More crops per drop

Over the coming decades, rises in global demand for food, fibre, feed and fuel are predicted to cause large increases in the amount of water used by agriculture. Currently, agriculture world-wide uses (pdf file 625KB) 6,800 cubic kilometre of water annually (km3/y), but by 2050 global water use in farming will need to rise to dramatically to 12,600km3/y unless substantial improvements occur in the water-use efficiency of farming.

After allowing for adequate environmental river flows, the ceiling of global usable fresh water river flows - termed blue water - is estimated to be only 8,700km3/y. This ceiling implies increased future water needs of agriculture cannot be met by increased irrigation, and there is widespread attention being given to solving global water scarcity problems. Nobel Prize winner Norman Borlaug (pdf file 151KB) has pressed for a “Blue Revolution” in water use - a revolution to get more crop per drop of water.

To measure a farmer’s ability to produce “more crop per drop”, agricultural scientists now use the term “water productivity”. Farm water productivity can be as high as 20kg/cubic metre water with cereals, or about 10kg/cubic metre with oilseeds and legumes, but such high efficiencies are obtained only in the best managed crops. Almost any factor that can influence crop yield or vigour will influence water efficiency.

Hence soil and stubble management, by increasing infiltration and water storage in the soil and decreasing evaporative losses from the surface, can improve water productivity. Timeliness of sowing, evenness of establishment, use of herbicides, management of nutrients, management of disease, crop rotation, choice of crop or plant variety, and breeding for particular traits all play a part. In short, skilful farming and good crop science save water.

Management of water demand from agriculture has usually focused on the blue water withdrawals - that is irrigation water withdrawals from dams, rivers and streams, with some 1,800km3/y of this blue water already used for irrigation.When one considers foreseeable water demand by industry and domestic users in 2050, the severe challenge of meeting the projected water demand by agriculture by 2050 from blue water resources alone becomes obvious.

Difficulties with future expansion of blue water withdrawals for farming are particularly acute in Australia, a country with relatively low annual rainfall, where already strong pressures for increased environmental river flows in the Murray-Darling system are putting pressures on available blue water for crop irrigation.

Focusing only on blue water resources - rivers and irrigation - can miss the water conservation opportunities from better on-farm water productivity. Globally, and especially in Australia, the hydrological cycle has more substantial water assets in than the blue water flows. There is another valuable water resource - totalling 72,000km3/y of water- in the invisible water vapour section of the global water cycle where rain precipitation is returned from the soil in fields, pastures or woodlands to the atmosphere, for which the term “green water” has been coined. Green water flows are 65 per cent of total precipitation on land, while realistically accessible blue water flows are only 8 per cent of total precipitation.

Green water includes both vapour evaporation from the soil and transpiration plant water vapour through leaf stomatal pores: 6,800km3/y of global green water vapour flow is from croplands. In Australia, total rainfall is 3,314km3/y and total green water 3,100km3/y, reflecting high vapour flows from evaporation and transpiration through vegetation, which is highly relevant to issues raised by deforestation and soil salinity. Total Australian cropland green water flow is 45km3/y, and blue irrigation water 10.4km3/y. The importance of green water in Australia is illustrated by wheat, canola and other major rain-fed crops that rely exclusively on green water flows.

There are many facets to green water conservation. Depending on the scale and type of process being considered - microscopic level inside a leaf, a field in a farm, a catchment, irrigated versus rain-fed cropping, or regional or global agriculture, somewhat different ways of assessing water productivity can be used.

For instance, inside the leaf, water efficiency might be assessed by the ratio between photosynthetic capture of carbon dioxide gas in leaves to water vapour losses from the stomatal pores. On the other hand, in discussing farm level efficiency of rain-fed agriculture, water productivity would be measured by ratio of crop yield to total rainfall in a season.

Scientific improvement of green water-use efficiency draws on the expertise of farmers, agronomists and biologists, and rests much more on knowledge and know-how, and the diffusion of better techniques and practices, rather than capital investment in dams, channels, and irrigation scheme used for blue water management. The good news is that Australia has strong local expertise, a long track record in this area, and broad expanses of rain-fed crops.

The Australian wheat industry provides superb examples of significant savings of green water resources from innovation extending over more than 100 years.

Since 1900, the rain-fed wheat industry has made water productivity gains at a steady rate of 1 per cent a year for over a century from ongoing wheat breeding and selection of genetically improved wheat varieties. These improvements resulted from genetics experiments that were largely intended to give higher economic returns to the farmer and increased cereal yields per hectare, but because wheat farming in Australia predominantly relies on soil moisture during a relatively dry growing season, the crop yield gains provided water efficiency gains too.

Recently new wheat varieties have been released that have been deliberately bred for better water conservation. The story of these varieties starts around 1980 when CSIRO scientists Graeme Farquhar and Richard Richards commenced fundamental research to gain understanding of the relationship between water losses and carbon dioxide gain through the stomata (pores) at the surfaces of the wheat leaf. Their ground-breaking work reinforced the idea that water loss from the leaf is necessarily coupled to carbon dioxide uptake through the same stomatal pores, and they established that isotopic composition (C12 versus C13) of plant carbon fixed from the atmosphere correlates with water-use efficiency of different wheat varieties.

