Vapour value

April 1st, 2014

Charlie Paton, Managing Director of Seawater Greenhouse Ltd., United Kingdom

For the cultivation of crops in hot, arid regions, evaporating seawater has the same if not greater value than providing fresh water through desalination, yet at a fraction of the cost.

Increased demand for food and a lack of water to grow it are two of the most widely predicted scenarios of the 21st Century.1 Solutions to this dilemma that we are frequently urged to adopt include eating less meat and increased use of irrigation. Here we use the Horn of Africa as an example to argue that both have flaws and that considering the issue from the plant and the planets’ perspective provides a more restorative solution.

The Horn of Africa suffers from chronic food insecurity. Causes include; drought, over-grazing, deforestation, desertification, social and political conflict, epidemics, and poverty.2 Conversely, agriculture itself can be the cause of food insecurity when it contributes to pollution, erosion, depletion of resources and loss of biodiversity.

It is curious to note that one of the poorest and most water stressed countries in the world should also hold the record of being the worlds’ largest exporter of live animals. Some 66% of the population of Somalia are semi-nomadic pastoralists, raising camels, goats and sheep. These are reared primarily for the markets in the Arabian Peninsula2 and make up over 90% of the country’s exports.3

Agricultural productivity in the Horn of Africa is hampered by high temperatures, erratic rainfall and strong winds. For these reasons, evaporation exceeds precipitation several-fold and as a consequence, agricultural productivity is low. Annual crop yields in Somalia average 5 tons per hectare and only 1.6% of the 600,000 km2 land area is cultivated.

A rising population and unsustainable farming practices are causing the depletion of vegetation, which in turn leads to reduced rainfall and the consequent inability to cultivate crops. The demand for food is likely to exceed availability in the foreseeable future, and shortfalls contribute to poverty and political unrest. The regions’ instability has made it a focus for humanitarian assistance and Somalia, along with Ethiopia and Sudan, top the list of emergency food aid recipients.4

Since the dawn of agriculture in Mesopotamia, there has been a net flow of nutrients, water and minerals from the land to the sea. Where surface water is insufficient, ground water is pumped from ever increasing depths. Now, some 8000 years later, recent satellite data reveals that the Tigris and Euphrates river basins have lost 144 cubic kilometres of water in just 6 years between 2003-2009. Shrinking aquifers have been observed beneath most of the world’s major agricultural regions including the Central Valley in California North China Plain, North Africa, southern Europe and the Upper Ganges in India and Pakistan.5

There is a growing realisation that this flow should be reversed if we are to have any chance of feeding our expanding populations. After all, the volume of fresh water on the planet is some 40 times less than that of sea water and of that, only 1% is easily accessible.

Finding a solution

Desalination has been considered prohibitively expensive to use for irrigation, however, costs have fallen dramatically over the past two decades together with a corresponding reduction in energy requirements. In situations where water use efficiency is practiced, it now competes favourably with abstraction of aquifer water taken from deep in the ground, especially where abstraction rates exceed infiltration leading to depletion of the groundwater. Converting seawater into water vapour in a greenhouse system is much cheaper still and arguably has an equal or greater value than liquid water as it can reduce plant transpiration by ten-fold or more (see Seawater Greenhouse: A new approach to restorative agriculture).

The benefit of reducing the transpiration rate can be illustrated by looking at how much fresh water it takes to grow a kilo of tomatoes. A typical value for a field grown crop is 200 litres although it could be 600 litres or more in a hot, arid climate. However, it can also be as little as 15 litres inside a greenhouse that is designed to optimise growth and water use efficiency (see Figure 1).

Improved water use efficiency goes hand in hand with achieving both higher yields and higher quality as the plant is not stressed by excessive transpiration. A plants’ water use efficiency is determined by the rate of transpiration which in turn is governed by the amount of sunlight it receives and the water deficit or ‘dryness’ of the air. The transpiration rate is minimised by reducing solar radiation and by cooling and raising the humidity of the air. Cooling is usually achieved by misting or evaporating water into the air, but where fresh water is used for this purpose, the crop will benefit but the water use efficiency is lost.

