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Flows of water and nutrients to Lake Tarawera and connected lakes

Lake Tarawera is a large, deep lake located within the Taupō Volcanic zone, 12 km south-east of Rotorua. Seven smaller lakes within the “Greater Lake Tarawera catchment” (Lakes Ōkareka, Ōkaro, Ōkataina, Rerewhakaaitu, Rotokakahi, Rotomahana and Tikitapu) contribute flows to Lake Tarawera via groundwater and/or surface water. The quantity and quality of flows from these connected lakes is important in assessing hydrological inputs to Lake Tarawera. Groundwater is recognised as a large component of the flow to Lake Tarawera, and previous investigations have been undertaken in the Greater Lake Tarawera catchment to assess groundwater resources and groundwater quality. Despite the extensive work of past researchers, there remain substantial uncertainties on the hydrology and chemistry of groundwater fluxes to Lake Tarawera. This research aims to improve understanding of groundwater flows in the Greater Lake Tarawera catchment using a water balance approach and to assess and describe the fluxes of water and nutrient loads to Lake Tarawera and hydraulically connected lakes under different climate and land use scenarios. The stable isotopes of water, ẟ²H and ẟ¹⁸O, in groundwater and surface water have been used alongside water balances to supplement understanding of groundwater flow paths and hydraulic connections between lakes. While previous research has focused on annual means to describe water fluxes and nutrient loads, with no expression of uncertainty, this research describes a range of annual water fluxes and nutrient loads. The ranges bracket the feasible possibilities of fluxes and loads, and account for interannual climatic variation, scientific uncertainties, and land use change, providing a means for policy makers to test outcomes of potential policy decisions. Analysis of stable isotopes of water have been undertaken from groundwater collected from 11 locations and surface water from 26 locations. Isotopic analysis suggests that the location of lake-to-lake groundwater connections are not widespread and isolated to localised areas. Flow gains observed in the Tarawera River downstream of the Tarawera Falls are largely sourced from rainfall. Annualised water balances and nutrient loads from 1972 to 2018 have been quantified for the Greater Tarawera Lakes and their catchments. The workflow has been automated using the R statistical computing language and a web-based tool that performs water balance calculations and nutrient load estimates has been developed using Shiny R technology. It is available for viewing at https://nickiwilson.shinyapps.io/TaraweraMsc/. Evaporation can have a significant effect on the water fluxes and nutrient load to the lakes. This research considers land use, land cover, and soil water stress in the calculation of evaporation and provides the most accurate evaporation estimations in the Greater Lake Tarawera catchment to date. Groundwater contributes 62% of the inflows to Lake Tarawera, and 70% of this groundwater is estimated to have been recharged from outside of its local catchment. Approximately 20% of the current nutrient load to Lake Tarawera is received from the catchments of its connected upstream lakes. Management of nutrient loads to Lake Tarawera should consider flow and nutrient loading in upstream lake catchments, and opportunities for improvement include better land management practices and the reticulation of wastewater.
Type of thesis
The University of Waikato
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