Resources
Below are some useful resources and reports from previous water quality studies on Lake Rotorua:
Websites:
2025
Gibson et al. (2025)
Downscaled CMIP6 future climate projections for New Zealand: climatology and extremes.
Click to expand: Summary
- National average warming of 3.1°C by end-of-century, with enhanced warming at high elevations compared to GCMs.
- Largest increases in summer mean temperatures, with daily maximum temperatures warming faster than minimums.
- Wetter winters and drier summers projected, with high uncertainty in summer precipitation changes.
- Substantial increases in heatwave frequency, duration .
2023
Özkundakci (2023)
Lake Rotorua science review, summary (Environmental Publication 2023/09).
Bay of Plenty Regional Council (Toi Moana). Whakatāne, New Zealand.
PDF
Click to expand: Summary
Brief summary text here.
2022
Hamill (2022)
Trend and State of Nutrients in Lake Rotorua Streams 2002–2022.
River Lake Ltd for Bay of Plenty Regional Council.
PDF
Click to expand: Summary
The report focuses on nine major streams contributing to Lake Rotorua’s water quality. - Nutrient trends were assessed using the seasonal Kendall test over three periods: 2002-2022, 2002-2010, and 2010-2019. - The analysis includes total nitrogen (TN), total phosphorus (TP), and dissolved reactive phosphorus (DRP) concentrations and loads.
Nutrient Concentrations and Stream Flows This section details the mean flow rates and nutrient concentrations in the streams entering Lake Rotorua.
- The mean flow to Lake Rotorua from 2002-2022 was 11.8 m³/s, with Hamurana being the largest contributor at 2.63 m³/s.
- The highest total nitrogen concentrations were found in Waiohewa (2839 mg/m³) and the highest total phosphorus concentrations in Waingaehe (123 mg/m³).
- Streams like Hamurana, Awahou, and Waingaehe showed high concentrations of dissolved reactive phosphorus (DRP), with ratios of DRP to TP between 80-90%.
Trends in Nutrient Concentrations Over Time This section discusses the observed trends in nutrient concentrations across the monitored streams.
- From 2002 to 2022, TN concentrations increased in Hamurana, Awahou, and Waiteti, particularly in Awahou and Waiteti since 2010.
- TN concentrations decreased in Puarenga and possibly Ngongotaha, but this trend did not continue post-2010.
- TP concentrations increased in Hamurana, Ngongotaha, Utuhina, Puarenga, Waingaehe, and Waiohewa, with trends continuing since 2010 in all but Waingaehe.
Nutrient Loads to Lake Rotorua
- The nine main streams transported an average of 410 t/y of nitrogen and 28 t/y of phosphorus from 2002-2022.
- The highest TN loads came from Awahou (71.1 t/y), Puarenga (67.4 t/y), and Hamurana (66.5 t/y).
- TP loads were primarily contributed by Hamurana (7.5 t/y), Puarenga (5.1 t/y), and Awahou (3.9 t/y).
Mangeya (2022)
Groundwater Modelling Report.
Jacobs for Bay of Plenty Regional Council (Toi Moana).
Click to expand: Summary
Project Background and Objectives
The Bay of Plenty Regional Council engaged Jacobs to develop a groundwater flow model for the Rotorua Water Management Area to enhance water management practices. The Rotorua Water Management Area covers approximately 1,100 km². The model aims to improve understanding of groundwater systems and surface water interactions. It will inform groundwater allocation limits and assess various abstraction scenarios.
Scope of Work for Groundwater Modelling
The project involved several key tasks to develop and document the groundwater model.
- Analyze available data and create a conceptual hydrogeological model.
- Develop and calibrate a transient three-dimensional numerical groundwater flow model using MODFLOW.
- Simulate predictive scenarios and document the model development and predictions.
- Deliver the numerical model and associated files to the Bay of Plenty Regional Council.
Limitations of the Groundwater Modelling
- The model does not account for certain factors that could affect groundwater and surface water quality.
