Lee Valley Irrigation

A proposed dam across the Lee Valley high above the Tasman districts Waimea Plains could provide a unique solution to seasonal water shortages and low river flows for the next 100 years. By storing water during wet periods and controlling its release during dry weather and at times of peak irrigation demand, the scheme could provide enough water for irrigating most of the Plain and for growing urban demand while still ensuring minimum flows and meeting the needs of recreational users and iwi.

The Waimea Water Augmentation Committee (WaiWAC) has been working on this project for seven years and is now at the point where detailed background work and planning has been done so that final approvals can be sought and finance raised. The Committees has been successful so far because right from the start efforts were made to involve representatives of all community interest groups.

The Waimea Plain in the Tasman District is a highly productive area of land boasting vineyards, market gardens, orchards, lifestyle blocks, a few larger farms and a handful of settlements and townships. Its soil qualities, contour and access to water along with the sunny Nelson climate have made it ideal for horticulture, so larger dairying and dry stock farms have been subdivided and land use has become more intensive in terms of higher value products as well capital investment in plant and infrastructure.

Water is the key to productivity. The Waimea River runs through the Plain it created into Tasman Bay, supplying many aquifers along the way. Users either pump directly from the river or from the aquifers, and in a good year there is enough to go around.

However, in an average to dry year water is the limiting factor. For historical reasons the 3800 hectares that are currently irrigated are way over-allocated, according to Joseph Thomas, a Tasman District Council hydrology engineer.

Originally acceptable minimum flow in the river was 225 litres per second but work done more recently says indicates it should be at least 800 litres to sustain life in the river and meet environmental, recreational and iwi requirements, he says.

To meet the Council's new criteria and to give security of supply, the available water should have been allocated for only about 1100 ha rather than the 3800ha that are currently irrigated.

The original water allocations were based on a design that envisaged water restrictions one year in 10, but recent experience is that demand exceeds supply in seven out of 10 years. It became clear some years ago that existing users were at risk of either losing their water rights or having severe restrictions imposed at key times, typically in late summer as many crops are nearing harvest.

Something obviously needed to be done, and a small group of concerned people got together to see what they could achieve. The result was WaiWAC The Waimea Water Augmentation Committee. A nearby smaller irrigation project that had faced a similar problem and found a solution provided information and experience that the Committee was able to learn from. They resolved to:

Involve as many community and interest groups as possible

Be totally open with their discussions and planning

Plan for a 100 year life of the project

The groups represented on the Committee include the Tasman District Council, Federated Farmers, horticultural and agribusiness interests, Forest & Bird, and local iwi.

The demand modelling was quite comprehensive and had to take into account what was likely to happen well into the future population growth, changes in the types and intensity of farming and horticulture, how the climate might change, what minimum river flows would be required etc. Surprisingly, projections were made with considerable confidence the current irrigated area is 3800 ha and its usage and shortfall are well documented; there is only 2000ha more in the district that can potentially be irrigated, so the total potential irrigation demand month by month could be calculated. Current urban usage is known and makes up only 15% of total demand so even if current population projections are a little off beam the effect on the total requirement will be small. Also known are the minimum flow levels in the river that are needed to preserve river life and provide for recreational and iwi needs.

All this has given the Committee a pretty good handle on what flows are needed and when. Climate data and rainfall records give a good indication of the variation in actual flows that might occur from year to year. Processing all the information they have been able to estimate quite accurately the volume of water that would need to be stored annually to meet demands in average and drought years for the next 50 years at least.

Of the many options looked at, the most promising and practical was to harvest and store rain in a high rainfall area that already feeds into the Waimea Plain river system. The upper reaches of the Lee Valley behind Brightwater form a suitable catchment across which a dam could be built.

