Precipitation distribution in the Lake Pukaki Catchment, New Zealand

The Lake Pukaki Catchment, New Zealand is nationally important for hydro electricity, conservation and tourism. From water balance estimates the 1359 km² catchment has an average annual precipitation of 3400 mm. Undercatch corrected precipitation measurements indicate an average annual precipitation of 1400 mm in the easily accessible south eastern half of the catchment. That leads to 5400 mm average annual precipitation across the remainder of the mountainous glaciated north western half of the catchment. The catchment is located within the southern hemisphere westerly wind belt at 43^o 48' S, is lee of the south-west to north-east running main divide of the Southern Alps and is less than 30 km from the Tasman Sea to the west. The predominant moisture laden westerly conditions explain the high precipitation in the north west of the catchment. The only long term precipitation gauge in this high precipitation region has an undercatch corrected 1971-2000 average annual precipitation total of 4700 mm. This single gauge site provides no information as to the distribution of precipitation in the upper catchment. Such information is required as it has considerable impact on glaciation, snow storage, runoff and lake inflows. Previously, precipitation distribution has been estimated to suit a dedicated application (e.g. runoff, glaciation, snow storage, avalanching, climatology, erosion) with a variety of results.

All available records from historic short and long term precipitation gauges together with ten new gauges have been analysed to estimate undercatch corrected 1971-2000 average annual precipitation normals. These normals have been interpolated to provide a new measurement-based precipitation surface for the catchment. Precipitation in excess of 10000 mm is estimated along the north western boundary of the catchment. This is similar to estimates made in other regions of the South Island of New Zealand, but never in a catchment lee to the main divide. The high magnitudes and horizontal precipitation gradient approaches values reported in Hawaii and Patagonia.

Average annual precipitation surfaces have limited value in applications requiring daily distributed precipitation estimates. NCEP/NCAR reanalysis enables a determination of daily synoptic air flow direction. Cluster analysis of precipitation parameters determined for 10^o sectors of this wind flow led to the identification of 5 wind direction classes. Precipitation distributions for these five different wind direction classes have been prepared. These distributions show that northerly conditions have a greater average daily magnitude of precipitation extending over a greater proportion of the upper catchment compared to the more predominant north westerly winds, while the southerly class results in a more even but lower magnitude precipitation field over the entire catchment. The wind classed precipitation distributions enable daily precipitation distributions to be determined given the synoptic flow direction and the gauge catch from at least one site in the catchment. These distributions are to replace average annual distributions currently used in snow storage models with a view to improving lake inflow forecasting.