These observations on mechanistic details of plant water-use efficiency fit with findings from plant physiology that plant growth is promoted by growing plants in humid air, and by enriching the atmosphere with gaseous carbon dioxide. The method of assessing water-use trait using isotope analysis of leaf material carbon is now generally referred to as the DELTA carbon technique.

DELTA carbon technology is eminently suitable for assessment of the numerous progeny that are generated in wheat breeding programs to producer superior crop performance, culminating in the release of Drysdale wheat variety in 2002 and Rees in 2004, about 20 years after the initial research:

The latest trials of a Graingene-bred water-efficient wheat variety have shown it has the potential to add millions of dollars to the value of the NSW wheat crop. In 12 independent field trials held across New South Wales in 2003, Drysdale wheat yielded an average of 23 per cent more grain than the current recommended variety Diamondbird, despite very dry conditions. "If Drysdale was sown throughout southern and central New South Wales, it could add hundreds of millions of dollars to the average crop value," says CSIRO's Dr Richard Richards.

Other genetic traits can be used by breeders to achieve green water efficiency indirectly. For instance one of the advantages of triazine herbicide tolerant canola, sown in winter cropping systems in the southern dry-land regions of Australia, is that it can be sown near to the break in season as possible, which is critical taking full advantage of soil moisture, particularly in dry regions.

Other traits that have allowed better green water productivity are green revolution cereal crops that matured in short or medium seasons (90-120 days) and therefore escape late-season drought. A trait called “Stay-green”, present in sorghum and maize, allows leaves to remain green even under drought conditions, and allows grain to fill even during drought.

Crop breeding is not the only way green water savings can be attained, and wheat is only one of many crops in which genetics is allowing savings of water resources.

One approach that has improved wheat yields, and water productivity, is rotation of wheat with lupins or canola, introduced in the 1980s, boosting yields by about 20 per cent. Conservation tillage or reduced use of the plough by drilling seed in stubble of the previous crop is another saver of water, which can give water saving from reduced evaporation, runoff and deep percolation. Herbicide tolerant varieties of soybeans, cotton, canola and maize, mostly produced using GM methods, are greatly facilitating conservation tillage (pdf file 130KB).

A similar green water conservation method is to avoid fallow soil by drilling seed into wheat stubble, as has been advocated recently for cotton by Narrabri, NSW agronomist Nilantha Hulugalle, who claims, “Standing wheat stubble can store up to 75mm more winter rainfall than when the soil surface is bare, because it improves rainfall infiltration and reduces evaporation”.

Outside Australia, there are many opportunities for green water savings. In sub- Saharan Africa for instance, most food production is rain-fed but water productivity is often very low, and evaporation rates often very high. Effective redirection of water vapour evaporating from the soil by: intercropping, better use of leaf canopy cover, conservation tillage, and increased use of fertiliser to improve yield and water productivity are some of the many approaches being successfully tried to bring about better crop and water efficiencies. Conservation tillage has given water gains with rain-fed wheat in Pakistan and maize in Tanzania.

Supplemental small-scale (blue water) irrigation of rain-fed agriculture is a particularly important tool to provide improved crop productivity for resource poor farmers. Even small volumes of stored water can be used very effectively when dry spells occur at times, when harvest yield is especially sensitive to water stress (such as at flowering in maize). Combined with drip irrigation, this can be effective in improving water efficiency.

Water efficiencies can even be obtained from international trade. Harvested Australian wheat exported to water scarce regions such as Iraq can in fact be regarded as “virtual water” exported from Australian rain-fed agriculture to regions with a water deficit. The concept of global trade in virtual water (pdf file 3.02MB) has been used to explain how food production can be shifted from water-rich regions such as North America to contribute to the water-deficient economies in Asia and the Middle-East.

Green water is invisible - as we actually cannot see the water transpiration stream emerging from a leaf, but there is much more of it than blue fresh-water. Water savings from improved green water productivity emerge from numerous changes in global agriculture where savings are achieved by trade in “virtual water,” and by options taken up by farmers that minimise irrigation by enabling farmers to operate more profitably without using irrigation.

An emphasis on improving green water productivity may well have more impact on ensuring sustainable use of water resources (pdf file 430KB) than investment in irrigation: it does not necessarily require heavy local capital investment, relies on technologies all farmers are familiar with, and, in the case of more water efficient crop varieties, has the potential to be distributed widely at low cost.

The green water concept helps highlight potentially overlooked opportunities for water conservation and improvement to water productivity. Sixty per cent of the world food production is rain-fed and totally dependent on green water and this rain-fed agriculture will remain the dominant food production resource for the foreseeable future. Green water savings are a major driver for reduced use of water in agriculture, and make possible both future food security and sound management of river catchments during the coming decades of this century.

Direct scientific approaches to improve crop water efficiency are still very much in their infancy; as with most agricultural innovation, there is a long lag time between basic science and the economic and environmental benefits and time is need to make sure innovation works effectively in the challenging context of farming.

The DELTA carbon technique, for instance, has yet to bear full fruit with other crops than Australian wheat varieties 25 years after its inception. Molecular genetics also is yet to be fully exploited for the options it promises for reducing crop loss from water stress, although there is a lot of promise in the pipeline. But because knowledge and seeds can easily spread from farmer to farmer, and farmers are using a technology they already are deeply familiar with, the benefits can be huge, and green water technological innovation by teamwork between scientists, farmers, and breeders, is living up to its name - it’s the green way to save natural resources.