Rotorua Taupo Nitrogen Model (ROTAN-Annual)


ROTAN-Annual is used for the prediction of nitrate inputs to lakes for various land use, rainfall, and mitigation scenarios to inform land use policy and decision making for improving environmental outcomes.


ROTAN is a daily/weekly time-step, GIS-based, rainfall-runoff-groundwater model which can predict the water flows and nitrogen concentrations in streams in daily or weekly time steps. Used at a catchment scale, it can be used to predict nitrogen loads to lakes. It can take account of time lags between nitrogen leaching, and its arrival at the lake.

The initial version of ROTAN (ROTAN-2011) was developed to model nitrogen in Lakes Rotorua and Taupo. It is a GIS-based catchment hydrology and water quality model that models the effects of land-use change and nitrogen mitigation measures such as riparian margins and plant uptake on nitrogen export to waterways. This initial model is based on the Scandanavian HBV and HBV-N models in which soils and aquifers are modelled as a series of fully-mixed reservoirs (buckets) connected in series and parallel.

The ROTAN-2011 model (Rutherford et al. 2008, 2009, and 2011) has been used to predict the effects of land use changes on nitrogen loads delivered to Lake Rotorua in surface and groundwater.  The original ROTAN-2011 model relied on OVERSEER v5 to estimate nitrogen losses from farmland. That version of OVERSEER is no longer supported and was replaced by OVERSEER v6. Therefore ROTAN-2011 needed to be re-calibrated before it could be used with OVERSEER v6.

This necessitated the redevelopment of ROTAN-2011. University of Waikato is reprogramming a new version (ROTAN-V2). However, in the interim NIWA was commissioned by BoPRC in March 2016 to develop a simplified
version (ROTAN-Annual) and calibrate it using:

  • OVERSEER v6 nitrogen losses
  • recent stream monitoring data
  • revised groundwater boundaries, and
  • updated land use information.

ROTAN-Annual is providing technical support to BoPRC and stakeholders during the plan change process for Lake Rotorua Catchment. ROTAN-Annual builds on modelling work undertaken in Hawke’s Bay during 2010-2011 (Rutherford 2013). This was undertaken to support the TRIM-Catchment model which was developed as a component of the TRIM (Tukituki River Model) nutrient-periphyton model. The concepts of TRIM-Catchment were further developed and the model re-coded to produce ROTAN-Annual.

ROTAN-Annual routes nitrogen through the catchment using mass flux which is consistent with OVERSEER which estimates annual nitrogen losses. Attenuation coefficients are calibrated using annual average stream concentrations (flux/flow).

In the model nitrogen travels to the lake by three pathways: ‘quickflow’ and ‘streamflow’ (that reach the lake within a year), and ‘slowflow’ (groundwater that may take decades to reach the lake). Nitrogen ‘loss’ along each pathway is quantified using separate attenuation coefficients.

ROTAN offers a range of outputs, including:

  • maps of flow and nutrient delivery pathways from each part of the catchment to the lake
  • tables for each part of the catchment, showing quantities of flow and nutrients, time delays and nutrient losses
  • tables and graphs showing historic trends in land use, stocking rate and fertiliser usage
  • tables and graphs showing the relationship between historic trends in land use, rainfall and stream nutrient concentration
  • predictions of nutrient inputs to lakes under various land use, rainfall and mitigation scenarios.

ROTAN-Annual is a closed source/use tool and is only intended to be available to NIWA research team.

State of Development Closed Beta
Current Development Activity Nil

Development Contact

Kit Rutherford

Main Developers

  • National Institute of Water and Atmospheric Research (NIWA)


Outcome Areas Environmental
Management Domains Freshwater
Intended End Users
  • Researcher
  • Waikato University
Spatial Resolutions 500-1000m
Spatial Extents Local (i.e. Catchment or District)
Spatial Dimensions 3D
Temporal Resolutions Days
Temporal Extents Days, Months
Steady State or Dynamic Dynamic
Level of Integration Environmental

Input & Output Data

Key Input Data Land Use, Rainfall, Land Cover, Potential Evaporation
Input Data Formats GIS compatible files
Key Output Data Runoff, Drainage water flows and nitrogen concentrations
Output Data Formats GIS Compatible Files


Open/Closed Source Closed Source
Licence Type Unknown
Licence Cost
(Non Commercial)
Not available for non-commercial use
Licence Cost
Not available for commercial use

User Information

Operating Systems MS Windows ArcGIS
Software Needed ArcGIS ArcGIS
User Interface Graphical desktop
Ease of Use Difficult Not intended for use outside the NIWA research group that developed it. Little work done on user manuals, help screens, or internal checking.
Use in Policy Process Plan (Policy Formulation), Review (Issue Identification)

Rucinski, D., S. Wadhwa, J. C. Rutherford. (2006). ROTAN - ROtorua and TAupo Nitrate Model. User's Guide. Hamilton, NIWA.

Support None outside NIWA
Users Forum None outside NIWA

Technical Considerations

Programming Language ESRI Visual Basic
Methods included for calibration and validation Calibrated by fitting to measured flows and nitrogen concentrations in streams, and to published annual nitrogen loads entering the lake.
Methods included for managing uncertainty Goodness of fit quantified (RMS error)
Analytical Techniques GIS
Model Structure


Based on the Scandanavian HBV and HBV-N models. Soils and aquifers are modelled as a series of fully-mixed reservoirs (buckets) connected in series and parallel.

Keywords Nitrogen, Land use, Lake, eutrophication
Linkages to other Models
Key References

Rutherford, J. C. (2016). Predicting nitrogen loads to Lake Rotorua using the ROTAN-Annual. Napier. NIWA Client Report 2016102HN.

Rutherford, J.C. (2013) Effects of land use on nutrients. Phase 2 studies in the Tukituki River, Hawke’s Bay. NIWA Client Report HAM2013-025. Hamilton.

Rutherford, J. C., C. C. Palliser, et al. (2011). Prediction of nitrogen loads to Lake Rotorua using the ROTAN model. Hamilton, NIWA: NIWA Client Report HAM2010-134. 178.

Rutherford, J. C., C. C. Palliser, et al. (2009). Nitrogen exports from the Rotorua catchment - calibration of the ROTAN model. NIWA Client Report HAM2009-019, NIWA, Hamilton: 62.

Rutherford, J.C., Tait, A., Palliser, C.C., Wadhwa, S., Rucinski, D. (2008) Water balance modelling in the Lake Rotorua catchment. NIWA Client Report HAM2008-048. Hamilton.

Associated Case Studies

Other Key Case Studies

Nitrogen exports from Lake Rotorua catchment - Calibration of the ROTAN Model. Rutherford, J. C., C. C. Palliser, et al. (2009)