Using seawater for evaporative cooling overcomes the problem and provides significant additional benefits. Saline water has a powerful biocidal effect on airborne pests and disease, reducing or eliminating the need for pesticides. High rates of ventilation translate to more benign conditions for plants growing in the lee of the greenhouse where they derive the additional benefit of wind protection from the greenhouse structure.

Clearly there is a capital cost to achieving water use efficiency as a greenhouse or shade house structure is needed to integrate and manage the growing environment. Generally the more sophisticated the greenhouse, the higher the yields will be, but this is not always the case. Much depends on the climate and the extent to which conditions need to be moderated. Simple shade net structures that are increasingly used in hot, arid regions such as Mexico, Israel and Turkey have costs in the range of $10-20 per m2 while high tech heated glasshouses that are common in Northern climates have costs that are ten-fold higher. In sunny locations that are close to sea level, it is much cheaper to cool a greenhouse than it is to provide heat and light somewhere cool.

So where are appropriate locations for such approaches? Low lying coastal plains where agriculture or grass-fed pastoralism is practiced and threatened offers the easiest and fastest route to food security. Water is heavy and pumping it uphill carries an energy cost that increases with height. Water vapour by contrast is light and is carried by the wind. The solution is to tip the balance in locations where it is most practical and economic. Reversing the ‘evaporation-exceeds-precipitation’ ratio will also help restore vegetation, especially where the prevailing wind is onshore and carries the water vapour to where it is most needed, and ultimately where it augments rainfall.

In fragile climates, yields from rain fed agriculture are marginal and carry a high risk should the rains fail. Irrigation with ground water mitigates this risk but it carries the long term penalty of over-abstraction from a limited resource. As grass is one of the hardiest and most resilient plants, grass fed pastoralism has traditionally provided the most viable route to economic security in arid regions. However it is no longer sufficient to meet the needs of a growing population and overgrazing leads to a loss of biodiversity and a reduction in rainfall as a consequence. In Somalia for example, the value of imported food is still higher than that of exports, yet it is a country where some 80% of the population are engaged in agriculture.6 The solution must be to stop using the sea as a sink but as a source. After all we are not short of water, it is just in the wrong place and too salty.

References:

1. The government’s chief science advisor Professor John Beddington has warned that growing world population will cause a “perfect storm” of food, energy and water shortages by 2030.

http://news.bbc.co.uk/1/hi/sci/tech/7952348.stm

2. The Horn of Africa is one of the most food-insecure regions in the world

http://www.fao.org/docrep/003/x8530e/x8530e02.htm

3. Central Bank of Somalia

http://www.somalbanca.org/economy-and-finance.html

4. World Food Program 2009

http://home.wfp.org/stellent/groups/public/documents/newsroom/wfp223562.pdf

5. Groundwater use is unsustainable in many of the world’s major agricultural zones.

http://www.nature.com/news/demand-for-water-outstrips-supply-1.11143

6. Central Bank of Somalia

http://www.somalbanca.org/economy-and-finance.html

7. O. van Kooten and E. Heuvelink C. Stanghellini, “New Developments in Greenhouse Technology can Mitigate the Water Shortage Problem of the 21st Century”. Wageningen University.

Charles Paton is a product developer, maker and designer and is a Royal Designer for Industry. For the past 20 years he has been developing his Seawater Greenhouse concept, designed to produce food and water on barren land in hot and arid coastal regions. It harnesses solar energy and seawater in a unique and inspired way to create a virtuous cycle that produces fresh food and fresh potable water in locations where food and water shortages are a significant problem. Charlie has built the pilot projects in four locations around the world to prove his concept; simple and elegant it is potentially life changing for huge numbers of people living in deprived coastal regions. The design has received many awards and it has attracted attention from all over the world, including from the National Geographic and New Scientist. For more information see the Seawater Greenhouse website.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.