Water, water, water!

Thornleigh is being geared up to grow lots and lots of yummy things: Mangoes, pumpkins, tomatoes, avocados, rock-melons and so forth. All these yummy things will give Lord Howe Island visitors and residents the energy they need to perform strenuous island activities. For example, swimming with turtles at Old Settlement Beach. But where do all those vegetables, fruits, and other plants get all that energy from?

A turtle saying hello at Old Settlement beach
A turtle saying hello at Old Settlement beach

Photosynthesis, of course! Plants fuel their activities by converting light energy from the sun into chemical energy in the form of carbohydrates. Photosynthesis requires carbon dioxide, water, and photons. Cleverly, we have worked out that we can get photons by sticking Thornleigh’s crops out in the sun. Around about one sextillion (that’s 10^21!) photons per second hit every square metre of Thornleigh’s garden beds during the day. We’ve got photons, then!

Photosynthesised yumminess
Photosynthesised yumminess

How about carbon dioxide? A little back-of-the napkin maths reassures us that there are approximately 3.9 tonnes of carbon dioxide swirling around Thornleigh’s fields at any moment. If we do get worried that there isn’t enough carbon dioxide in Earth’s atmosphere, we could always leave Matilda idling in the paddock. We’ve now managed to acquire two out of three components necessary for photosynthesis. And we didn’t even have to do anything! This farming thing isn’t all that hard, so far. But of course, the third component is the tricky one.

Sweet potatoes sucking in carbon dioxide
Sweet potatoes sucking in carbon dioxide

There are two wells on Thornleigh, both of which may be used to access Lord Howe Island’s groundwater reserves. They are very old, the first being established by Phillip John English Dignam in the 1890’s. In the early 20th century his son, a young Phillip William Dignam, hauled water from the wells to the fields in a wheelbarrow. Later, Phillip W. employed flood irrigation techniques: Filling the trenches between planted rows with water.

Phil working in the Thornleigh gardens, mid 20th century
Phil working in the Thornleigh gardens, mid 20th century

Somewhat surprisingly, Robert and Lindy have not been able to convince their sons to spend all day carting water to the fields in wheelbarrows. Flooding is also much less efficient than modern irrigation methods, such as drip irrigation. The combination of drip lines, pipes, and pumps could supply Thornleigh’s fields with water, completing the trifecta of components we need for photosynthesis. But is using groundwater for general irrigation a good idea?

Irrigate me!
Irrigate me!

Groundwater is not an infinite resource: It varies in quantity and quality.  The amount of groundwater available depends on the volume of rainfall in local catchment areas. Lord Howe Island receives plenty of rain: The yearly average at the centre of the island is 1499mm, compared to 1212mm in Sydney’s CBD and 662mm in Melbourne’s botanical gardens. Slightly less rain falls in the northern settled areas than the south, where mounts Lidgebird and Gower induce orographic precipitaion. Look closely at the graphs below and you’ll notice another important characteristic: Rainfall drops significantly in the summer months.

Thornleigh
Map showing rainfall at different Lord Howe Island BOM monitoring stations

The drier summer months coincide with the peak season of Lord Howe Island’s most important industry: Tourism. During the tourist season, the population of the island doubles. Suddenly, there are 400 more people flushing toilets, taking showers, and washing clothes. Almost every residence and business on the island uses rainwater tanks, but in summer groundwater is an essential component of the water supply, especially for the big guest houses.

To keep the lagoon looking this beautiful, Lord Howe Island needs to manage water supplies carefully
To keep the lagoon looking this beautiful, Lord Howe Island needs to manage water supplies carefully

Back in 1996, groundwater studies revealed that during the summer months, groundwater extraction and septic waste disposal were causing elevated salinity and nutrient levels in Lord Howe Island groundwater. This poses potential risks to human health and, via runoff, the health of the islan’ds marine environment. Thornleigh is right in the middle of one of the zones identified as showing signs of strain. Evidently, a restored heritage farm pumping thousands of litres of water out of the ground in summer is not what Lord Howe Island’s water supply needs! But we need to photosynthesise in summer, so what to do?

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At first, the answer seems simple. Water tanks! By storing bucket-loads of rain water in the winter, Thornleigh’s drip irrigation systems could hum away through the summer without putting any additional strain on the island’s groundwater supplies. Polyurethane tanks of all shapes and sizes can be had for reasonable prices. To store rainwater, we could order a whole bunch of these tanks, attach them to shed roofs across the property, and complete the photosynthesis equation.

Brenden re-positioning a polyurethane rainwater tank
Brenden re-positioning a polyurethane rainwater tank

There is a hitch though: Putting a water tank on Lord Howe Island is not like putting a water tank on the mainland. It’s not possible to order a tank and have it delivered on the back of a truck. Everything bigger than a human that comes to the island needs to come via ship, and that is an expensive exercise.

The Island Trader visits Lord Howe every few weeks, carrying essential cargo
The Island Trader visits Lord Howe every few weeks, carrying essential cargo

Polyutherane tanks are monolithic, and take up just as much space in a cargo hold as they do when installed. We will need well over one hundred thousands litres of water to reliably get through summer. Shipping that much plastic is just not feasible! Fortunately, there’s another option: The Stockman tank from Tankworks.

920kgs of environmentally responsible farming!
920kgs of environmentally responsible farming!

Stockman tanks are low profile, high volume, sturdy water tanks that come flat-packed in kit form. Instead of filling a ship’s cargo hold with plastic tanks full of air, the Stockman can be transported efficiently in a crate.

Robert Jeremy (left) and Rob "Bert" Simpson during tank 1's construction
Robert Jeremy (left) and Rob “Bert” Simpson during tank 1’s construction

The tanks selected for Thornleigh are 1.9 metres tall, 7.2m in diameter, and have a capacity of 77,000 litres. They are made of corrugated galvanised steel, and feature a polypropylene based woven fabric bladder.

Brenden enduring searing summer heat to install the bladder inside tank 1
Brenden enduring searing summer heat to install the bladder inside tank 1

Both tanks are now completed, in time for the 2015 winter rains. Their combined 154,000 litre capacity will ensure that Thornleigh’s drip irrigation systems can continue to feed water to the gardens through the summer months.

Brenden (Left) and Robert working on tank 2 under the supervision of Bill Retmock
Brenden (Left) and Robert working on tank 2 under the supervision of Bill Retmock

For more information about the Stockman tanks, head over to the Tankworks website. If you would like to hear more news from Thornleigh, sign up for the Thornleigh newsletter. See more photos of the tanks under construction in an extended album, right here on the Thornleigh website.