Such a scheme would have many advantages. No pipes would be needed to deliver water to the Plain since the river system already does this. Users could use their existing pumps and bores and so the only new infrastructure needed would be for areas not already irrigated. The amount of water released from the dam could be varied as necessary to maintain at least the minimum river flows that would ensure there was sufficient water for all concerned in the river and the aquifers it feeds. Also, in very dry summers there would be no risk that the lack of flow would allow seawater to seep inland and contaminate aquifers, as can happen at present. The guaranteed availability of water would increase the productivity of the land and give growers the confidence to invest in a wider range of intensive cropping. Security of water would also benefit the urban population in Tasman and possibly even provide for the future demands of Nelson city.

Another possibility for the scheme is a small hydroelectric power plant that could be driven by the continuous flow from the dam. If a dam of the preferred size goes ahead such a plant could generate around 6.3 GW hours which could go directly into the substation of the nearby town of Brightwater and supply that community with all their power needs throughout the year.

Meeting the expected water demand scenario would require a dam 52m high forming a 65ha lake with a volume of 13.4 million m when full. The entire footprint of the dam and the buffers etc would be about 107 ha. At least 500L/sec would be continuously released from the dam, allowing the operation of a hydro turbine. Sensors at strategic points in the catchment could provide real-time information and allow close control of water release to meet the actual needs during dry periods. From the dam the water would flow 12km through natural watercourses to the Plain where it would top up the river and aquifers.

The operation of the dam is run of the river (there are no pipes from the dam to the people that need the water), and one of the positive things that the project is offering is the guarantee that below the dam there will always be at least 500 L of water let out of the dam per second, and there is the proposition that we can embed a small hydro electricity plant because whether you let it go over the gate or put it through a turbine learned water still goes into the river.

Why dont they just get in and do it? After all, if the dam doesn't happen there is the possibility that 70% of the irrigation water allocation may have to be rescinded flows to protect the river with sufficient water, and that would ruin many businesses. There would be fighting in court over water flows and allocation rights, land values would drop by about $160 million, and production lost would amount to something like $440 million over the 25 year payback life of the dam. Ten years ago the total economic worth of water on the Waimea Plains was estimated to be $250 million, and it would be much more today and will increase in the future if water supply is secured. Land values are currently around $70,000 per hectare and to invest in an intensive pipfruit operation you might spend $60-$70,000 in development on top of that. At $140,000 per hectare capital value you can't afford for your crop to fail so reliability of water supply is very important particularly through the January February period when most people are finishing their crops.

If the region prospers, the people prosper, and if the people prosper the country prospers, and with this project there would be significant inter-generational and strategic benefits. You cant drink oil or minerals and they are not essential for growing food.

It also seems that everyone wants the project to go ahead in some form. The Committee has made great efforts to involve all interest groups and to be totally open with the information that has come from surveys and studies of the feasibility of various options. Many public discussions and field days have been held, and over 90% of the local population is on board with the project.

The sticking point is the cost and who should pay. User pays? Thats all very well, but the strategic and economic importance of the water supply to the region has national as well as local significance, and there is a good argument for both local and central government support.

To date, various investigation studies have cost several million dollars, and this has been paid for through levies on water users and land owners along with funding from the Sustainable Farming Funded and local Councils. The committee wants the payback period to be 25 years, but just how much each interest group should pay is the subject of much debate at the moment. There are also options to reduce the cost, but each comes with a price that includes reduced benefits and a shorter life for the scheme.

Murray King, chair of the Committee is quick to point out that the project has quite some way to go yet before it can be seen as a success.

We haven't completed its yet, and until we have done that we need to be reserved about the outcome. But the key point really is that we have taken a collaborative approach designed so that we should all be winners and no losers. We have taken a more modern view of water management and included all people at an early stage rather than having to consult with them at the end of the process, which is where problems have occurred in the past, he says.

We have a had a good relationship with iwi and we invited them to join the committee right at the start. DOC and fish and game have been involved right from the start so we have covered recreational environmental and cultural groups as much as possible. At the end of the day everyone wants the same thing even though they may not recognise it, regular water and a healthy river.

The second point is that projects such as these take a long time. The Committee was formed seven years ago, and if all goes according to plant it will be another five years before the scheme is fully operational assuming funding and consent approvals are forthcoming.

One thing is for sure doing nothing is not an option.