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Factors contributing to the unnaturally low water table of Moanatuatua Scientific Reserve, Waikato, New Zealand

Water table regime in peatlands (depth below the peat surface) is an important driver in biochemical processes. A deep water table can lead to increased rates of peat decomposition resulting in surface subsidence, release of carbon and changes to the vegetation cover. This study focused on the factors responsible for the unnaturally deep and highly fluctuating water table regime at Moanatuatua, a remnant peat bog, in contrast to the hydrologically pristine Kopuatai bog, with a shallow, more stable water table. Moanatuatua Scientific Reserve in the Hamilton basin is the 1.1 km2 remnant of a former 75 km2 raised peat bog, and Kopuatai bog covers an area of 96 km2 on the Hauraki plains. Native bog vegetation in the Waikato is dominated by two species from the Restionaceae family of vascular plants, Empodisma robustum and Sporadanthus ferrugineus. Water table measurements at Moanatuatua were obtained from nine pressure transducers across an east to west transect of the bog. At Kopuatai, water table measurements were taken from a single reference site. At both bogs evaporation and rainfall were measured by eddy covariance towers. Three years of measurements (1 September 2015 – 31 August 2018) were used to compare the two sites. The water table at Moanatuatua was consistently very deep for a peat bog and had a strong seasonal cycle resulting in a deeper water table in summer and a shallower water table in winter, with a mean depth of -601 mm below the surface for the three-year study period. Kopuatai also showed a seasonal pattern although the water table did not reach the same extreme depths with a mean water table depth of -25 mm. The water table at Moanatuatua followed the domed shape of the peat surface, but remained well below the peat surface for the duration of the study. The pattern of water table depth across Moanatuatua indicated that the border drains do not cause a drawdown effect on the water table across the entire transect of the bog. Similar water table depths were found 18 years ago suggesting that the hydrological regime at Moanatuatua has not altered much in this time. Evaporation rates from Moanatuatua were higher than Kopuatai. Mean daily evaporation from Moanatuatua was 2.2 mm and Kopuatai 1.8 mm for the entire study period. Eliminating wet canopy influences the evaporation rate at Moanatuatua was 2.0 mm day-1 and at Kopuatai of 1.45 mm day-1. Mean annual water balance inputs into the bog (rainfall – evaporation) were 587 mm at Moanatuatua and 872 mm at Kopuatai. If the late successional vegetation at Moanatuatua were replaced with early successional vegetation as present at Kopuatai, it is estimated that an average water balance would have been of 741 mm, resulting in more water available for potential water table recharge.
Type of thesis
Daws, C. M. (2018). Factors contributing to the unnaturally low water table of Moanatuatua Scientific Reserve, Waikato, New Zealand (Thesis, Master of Environmental Sciences (MEnvSci)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12635
The University of Waikato
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