4. Assumptions and limitations
The following is a list of assumptions and limitations that evolved during the development of this report on groundwater knowledge for the Waitaki Catchment. The limitations are mainly data specific; however interpretation or estimation of aquifer functionality based on limited information has been provided in this report.
- There is limited borehole information in the upper Waitaki Catchment including the Hakataramea Valley. Some areas, namely the Omarama and Twizel townships, have a reasonable number of bores drilled nearby. However, there are vast areas where no data for groundwater level, aquifer occurrence or aquifer hydraulic characteristics exists, such as the Tekapo and Twizel Basins. Thus, hydrogeological interpretation of the Tekapo and Twizel Basins is based on few widely distributed data points, including interpolation of surface water features and visual observations made during SKM's site visit.
- The upper extents of the Tekapo and Twizel aquifer basins are poorly controlled. The exact location of groundwater divides in those areas is unclear.
- Multiple layered aquifer horizons in the upper Waitaki Catchment are possible. However, there is insufficient data to assess the presence or extent of confined or semi-confined aquifers in groundwater basins.
- Virtually no data is available for Benmore Terrace, Aviemore and Waitaki terraces, and the Hakataramea Valley.
- Stream flow loses and gains for the majority of aquifer basins have not been specifically investigated. However, other environmental considerations such as depth to water table and piezometric contouring of groundwater, provides some information to determine the potential flow loss/gain areas of streams. Aquifers with reasonable reporting of stream flow loss/gains include the Maerewhenua and lower Waitaki basins.
- Aquifer hydraulic properties and saturated thicknesses is not well known in the upper Waitaki Catchment and the Hakataramea Valley. Hence, estimates of hydraulic conductivity, aquifer saturated thickness and specific yield, used to calculate total stored groundwater volumes in this report are considered broad estimates only.
- Rainfall recharge infiltration rates have not been investigated for the upper Waitaki Catchment. The infiltration rate estimate used in this report for the upper Waitaki Catchment reflects a typical Central Otago-Upper Clutha Valley situation, based on research done there. As such, the recharge estimate is a broad attempt to quantify the potential amount of rainfall recharge to the surface of each aquifer basin. Given the rainfall gradient in the upper Waitaki Catchment, rainfall infiltration may be highly variable.
- Recharge inputs to the aquifer basins from elevated catchments in the form of groundwater throughflow or surface runoff infiltration is not known. Further assessment of the significance of these recharge contributions would help to refine the total rainfall recharge estimate. It is considered that by omitting this recharge component, the rainfall estimates provided for each basin are conservative.
- Irrigation use and potential recharge from excess irrigation infiltration has not been quantified in this report for the upper Waitaki Catchment, as there does not appear to be any published data to make any reliable estimates. Due to the limited irrigation in the upper Waitaki Catchment, it is regarded as a minor recharge input to groundwater.
- Time dependent groundwater level data to assess the seasonal variability of groundwater and response to recharge events in the upper Waitaki Catchment does not exist. There is however data for the lower Waitaki Catchment (either manual or automated measurements).
