Water flows vary naturally through time and between water bodies due to climate, topography, land cover, and underlying geology. Natural variability in the flow of water is important for the health and mauri of freshwater ecosystems, and the services they provide.
Climate is a natural driver of flow regimes, including the availability of water, which is predicted to change in the future due to the effects of climate change (see Projected effects of climate change).
Projected effects of climate change
New Zealand’s climate is projected to get warmer through the 21st century (Reisinger et al, 2014). This warming is expected to cause rising snow lines, reduced glacier volumes, increased westerly winds, and changes in the frequency and severity of droughts, rainfall patterns, and evaporation rates. Water flows and water availability are projected to change because of these factors, consequently affecting our freshwater ecosystems and species (Robertson et al, 2016).
A warmer climate will lead to higher water temperatures, which have implications for water quality, including reduced levels of dissolved oxygen and potential increases in algal blooms (Hamilton et al, 2013). This may affect the number and distribution of many freshwater species.
In the South Island, annual rainfall is projected to increase in the west and south and decrease in the northeast (Ministry for the Environment, 2016). In the North Island, annual rainfall is projected to increase in the west, and decrease in the east and north (Ministry for the Environment, 2016). Changing rainfall patterns and increasing evaporation rates may mean irrigation demand will increase in some parts of the country. Water quality may deteriorate because of lower flows (due to reduced rainfall) in areas that get dryer. Higher erosion rates may ensue (due to increased rainfall) in wetter areas, although this will depend on other factors, such as vegetation growth and cover. New Zealand’s hydroelectric power generation is also sensitive to changes in rainfall patterns and the availability of water.
New Zealand has plenty of fresh water but it’s not always where and when we need it
Rainfall and snow melt influence the amount of water in our rivers, lakes, and aquifers. By international standards, New Zealand has a plentiful supply of fresh water (OECD, 2007). Our low average population density and high average rainfall mean the amount of fresh water per person is higher than most countries. We have approximately 711 billion cubic metres stored as groundwater in aquifers, 320 billion cubic metres in lakes, and 440 billion cubic metres flowing through rivers and streams each year (Ministry for the Environment and Statistics NZ, 2015).
Our rainfall varies year on year. On average, New Zealand receives 550 billion cubic metres of precipitation a year – enough to fill Lake Taupo nine times over (Collins et al, 2015). Rainfall replenishes our rivers, streams, lakes, and groundwater, but it is not uniform across the country. Rainfall is generally much higher on the western side of the North and South islands – the West Coast of the South Island received 26 percent of national precipitation between 1994 and 2014 (Collins et al, 2015). After standardising for different catchment sizes, natural river flows are higher in some catchments than others. For example, river flows are naturally lower on the eastern side of the South Island compared with the western side, except for the large rivers that flow east from the Southern Alps, such as Waitaki, Rakaia, and Rangitata (Booker, 2015).
For more detail see Environmental indicators Te taiao Aotearoa: Geographic pattern of natural river flows [Stats NZ] and Water physical stocks: precipitation and evapotranspiration [Stats NZ].