New Zealand is water
wealthy with a wide diversity of lakes and constantly flowing
rivers. Over 200,000 kilometres of rivers network through New
Zealand's mountainous terrain. They tend to be short and drain
directly into the sea from steep sloped terrain. The longest river,
the Waikato, is only 400 kilometres long - a very short river
by world standards.
Over 300 cubic kilometres of rain fall on New Zealand, of which and estimated 2 cubic kilometres are used for domestic, agricultural, and industrial purposes. Hydroelectric power generation uses about 100 cubic kilometers of water but of course this water may be used several times as it flows through dams and down rivers.
The highest levels of rainfall are on the windward, forested slopes of mountains where sea breezes are lifted into the cooler highlands. The higher the mountains, the higher the rainfall. The local difference in rainfall may vary between 12 meters per annum on the windward slopes of the higher mountains of South Island to less than 400mm per year in the leeward rain shadow. Forest mountain slopes are, therefore, of great importance to the supply of water for lower regions. Fresh water springs and lakes of the mountains flow into the drier flatlands and into ground water reservoirs.
New Zealand is subject to long periods of drought followed by heavy rains. Deforestation and agriculture has upset the natural capacity of the land to generate, catch and hold water in the rainy season for slow discharge during the dry season. During the droughts, soil becomes dry and powdery and much is lost through wind erosion. Wildlife does not fare as well as people during droughts. When surface lakes and rivers dry up, wildlife have no place to drink. This adds to a range of other stresses on declining populations of wildlife. Lowering of river and reservoir levels by drought is amplified by use of the surface water supplies for agriculture and other human activities.
Loss of wetlands and forests also results in rapid run-off of stormwater. Heavy rains may swell river volumes by over 20 times, resulting in flooding and extensive damage to the homes and businesses built on the river flood plains.