The rapid increase in irrigation for agriculture over the past two decades is placing considerable stress on many of our rivers and streams and the freshwater ecosystems they support.
These delicate systems are a complex web of interactions between animals, plants, bed and banks and flowing water providing many services to communities, individuals and to the economy.
Amenity values of rivers include swimming holes, areas for children to play and explore, relaxing spots to cast a fly and perhaps catch a trout, and areas to picnic or just meditate on life.
Of course, river ecosystems also provide habitats for a range of native fish and wildlife and so serve a vital conservation purpose.
But to be able to enjoy these natural amenities and to maintain biodiversity, we need both clean and properly functioning streams and rivers.
This requires a commitment to setting limits on the water taken for irrigation, particularly during high summer when the natural ecosystems are already under pressure.
To help protect river values the Resource Management Act provides for regional councils to set minimum flows and allocation limits to control the amount of water taken out of individual rivers for irrigation and other uses.
A minimum flow is a level below which no further water can be taken, with a few exceptions such as drinking water and fighting fires, while allocation limits determine the total amount of water that is available for use.
These restrictive measures are being set for several Otago rivers that are under irrigation pressure, such as the Lindis, Cardrona and Manuherikia, and other systems, such as the Pomahaka, where there is less demand for river water.
Minimum flows are of greatest importance during low-flow periods over dry summers, when rivers are at their lowest and demand on water is at its greatest.
During these periods there is a very real danger of water allocation takes dropping the level of a river or stream dangerously low.
At times of low flow fish stocks are stressed by reduced habitat, loss of refuge areas, high water temperatures, and increased predation.
A minimum flow provides a base level aimed at preserving ecosystem health and natural character.
The complex nature of river ecosystems makes it hard to predict the impacts of water takes.
Yet demand for irrigation is significant and intense, resulting in pressure to set minimum flow levels well below what would be deemed a natural flow.
The optimum minimum flow would be the one an ecosystem system would reach naturally during a summer low-flow period, providing strong protection to the natural character of a stream or river by buffering against artificial reductions caused by water takes.
This natural low can be calculated by averaging the seven lowest consecutive days for each year on record, what is known as the mean annual low flow, or Malf.
The process of setting a minimum flow can be contentious, considering the number of factors that need to be taken into account.
Demand for water, existing water uses, environmental, social, cultural and economic costs and benefits all need to be weighed up.
Too often, however, too much emphasis is placed on the economic side of the ledger, without due consideration given to how this affects the other equally important - but less tangible - aspects.
Assessing the needs of fish stocks and fisheries involves estimating the habitat for different fish species through computer modelling.
Trout are good examples, as they usually have the highest requirements in terms of flow level and water quality and so, if they are looked after properly, the flow-on effects should take care of the rest of the ecosystem.
Trout also provide their own economic benefits. Sport fishing is a hugely popular activity, bringing many millions of dollars a year to New Zealand's economy.
Habitat modelling for the Lower Pomahaka found that optimum flows for adult brown trout are between 13cumecs (cubic meters per second) and 7.5cumecs, below which their habitat availability starts to decline sharply.
The Malf for the lower Pomahaka was calculated at 4.3cumecs, far below an adult brown trout's ideal water level, demonstrating that summer low flows can be a natural period of stress in a system even without irrigation pressure.
The Pomahaka is considered to be a regionally significant trout fishery as well as providing a home to more than nine species of native fish and so needs a minimum flow that will adequately reflect and protect these values.
New fish-habitat modelling methods have also shown that traditional modelling surveys may be too simplistic and inadequate indicators of the necessary flow level for trout.
In a recent study by Niwa on the Mataura River, it was shown that by using a new "energy intake" model there was a 40% increase in the recommended minimum river flow for trout from 10.2cumecs to 14cumecs, much closer to the Malf of 16.9cumecs.
This could mean several proposed minimum flows inOtago may be inadequate, at least where trout fisheries are concerned.
The interpretation of science is where much of the conflict over minimum flows emerges, as groups such as Fish and Game and Forest and Bird push for higher minimum flows to protect habitat for trout and native fish while irrigators lobby for lower minimum flows so they can have more water available for agriculture.
Regional council decisions often seem to favour irrigation supply. Past minimum flows have been set as low as 60% of the Malf.
One Government policy initiative that could help is the Proposed National Environmental Standard on Ecological Flows and Water Levels (Proposed NES), which would apply to all streams and rivers across New Zealand that do not already have minimum flows.
The proposed standard is a minimum of 80% of the Malf for larger rivers and 90% for smaller rivers and streams.
This would provide a precautionary protective measure for river flows, protecting the character of our rivers, while also ensuring the retention of native fish, trout and salmon.
Unfortunately, the NES has stalled at the Government level and is unlikely to be put in place any time soon.
Otago rivers are important for everybody in the community to enjoy and not just as a source of water for irrigation.
It is essential that we look after and protect them as a source of drinking water, for their rich and complex ecology, and for the enjoyment of being able to fish, swim and play in some of the highest quality water in the world.
Asher Flatt is a recent MSc graduate from the science and communication programme at the University of Otago. He worked for Otago Fish and Game over summer.