- Excludes the impact of groundwater extraction on water quality. Does not consider density effects from variable groundwater salinity or temperature. Ignores two-phase fluids in the active geothermal reservoir.
2021
McBride, MacCormick, and Verburg (2021)
Estimated catchment loads of nitrogen and phosphorus to the Rotorua Te Arawa Lakes.
ERI report 143, University of Waikato.
PDF
Click to expand: Summary
Importance and Context
Rotorua Te Arawa Lakes are culturally, historically, and ecologically significant.
Water quality varies with catchment characteristics: undeveloped (e.g., Tikitapu, Okataina) vs. highly developed agricultural/forestry (e.g., Rotorua, Rerewhakaaitu, Okaro).
Lake water quality is strongly influenced by nutrient inputs from surface and groundwater flows.
Methodology for Nutrient Loads
Used updated export coefficient approach and Overseer® modelling for nitrogen (N) and phosphorus (P).
Land use areas calculated for surface and groundwater catchments.
Nutrient loss rates estimated per land use, accounting for local factors (e.g., pest impacts, forest removal).
Additional sources included geothermal inputs, septic systems, municipal wastewater, atmospheric deposition, and hydrological connections among lakes.
Attenuation estimated from physiography to produce a consistent regional nutrient budget.
Key Findings and Implications
Agricultural land contributes disproportionately to N and P loads.
Geothermal, atmospheric, and connected-lake sources are significant in some catchments.
Many catchments exceed ‘reference’ nutrient loads by 2–3×, complicating restoration.
Provides basis for assessing internal lake loading and guiding catchment/lake management strategies.
Example Nutrient Loads (kg/yr)
- Lake Rotorua: N 721,427, P 56,760.5
2019
McBride, Abell, and Hamilton (2019)
Long-term nutrient loads and water quality for Lake Rotorua: 1965–2017.
ERI Report 123, University of Waikato.
PDF
Click to expand: Summary
Summary…
2018
Me et al. (2018)
Modeling the effects of climate change on water quality of Lake Rotorua, New Zealand.
Click to expand: Summary
Modelling Hydrology and Water Quality This study integrates a catchment model with a lake water quality model to assess the impacts of nutrient load reductions and climate change on a eutrophic lake.
- The study combines SWAT and DYRESM–CAEDYM models to simulate nutrient dynamics and water quality.
- The models performed satisfactorily in simulating observed data, indicating accurate representation of key processes.
- Nutrient load reductions from agricultural practices had minor effects on lake nutrient concentrations.
- Climate change projections for 2090 showed significant impacts on nutrient levels and water quality.
Nutrient Load Reductions and Their Effects The study evaluates how reducing nutrient loads affects the trophic state of Lake Rotorua.
- Simulations indicated that reducing fertilizer and wastewater irrigation had limited impact on lake nutrient concentrations.
- The projected climate change for 2090 is expected to increase internal nutrient loading significantly.
- Polymictic lakes like Lake Rotorua are particularly vulnerable to eutrophication due to climate change.
Climate Change Impacts on Water Quality This section discusses the influence of climate change on hydrological cycles and nutrient dynamics.
- Increased precipitation is predicted to raise total phosphorus loads by 3.3%–16.5% in Danish streams.
- A study in Slovenia projected total nitrogen losses could increase by 5.3% to 80.2% due to climate change.
- Climate change is expected to directly affect lake water temperature and stratification, altering nutrient dynamics.
2015
Abell, McBride, and Hamilton (2015)
Lake Rotorua Treated Wastewater Discharge: Environmental Effects Study ERI Report 80, University of Waikato. PDF
Click to expand: Summary
Key Findings
- The treated wastewater discharge contributes a measurable nutrient load to Lake Rotorua, which may influence the lake’s trophic status, algal growth, and water-quality trends.
- There is evidence of spatial variability in how the discharge impacts the lake — e.g., nearer to the discharge site showing different water quality versus more remote parts of the lake.
