A water quality model for Lake Tikitapu

Abstract

Lake Tikitapu is a small lake in the Rotorua Te Arawa Lakes region which is extensively used for recreation and sporting events, and is of cultural significance to Iwi (Te Arawa). It is an attractive and popular lake with oligotrophic-mesotrophic water quality. Tikitapu's target Trophic Level Index (TLI; Burns 1999) is 2.7, whereas a TLI of approximately 3 was observed between 2000 and 2010. An Action Plan has been established for Lake Tikitapu (BoPRC 2011) and the proposed actions have now been completed (as at 2015), including the reticulation of the lakeside wastewater systems. This report describes the establishment of a one-dimensional hydrodynamic-ecological computer water quality model (DYRESM-CAEDYM) for Lake Tikitapu. The modelling process includes calculations for catchment and lake water balances, as well as estimation of nutrient loads to the lake. The simulation period spans from 2001 to 2010. The model has been calibrated and validated, and model performance is acceptable relative to other DYRESM-CAEDYM lake applications documented in the literature. The established model can be used for simulating scenarios of lake management actions, and can be considered a 'decision support tool'. Initial simulations are presented for increased diffusion of silica from lake sediments (C. Hendy, pers. comm.), and for reticulation of lakeside wastewater systems. Septic systems comprise a large fraction of estimated annual nutrient loads, particularly for phosphorus. As such, simulation of the removal of septic tank nutrient loads indicated that this action alone may be sufficient to meet the TLI target for the lake.