Introduction

Australia is the driest continent on Earth, other than Antarctica, but has mean annual rainfall ranging from 3200-4000 mm in the tropical north-east and south-west Tasmania to <200 mm in the dry interior (Figure 1).

Rainfall in most areas of New Zealand (NZ) varies from 600 mm to 1600 mm, but extremes of >6000 mm are noted on the mountainous west coast of South Island (Figure 2).

Although these Southern Hemisphere neighbors have different water resource issues many of their lakes and reservoirs have undergone similar changes since European colonization, including modified hydrology and catchment land use, and the ensuing changes in timing and magnitude of water delivery, as well as deterioration of water quality.

The Australian continent is characterized by enormous climatic diversity and is also subject to highly variable seasonal and interannual climatic conditions. Rainfall and streamflow variability in Australia are greater than elsewhere in the world, except for South Africa, which shares similar hydrology. By contrast, New Zealand has a maritime climate characterized by small seasonal and interannual variations in rainfall, and lower amplitudes of variation in temperature and humidity. The maritime climate produces deeper surface mixing in lakes in NZ than those of comparable latitudes in the Northern Hemisphere.

A major influence on climate in the southern part of Australia is El Nino. The El Nino/Southern Oscillation (ENSO) phenomenon results from interactions between large scale ocean and atmospheric circulation processes in the equatorial Pacific Ocean. El Nino is used to describe the appearance of warm water along the coast of Peru and Ecuador, and the Southern Oscillation is the alternation of atmospheric pressure differences between the Australia-Indonesia region and the eastern tropical Pacific Ocean. El Nino conditions typically result in low rainfall throughout southern Australia and tend to be associated with Australia's worst droughts. The opposing condition is La Nina, in which above average rainfall would be expected. The ENSO effect accounts for only a small fraction of annual variations in climate in NZ, with El Nino tending to increase the intensity of the predominant westerly winds, which reduces rainfall in the more arid eastern regions of the country.

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