are building on the material previously collected by NIWA in 2014.


Results from the first tranche of survey responses received during September and October 2017 are summarised below. A paper containing more detail will be made available in due course.

Development of guidance came out as the top priority, closely followed by resolving outstanding technical methodological issues. Providing centralised access to information was the third priority but ranked considerably lower than the first two. Training was the lowest priority, although those who ranked it highly tended to work for smaller territorial authorities.

Top rainfall priorities were (in order):

  • Develop a range of temporal patterns of rainfall for use around NZ (including nested rainfall patterns), using all available data and models (e.g. building on NIWA’s High Intensity Rainfall Design System (HIRDS)).
  • Update intensity-frequency-duration (IDF) information across NZ, building on HIRDS).
  • Use existing rainfall data to map spatial variability across NZ. Identifying gaps in data to further this work.

Feedback from the 2017 Hydrological Society conference also indicated that there are data quality and accessibility issues that we should consider in any rainfall work that is done. This issue also came through strongly in survey responses.

NIWA has been working on version four of HIRDS (HIRDSv4) which is due for release in 2018.While HIRDSv4 will resolve some of the rainfall issues identified in the survey it will not resolve all of them. A technical project is now underway to identify specific priorities for future work – see below.

Top runoff priorities were (in order):

  • Large to extreme floods - Methods and guidance for estimating flood flows covering the range of available models and data. Priority should be given to methods and guidance for urban areas.
  • Loss models for catchment simulation – Guidance on which methods should be applied in specific circumstances and when particular methods are not appropriate (e.g. guidance on when the SCS method is/is not appropriate). Resolving specific technical issues in this topic area was also deemed important such as confirming curve numbers across different parts of NZ, by calibration to measured flows, in cases where SCS is deemed to be appropriate.
  • Regional flood methods - Guidance on which methods should be applied in specific circumstances and when particular methods are not appropriate (e.g. when and how the rational method should/should not be applied). Resolving specific technical issues in this topic area was also deemed important such as detailing methodologies for flood estimation at a regional / national scale in areas where data is sparse.

Finally, a strong theme in the responses was that all guidance should be clear about how technical information should be used in decision-making, taking account of uncertainty in the information and the risks involved in specific decisions. Respondents wanted to see pragmatic and practical guidance that would be clear and understandable to a variety of end-users not just technical experts (i.e. not like the ARR). Guidance need to include discussion about the outcomes and effects of the modelling in terms of design of structures, risk management practices and policies (e.g. planning controls) and communications.

Work now underway

The Steering Group has initiated four projects which are all being progressed by volunteers:

1. Assess HIRDSv4 to:

  • Determine what it now includes and what priority areas it still does not include.
  • Scope out guidance needs especially around understanding data sources and their quality, and how to appropriately use different sources given their limitations.
  • Identify ways to help users see ‘inside the black box’ i.e. presenting gauges and other information visually.
  • View the High Intensity Rainfall Design System v4 document here.

2. Scope a national skeleton guidance document that goes beyond chapters headings (which has already been done) and sets out:

  • The contexts / decisions where rainfall runoff analyses are used.
  • What techniques / methods / models should be used and should not be used.
  • What input data is needed and related issues / limitations.

3. Scope a high-level business case to use for communicating with potential funders.

  • Pieces of “the story” have been described before but material is currently dispersed and language used was either too technical or too vague. Pull it together into a coherent story that is pitched at a level that decision-makers understand (describing the tangible benefits).
  • An attempt will be made to wrap some costs around aspects of “the story” to demonstrate the scale of impacts to NZ from not having national guidelines (and the benefits of investing in them).

4.Gather all the existing guidance documents in use and upload those onto a web-based, spatially presented platform for later gap analysis.

  • A big thanks to Auckland Council for their assistance with setting up the platform (a special shout out to Ken Williams).


Please get in touch with us to be added to the email list to receive periodic updates about this work.

If you know others (members or non-members) who may have an interest in this work, please send them the link to this web page and ask them to sign-up for updates.

The Water New Zealand contact for this work is Noel Roberts (Technical Manager) at noel.roberts@waternz.org.nz or 04 495 0892.