From the grower’s perspective, WUE is quite simply the amount of harvestable product per unit of water consumed by the crop, or, in strictly economic terms, profit per unit of water applied. WUE only becomes truly important if crop yield is limited by the amount of water available.
Everyone now recognises that global carbon dioxide (CO2) levels and average temperatures are rising, and even the not-for-turning George Bush seems to be coming around to the idea that burning fossil fuels might have something to do with it. It is predicted that summers will become drier and that very dry summers, such as those experienced in 1976, 1995 and 2003, will become more frequent.
Climate change will affect water use in several ways. First, just to prove that it is not all bad news, there is a benefit, because increased CO2 improves growth by enhancing photosynthesis - after all, that is why glasshouses are pumped full of CO2. There are further benefits, because higher CO2 levels also tend to close stomata and slow down the rate of water loss from the leaves. So high CO2 levels give more food for less water consumed, or, in other words, a big boost to WUE.
On the down-side, loss of water from crops (evapotranspiration) will be greater due to the warmer, sunnier weather, but yields could go up or down with the higher temperatures. In the spring, higher temperatures could boost growth, but, in the peak of the summer, temperatures may climb too high and inhibit growth. Most importantly, on average there will be less rainfall during the critical growing period, and crops will suffer from soil moisture deficits. Drought stress caused by higher evapotranspiration and reduced summer rainfall will probably override any growth benefits from the higher CO2 levels, unless irrigation can be stepped up to compensate. However, new regulations in the form of the EU Water Framework Directive (WFD) could restrict increases in irrigation.
The WFD aims to establish a “good” status in all surface and ground waters by 2015, and it will achieve this partly by restricting water abstractions. The Environment Agency (EA) will only grant new licences where water can be abstracted without causing environmental damage, and the renewal of existing licences requires that the applicant can demonstrate efficient use of water. Licence trading within each catchment could soon become a reality, and trading could shift water use towards crops (or other uses) with the highest economic return.
One danger is that the desire in the EU to protect the ecology of rivers and lakes could limit irrigation to the point where crop yields decline, forcing the EU to become more dependent on food imports. The amount of water needed to grow a particular agricultural product can be viewed as “embedded” or “virtual” water. For example, taking global averages, it takes 1,334 tonnes of water to produce a tonne of wheat (this includes evapotranspiration and irrigation) and an amazing 15,497t of water to produce a tonne of beef (including all the water needed to produce the fodder). These amounts of water can be viewed as embedded in the food product as virtual water, and from the import and export of goods the flow of virtual water can be worked out for each country. It turns out that the UK is the third-biggest net importer of virtual water, behind Japan and Italy, and, for every cubic metre of virtual water we consume in home-grown agricultural products, we import 2.7m3 of virtual water in the form of agricultural products from overseas. So, the UK is highly dependent on the exploitation of overseas water for our food - this is no problem, provided that the imported food is produced in a sustainable way, but, in many regions around the world, water is abstracted from rivers until they run dry, and ancient aquifers are being mined for water, with water tables dropping year-on-year with no prospect of replacement. So, for reasons of global environmental protection and food security, we should seek to maximise the sustainable outputs from agriculture in this country, and insist that imported products are produced sustainably. Improving WUE at home and contributing to the global R&D effort in this area are important ways forward.
GROWER BENEFITS
The effects of climate change on water supplies could also provide economic opportunities for UK growers; crop production in southern Europe, a significant proportion of which ends up in UK supermarkets, could be more heavily affected by drought and high temperatures than the UK. Spain, for instance, has been in a three-year drought, only broken by heavy rains and floods earlier this year that damaged an estimated half a million hectares of crops. So, if UK growers can maintain or expand production by making good use of our water resources, they may benefit in the future from higher prices, reduced imports, and even potentially new export markets.
The EA has powers to regulate water abstraction, but changes in land use could also have a large effect on hydrology and the ecology of water courses. The deforestation of much of England and conversion to crop land many hundreds of years ago reduced evapotranspiration and allowed more rainfall to reach rivers and ground waters by run-off and deep percolation. Now, with the prospect of a massive expansion of the area of water-hungry energy crops such as willow, miscanthus or poplar (grown as mitigation for rising CO2), we could end up reducing ground water recharge and river flows again at a time when rainfall is declining. This will potentially lead to more EA restrictions on abstraction for irrigated crops; if we are to produce both food and energy crops in the UK whilst keeping to the WFD, then efficient use of water will become increasingly important.
THREE APPROACHES
So what can be done to sustain production of field crops as summer water availability declines? There are only three approaches;
(i) build reservoirs to store water abstracted in the winter,
(ii) improve WUE by optimising growing and irrigation systems, and
(iii) breed new cultivars with improved WUE.
Regarding approach (ii), there is a great deal of research aimed at improving irrigation scheduling and uniformity, but it must not be forgotten that, in general, good agronomic practices that increase yield will also increase WUE because it is simply the ratio of yield to water used - if growth is limited by other factors such as disease or nutrition, then the water applied may be wasted. Different growing systems will also have inherently different efficiencies; for soft fruit and salads, one way to greatly increase WUE is to intensify production with polytunnels; the more humid environment and lower wind speeds under polythene tend to reduce evapotranspiration and together with high yields over a longer season will give high WUE. If rain water can be harvested from polythene using the guttering now available with polytunnels, then abstraction for irrigation can be kept to a minimum. Mulches are a good method for increasing WUE, as they reduce non-productive evaporation from soils (particularly effective in crops with incomplete ground cover), and if green waste mulches are used regularly they will gradually increase the water holding capacity of lighter soils, so that less water is lost by percolation. The recycling of green waste has increased tenfold in the last 10 years; in 2005-06, 9.6 per cent of household green waste was recycled in England, equivalent to 2.4 million tonnes; this provides an opportunity to increase the use of mulches and this has been taken up by some salad growers, primarily to suppress weeds and to condition soils.
Potato crops consume around half of the irrigation water applied to field crops in England, and around half of that is for prevention of common scab. Potato growers are fairly advanced in scheduling irrigation to meet the demands of the crop, but one factor that could improve WUE is to avoid soil compaction by careful soil management, so that roots have access to more of the water stored in the soil.
RESEARCH FOCUS
At Warwick HRI, research is focused on helping breeders to develop cultivars that are more water-use efficient; for example, we are addressing the issue of soil compaction by investigating the genetics of how roots can penetrate compacted soils, with the eventual aim of helping breeders develop deeper-rooting potato cultivars. Breeding for resistance to common scab, although a challenge, could also have a big impact on the amount of water needed by potato growers.
In other projects at Warwick HRI, we are investigating the genetics of WUE in brassicas; by measuring the ratio between photosynthesis and transpiration in a diverse collection of brassicas held at Wellesbourne, we have identified, using conventional methods, a particular region of brassica DNA that can increase WUE by around 20 per cent. By using genetic engineering we can do even better; we have increased WUE by 50 per cent in tomato plants, by engineering them to make more of the hormone that is normally produced by plants under drought.
Finally, a good way for individual growers to get to grips with improving WUE and to share best practice is to
join the UK Irrigation Association
(http://www.ukia.org/), or to join (or set-up) a regional Water Abstractor Group (WAG). A network of WAGs could develop across critical catchments
(see http://www.ukia.org/eeda.htm), and could give growers a collective voice on water issues, something that will become crucial as competition for water heats up.
Contact: Dr Andrew J Thompson
(a.j.thompson@warwick.ac.uk)
http://www2.warwick.ac.uk/fac/sci/whri/research/plantwateruse/
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