More crops per drop

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.