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Simulating discharge and pollutants from the Waipa Stream catchment under different irrigation scenarios using the SAWT model

The water quality of Lake Rotorua has declined over several decades due to increasing inputs of nitrogen (N) and phosphorus (P). Prior to 1991, the Rotorua City wastewater treatment plant was a significant contributor to this decline through direct discharge of treated municipal wastewater to the lake. In 1991 the Rotorua Lakes Council (RLC) halted wastewater discharge to Lake Rotorua and commenced land disposal of treated wastewater by spay irrigation to 193 ha of the Whakarewarewa Forest. It was intended that plant uptake and microbial denitrification would reduce N levels, and P would be removed through absorption and retention to clay soil particles. The aim was to reduce wastewater nutrient loads to the point that wastewater runoff entering the Waipa Stream, and ultimately Lake Rotorua, would have a minimal impact on the trophic state of Lake Rotorua. Resource consent conditions for wastewater disposal to the Whakarewarewa Forest restricted total nitrogen (TN) and total phosphorus (TP) wastewater losses to the Waipa Stream to 30 tonnes yr⁻¹ and 3 tonnes yr⁻¹, respectively. Previous assessments of nutrient losses have indicated some non-compliance with the TN consent limit, there has also been an increase in TP loading to the Waipa Stream since 2002. These increases may be caused by a number of factors including the application rate of wastewater, increases in the nutrient load from non-irrigated areas within the catchment, forest operations not associated with wastewater irrigation and altered rainfall patterns. The Rotorua Lakes Council contracted the University of Waikato to provide an assessment of possible causes for the increased nutrient loading, and to investigate how varying the timing of wastewater irrigation regimes could affect nutrient loading to the Waipa Stream. The Soil and Water Assessment Tool (SWAT) is a GIS based catchment model used to simulate water fluxes and contaminant transport. The model was parameterised using hydrological and water quality values from relevant literature, and values derived from model auto-calibration within fixed ranges using a Sequential Uncertainty Fitting (SUFI-2) procedure followed by manual adjustment. Model results showed a high correspondence (r > 0.8; p < 0.001) between measurements and simulations of discharge. However, the model underestimated several peaks of weekly mean TSS load during high rainfall events which were found to be related to harvesting of forest blocks. Underestimates of weekly mean TP peaks were also observed and appeared to be either a lagged response to high rainfall, high TP concentrations in the irrigated wastewater or harvesting of several forest blocks concurrently. Most peaks in hourly routing of NO₃-N and TN loads averaged to weekly time scale also corresponded to high rainfall as well as to high TN concentrations in the wastewater. The effects of various wastewater irrigation scenarios on downstream water quality were also simulated using SWAT. Daily wastewater irrigation was the most effective scenario for reducing nutrient leaching and avoiding soil nutrient saturation when compared to weekly irrigation (i.e., identical total volume on a weekly basis). Cessation of irrigation on high rainfall days, with reassignment of the irrigation to the first subsequent low-rainfall day, produced unexpected small increases in annual TP (+5%) and TN loading (+2.5%). Simulations of increased area of irrigation or ceasing irrigation resulted in reductions of nutrient loads to the Waipa Stream. These results indicate that harvesting of plantation forest blocks in the Waipa sub-catchment and wastewater irrigation during high rainfall were contributing to higher than estimated TSS and TP loading to the Waipa Stream. Modelling of variations in wastewater irrigation frequency showed that reductions in TP and TN loadings to the Waipa Stream could be achieved by switching to a programme of daily rather than weekly irrigation and avoidance of irrigation during or the day following heavy rainfall. These findings support the decision to switch irrigation frequency from weekly to daily and are likely to assist in reducing the impact of anthropogenic nutrient loading on Lake Rotorua.
Type of thesis
ERI report
Me, W., Hamilton, D. P., & Abell, J. M. (2017). Simulating discharge and pollutants from the Waipa Stream catchment under different irrigation scenarios using the SAWT model. ERI report No. 98. Client report prepared for Rotorua Lakes Council. Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato
© 2017 copyright with the authors.