The hydrological model TopNet was developed at NIWA and is commonly used in New Zealand. Typical applications include flood forecasting, irrigation planning and estimates of climate change impacts on river flows.


TopNet is a semi-distributed hydrological model for simulating catchment water balance and river flow.

TopNet uses a modelling approach which combines an understanding of hydrology and soil science to calculate the water available in various locations (soil, lakes, streams) over time. In simple terms, TopNet is a catchment water-balance calculator.

Using this computer-based model scenarios can be tested based on specific ‘variables’, such as size of reservoir, amount and timing of rainfall, size of the catchment, amount and timing of irrigation, and so forth. TopNet allows calculation of how these variables will interact over time. TopNet has resulted in the following:

  • New Zealand’s first national-scale water resources simulation model
  • The most detailed national-scale water resources map ever produced
  • The ability to estimate river flows in all ungauged catchments in New Zealand

Latest Version Version 11 (2011)
State of Development Stable Release
Current Development Activity Coupling with a glacier component and developing a retrospective streamflow assimilation scheme.

Development Contact

Hilary McMillan
+64 3 348 8987
National Institute of Water and Atmospheric Research (NIWA)
10 Kyle St
PO Box 8602
New Zealand 8044
New Zealand

Main Developers

  • National Institute of Water and Atmospheric Research (NIWA)


Outcome Areas Environmental
Management Domains Land
Subdomains Water Yield
Intended End Users Highly trained personnel / In-house specialist
Spatial Extents Local (i.e. Catchment or District), Regional, National
Spatial Dimensions 2D
Temporal Resolutions Hours
Temporal Extents Decades
Steady State or Dynamic Dynamic
Level of Integration Environmental

Input & Output Data

Key Input Data Land Use, Land Cover, Rainfall, Temperature, Potential Evaporation Topography, soils
Input Data Formats netcfd
Key Output Data Runoff, Drainage Soil moisture, snow
Output Data Formats netcdf


Open/Closed Source Closed Source

User Information

Operating Systems MS Windows, Linux
Software Needed netcdf libraries
User Interface Other (please specify) Command Line
Ease of Use Difficult There is a significant training overhead to learn to use Topnet. Specific implementations customised for users are much simpler to operate, and more limited in functionality.
Use in Policy Process Plan (Policy Formulation)

User manual and help files available on the NIWA website (

Support On research collaboration basis
Users Forum None

Technical Considerations

Programming Language Fortran and C
Methods included for calibration and validation (1) Manual calibration (2) NLFit: a parameter fitting Windows-based program that uses optimal parameter search algorithms.
Methods included for managing uncertainty Monte Carlo simulation: a large number of different parameter sets are tried and the best set selected based on an objective function (e.g. the Nash Sutcliffe criteria).
Keywords flooding, irrigation, climate change, rivers, flow
Key References

Clark, M.P.; Woods, R.A.; Zheng, X.; Ibbitt, R.P.; Slater, A.G.; Rupp, D.E.; Schmidt, J.; Uddstrom, M.J. 2008:. Hydrological data assimilation with the Ensemble Kalman Filter: use of streamflow observations to update states in a distributed hydrological model. Advances in Water Resources: 31, 1309-1324.

Associated Case Studies

TOPNET - National Water Accounts

Subsequent requirements extended the modelling window to mid-2014 and the TopNet model was re-run to include this additional time period.


Other Key Case Studies

Mostly used in New Zealand, but with applications overseas, such as in the United States of America and Chile.

The Water Accounts of New Zealand (2016)

A pressure-state-impact model for freshwater flows with example application to Canterbury, NIWA Client Report No: 2017071CH, Prepared for Ministry for the Environment, March 2017.

Impacts of climate change on river flows and floods (2009)

Irrigation forecasting - Wamakariri River (2009) -