- The effectiveness of the current wastewater treatment and discharge regime is assessed and found to have limitations: even treated wastewater can contribute significant nutrient inputs if volumes are large and/or treatment is not optimised.
- The report discusses the lake’s capacity to assimilate or dilute the nutrient load, and whether it is being exceeded or approaching thresholds that may degrade ecological health.
Implications & Recommendations
- Management of wastewater inputs to Lake Rotorua must account not just for concentration of nutrients in the effluent but for volume of discharge and how that interacts with lake hydrodynamics (mixing, stratification, residence time).
- Upgrading treatment processes, reducing nutrient concentrations further, or reducing the volume of discharge may be necessary to protect lake health.
- Monitoring of water quality and ecological responses (algal blooms, oxygen levels, benthic habitat) should continue and be linked explicitly to discharge data to evaluate trends.
- Lake management strategies should take a holistic view: catchment land-use, inflows, internal nutrient cycling, and wastewater discharges all interact to determine lake condition.
2013
Scholes (2013)
Trends and state of nutrients in Lake Rotorua streams 2013.
Bay of Plenty Regional Council.
PDF
Click to expand: Summary
Summary…
2012
Hamilton et al. (2012)
Predicting the effects of nutrient loads, management regimes and climate change on water quality of Lake Rotorua. (pers. comm. 2025-11-24)
Click to expand: Summary
Lake Rotorua’s Water Quality Challenges Lake Rotorua has faced significant water quality decline due to nutrient loads and land use changes, necessitating a comprehensive management strategy.
- Lake area: 80 km², mean depth: 10 m.
- Cultural significance to the Te Arawa people and vital for tourism and recreation.
- Water quality deterioration began in the 1960s, linked to treated wastewater inputs and land use changes from forest to pasture.
- Urban wastewater discharge ceased in 1991, but ongoing pastoral expansion, especially dairy farming, has continued to impact water quality.
- Legacy effects from historical land use and nutrient pools in sediments complicate current management efforts.
Modelling Approach for Water Quality Management A modelling approach is essential for assessing the impacts of land use, climate change, and management actions on Lake Rotorua’s water quality.
- Integrated multiple models: SimCLIM, ROTAN, CLUES, DYRESM-CAEDYM.
- SimCLIM predicts climate data (temperature, rainfall) for the Rotorua catchment up to 2100.
- ROTAN simulates streamflows and nitrate transport between surface water and groundwater.
- CLUES estimates annual average phosphorus loads in streams.
- DYRESM-CAEDYM models vertical distribution of water temperature and nutrient cycling relevant to trophic state.
Nutrient Load Reduction Requirements To improve water quality, significant reductions in nitrogen and phosphorus loads are necessary.
- At least 350 tonnes of nitrogen and a similar amount of phosphorus must be removed annually.
- Target Trophic Level Index (TLI) value set at 4.2 to restore water quality.
- Climate change is expected to exacerbate challenges, increasing stratification and internal nutrient loading.
- Internal nutrient release from sediments is comparable to external loads, complicating management.
Internal Nutrient Loading and Management Strategies Addressing internal nutrient loading is critical for achieving long-term water quality goals in Lake Rotorua.
- Internal loading from sediments contributes significantly to nutrient levels, especially during stratification.
- Chemical flocculants may reduce internal loading but have limited duration of effectiveness (approximately four years).
- Effective sediment capping can offset water quality deterioration but requires prior reduction of catchment nutrient loads.
2009
Burns, McIntosh, and Scholes (2009)
Managing the lakes of the Rotorua District, New Zealand.
Lake and Reservoir Management, 25(3), 284–296.
DOI
Click to expand: Summary
Summary…
2005
Rutherford (2005)
Modelling the Effects of Groundwater Lags on Nitrate Inputs to Lakes Rotorua & Taupō.
In MODSIM 2005, pp. 2749–2754.
PDF
Click to expand: Summary
Summary…