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.