Natural water temperature variations in the lower Waikato River, New Zealand

Natural temperature in a 30 km reach of the Waikato River during autumn 1984 varied synchronously across the channel and for practical purposes can be assumed transversely uniform (within 0.03°C). Significant differences between sites were found in the amplitude and phase of diurnal temperature cycles which resulted in instantaneous temperature differences along the channel of up to 0.5°C. These were caused by a reduction in mean depth and a change in channel orientation along the study reach. The observed lag between peak radiation and maximum water temperature yielded preliminary estimates of the surface heat exchange coefficient K = 155–670 W m −2 s −1 °C −1 but there is some doubt about the accuracy of these estimates. A heat budget model was used to predict longitudinal temperature changes. Model predictions were accurate only to within ± 0.2°C (RMS error) probably because meteorological data came from sites 40–50 km away from the study reach. Uncertainties in natural temperature of about 0.5°C will make it difficult to accurately estimate excess temperatures arising from waste heat inputs.