Discussion

The remaining areas are considered ungauged (Figure 6.15). This large area is made up of catchments which have been gauged historically but are currently not gauged and areas which have never been gauged (Figure 6.15). Regionalisation is used to estimate the inflow from these areas. Regionalisation essentially applies calibrated parameter sets from gauged catchments to ungauged areas. The selection of an appropriate catchment is generally achieved by determining a relationship between the physical characteristics of the gauged catchment to the ungauged areas (Croke and Norton, 2004; Seibert, 1999). Parameter values can be adjusted based on the relationships found. If the gauged catchment is too dissimilar to the ungauged area then the modelled output will be prone to error (Blöschl and Sivapalan, 1995).

As a first step, streamflow time series for all historically gauged catchments are correlated. Catchments which show significantly strong relationships ( 0.8) are assessed for their hydrologic and physical similarities. This provides an indication of possible suitable parameter sets for estimation of streamflow in currently ungauged areas.

Significantly high streamflow cross-correlations (Table 6.5) are found between the four catchments of the western bays. Despite high streamflow correlations, the Whareroa displayed some hydrologic dissimilarity to the other three catchments.

The Whareroa catchment had lower variability, lower climatic indices and specific discharge. The Kuratau, Whanganui and Waihaha catchments, on the other hand, show some hydrological similarity in terms of wetness ratio, runoff coefficients and

144 | A rainfall-runoff model for the Lake Taupo catchment

Figure 6.15 Ungauged areas of the Lake Taupo catchment. The Waihaha and Whanganui catchments are estimated directly using the parameters of the Kuratau catchment.

Figure 6.16 Ungauged areas estimated using the regionalised Kuratau parameters and regionalised Tauranga-Taupo parameters.

A rainfall-runoff model for the Lake Taupo catchment | 145 Table 6.5 Cross-correlations of streamflow data between the Kuratau, Whareroa, Whanganui and Waihaha catchments.

Kuratau Whareroa Whanganui Waihaha

Kuratau 1

Whareroa 0.8661 1

Whanganui 0.9542 0.8839 1

Waihaha 0.8589 0.7947 0.9684 1

specific discharges. Flow variability is also similar between the three but over the low flow conditions, the Kuratau is less variable. For these reasons, the parameters of the Kuratau catchment are used to estimate inflow from the currently ungauged Whanganui and Waihaha catchments. The Whareroa catchment is included in the larger ungauged area of Lake Taupo.

Streamflow from the remaining ungauged areas (1287 km², Figure 6.16) is estimated from regionalised parameters of calibrated catchments which are the most physically similar to the ungauged areas. Based on this, the ungauged areas are divided into two sub-areas. The first area includes the northern bays and the remaining ungauged section of western areas. The area south of Taupo township and the Waitahanui catchment are included in this area. This accounts for approximately 75% (1003 km²) of the remaining ungauged areas. The parameters of the Kuratau catchment are regionalised for this area. The Kuratau parameters are chosen because this catchment is quite representative of the diverse land cover and soils across this area. For the area south of the Hinemaiaia catchment around to and including the Waihi catchment, the parameters of the Tauranga-Taupo catchment are selected for regionalisation. This ungauged area (284 km²) drains some of the steep slopes of the Kaimanawa Ranges and smaller volcanoes (e.g. Pihanga) near Turangi. The geology of the area is very similar to the Tauranga-Taupo catchment, which is considered a significant influence on runoff generation (see Chapter 5).

While these parameter sets would normally be adjusted to account for differences between the sub-catchments, it is assumed that the parameter sets used are adequate for the purpose of this study, although this is an area that could benefit from further study. The only parameters that are calibrated are the rainfall multiplier and lag times. Calibration, however, requires a streamflow time series. In this study, this time series is derived from the residual between the change in lake

146 | A rainfall-runoff model for the Lake Taupo catchment

level ( ), outflow ( ), gauged inflows ( ), lake evaporation ( ) and direct lake rainfall ( ) so that

where is derived from the lake level time series generated using a Butterworth filter to remove unwanted seiche and other oscillatory effects, as described in Section 4.4. However, while the filter has removed many of the high frequency oscillations, many other longer period oscillations still exist. As a result, the filtered lake level still shows some considerable fluctuations in lake level which correspond to considerable negative fluxes that are not physically realistic. This is reflected in the derived ungauged time series illustrated in Figure 6.17 (top).

Closer inspection of the filtered average lake level time series show that these oscillations generally balance out over time. These „see-saw‟ oscillations (Thompson and Ibbitt, 1978) are a result of changes in the driving force of the seiches (Figure 6.17, lower). For example, a change in wind direction will release water which has been pushed toward a certain direction causing some „sloshing‟ back and forth.

Similarly, barometric changes alter lake levels as the pressure gradient passes over the lake, after which normal levels return (Thompson and Ibbitt, 1978).

Figure 6.17 Upper: Ungauged inflow time series derived from known inflows, outflow, lake level change and direct precipitation and potential evapotranspiration. Lower: these oscillations generally balance out as a flux in one direction is often followed by a corresponding flux in the opposite direction. Most

A rainfall-runoff model for the Lake Taupo catchment | 147 Since the model in this study is not permitted to predict negative inflow to Lake Taupo there will be some discrepancy between simulated and „observed‟ inflow from ungauged areas. Calibration is undertaken on the cumulative distribution of modelled and observed time series to ensure the overall mass balance is achieved.

Since these ungauged areas cover some considerable distances, rainfall is based on area averaged volumes using Thiessen polygons.