3 Mean and Median Flow Maps
3.1 General
A series of maps (Appendix B) has been produced to show the geographical distribution of flows in the Waitaki Catchment:
- Figure 1 - Mean flow by zone shows the mean flows, including the seasonal distribution of flows, on an area-by-area (or zone) basis on a 1:600,00 scale (at A3 size) map of the Waitaki Catchment. Details of the derivation of the mean flows for each zone are provided in Section 3.2.
- Figure 2 - Mean flow at flow recording sites shows the mean flow at key locations and long-term flow recording sites in the catchment. A tabulated summary for these sites is given in Section 3.3.
- Figure 3 - Median flow at flow recording sites shows the median flow at long-term flow recording sites. A tabulated summary for these sites is given in Section 3.3.
To allow a meaningful comparison of flow statistics from different sites, it is desirable to generate statistics from each record for a common data period. The period 1980 to 2003 (complete calendar years) has been selected as the reference data period for the following reasons:
- Apart from the Power Archive sites, many of the recording sites did not have records that extended significantly earlier than 1980. That is, and importantly, the majority of sites have data from 1980 to 2003.
- The Pukaki High Dam was built and commissioned about 1980, which resulted in a paradigm shift in the operation of the power infrastructure from 1980 onwards.
- Any trends in the flow data as a result of climate change would be better captured in the more recent data compared with earlier data. That is, earlier data would be less representative of expected flow regimes in the immediate future.
- Data was truncated at the end of 2003 because the records for the 2004 year were incomplete at the time of study.
There is a suggestion that the selected data period 1980 to 2003 encompassed a particularly wet period (compared with long-term average) because of its coincidence with the positive or wet cycle of the Pacific Decadal Oscillation (PDO) from 1978 to 1999. The PDO index has recently (1999/2000) shifted to a negative cycle, which some climatologists believe heralds a return to generally drier conditions that existed before 1977.
A brief assessment of the available long-term records indicates that inflows to the upper Waitaki lakes between 1980 and 2003 averaged about 2.4% greater than from 1925 to 2004 (varies from 0% to 5.3% depending on the lake concerned). Long term flow records in the upper Mackenzie area show a mean flow about 4% greater for the period 1980 to 2003 compared with the full record from the mid-1960s to 2004.
On the main stem of the Waitaki, there is a 3.6% greater mean flow for the period 1980 to 2003 compared with the period mid-960s to 2004. However, about 1.6% (or about 6 m3/s) of this increase is attributed to the flow lost to dead storage when the hydro lakes were raised (Lake Pukaki) or created (Benmore and Aviemore) by dam construction. Thus, the net increase in mean flow from natural variability is about 2%. It is worth noting that flow records for the Waitaki Dam have been excluded from this analysis because of a suspected over-prediction of flows in the latter half of that record (see, for example, Table 3.1).
While the increased mean flow in the reference period has been linked with the cyclical pattern in the PDO, the effects of long-term climate change from global warming cannot be ruled out as a contributing factor. The extent of current investigations in New Zealand to unravel the component effects of climate change and the PDO is unclear. However, the results from any such analysis would be tenuous given the significant year-to-year variability of the parameters involved.
3.2 Mean flows by zone
The Waitaki Catchment has been divided into nine broad areas or zones, each between about 900 km2 and 2000 km2, to show the relative magnitudes of the annual and seasonal mean flows generated in the various parts of the Waitaki Catchment.
Computation of the flow statistics for the three upper Waitaki lakes was relatively straightforward. However, derivation of the mean flow parameters for the other six zones required varying degrees of analysis and inference, which were based on:
- all available flow records within that zone, including Environment Canterbury's mean flow estimates for several temporary sites
- reference to mean annual rainfall contours (e.g. Figure A.1 in the Project Aqua - Waitaki River Hydrology Study)
- reference to previous hydrological study reports, especially the 1984 report by Jowett and the 1987 report by Young
- a broad overall catchment water balance analysis.
In particular, there was a distinct lack of flow recording sites in two of the zones, namely, Benmore and Aviemore. Thus, mean flow estimates in these zones have greater uncertainty.
The flow data plotted in Figure 3.1 is summarised in tabular format in Table 3.1 overleaf.
3.3 Summary flow statistics
Mean and median flows at recording sites are plotted on Figures 2 and 3 respectively. These estimates were obtained directly from the relevant TIDEDA database supplied to Tonkin & Taylor Ltd as part of this project. Where data is available, the flow for the reference period 1980 to 2003 has been shown, otherwise the flow for the full record period is given.
Four of the sites in Figure 2, that is, Grays River at Bridge, Stony River at Slip Panel Corner, Otamatapaio River at Top Bridge and Waitaki at River Mouth, are not flow recording sites. However, estimates have been given for the expected mean flow at these locations. The flow data plotted in Figures 2 and 3 is summarised in Table 3.2.
Table 3.1: Waitaki Catchment - mean flows by zone
View Waitaki Catchment - mean flows by zone (PDF 13 KB)
Table 3.2: Waitaki Catchment - flow statistics at flow recording sites
View Waitaki Catchment - flow statistics at flow recording sites (PDF 13 KB)
