Professor Wayne Meyer is Professor of Natural Resource Science at the University of Adelaide and former Deputy Chief and Business Director for Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water. He has received the CSIRO Medal for Research Achievement for his research in irrigation water management.
Prof. Meyer has estimated that, to produce 1 kilogram of product, it takes between 50,000 and 100,000 litres of water for beef compared to between 715 and 750 litres for wheat and between 1,550 and 2,000 litres for rice. 
David and Marcia Pimentel of Cornell University have reported that producing 1 kilogram of animal protein requires about 100 times more water than producing 1 kilogram of grain protein. Their estimates for 1 kilogram of beef range from 100,000 litres (relating to grain and hay for production systems that include intensive feedlots) to more than 200,000 litres (relating to forage production on rangelands). [2, 3] Elsewhere, David Pimentel and co-authors have cited figures of 43,000 litres for intensive production including feedlots and 120,000 – 200,000 litres for open rangeland production. 
Professor Meyer’s figures were originally derived for intensive production using irrigated pastures. Seemingly consistent with the findings of David and Marcia Pimentel, he has subsequently suggested that if the same exercise were conducted on rain fed, extensive meat production, there may be even more water involved. The reason is that feed conversion is likely to be lower, energy expended in gathering dry matter (including grass) would be greater and soil evaporation losses may even be higher than in a system involving irrigated pasture. 
It then becomes a question of the optimum use of the water, taking into account potential alternative uses.
Prof. Meyer has pointed out that water used for irrigation has many alternative uses, including keeping it in the river systems, keeping riverine and wetland ecosystems healthy and providing water for urban and industrial uses. He has noted that alternatives for rain fed areas are more restricted, but could include provision of run-off in catchment areas, growing native vegetation for conservation purposes and or for groundwater recharge. He has said:
“Using this logic there is little value in arguing that meat production does not embody a lot of water. More rationally the discussion can be about the value we place on the genuine alternatives for the use of this water.”
In areas where crops for human consumption can be grown, there are high opportunity costs in meat production, with the water requirement of animal-based foods being many times that of non-animal options for any given level of nutritional output.
In non-cropping areas, the choice can be as simple as steak dinners versus natural ecosystems. Alternatives are available for steak dinners but not for natural ecosystems.
Prof. Arjen Hoekstra of the University of Twente in the Netherlands and Prof. Ashok Chapagain of the University of Free State, South Africa, have estimated that, in Australia, 17,112 litres of water are required to produce 1 kilogram of beef.  Although lower than other estimates referred to in this article, their estimate is still many times higher than estimates for vegetables and grains.
Their figures for soy beans were 2,106 litres (Australia) and 1,789 (global average), and for paddy rice 1,022 litres (Australia) and 2,291 litres (global average).
Hoekstra and Chapagain are on the supervisory board of the Water Footprint Network, which is a non-profit foundation under Dutch law. The founding partners were: University of Twente, World Wildlife Fund, UNESCO-IHE Institute for Water Education, the Water Neutral Foundation, the World Business Council for Sustainable Development, the International Finance Corporation (part of the World Bank Group) and the Netherlands Water Partnership. 
In responding to queries regarding the differences between his figures and those of Prof. Meyer and Dr Pimentel, Prof Hoekstra has noted: 
His global average figures for chicken meat and pig meat are more than double those of soy beans, while the multiple for beef is more than eight.
Subsequent to those findings, Hoekstra published papers with Mesfin Mekonnen, also of the University of Twente. As with previous research, they included findings for “green” (rain) and “blue” (surface and ground) water, but added “grey” water, which is the volume of freshwater required to assimilate pollutants. They also included additional products, including nuts, whose water footprints were relatively high. 
Some global average figures for animal products were: beef 15,412 litres (similar to Hoekstra’s previous global average); sheep meat 10,412; pig meat 5,988; chicken meat 4,325; egg 3,265; and cow’s milk 1,020.
The approximate range of figures for other products are shown below.
- Legumes (including soy beans, shelled peanuts, dried beans and lentils but excluding fresh beans and peas for this comparison) from 2,000 litres to 7,000 (with soy beans 2,145 and soy curd or tofu 2,523).
- Fresh vegetables (including fresh beans and peas) from 200 to 2,200.
- Fresh fruit from 200 to 3,800.
- Grains from 1,400 to 3,100.
- Nuts (other than peanuts or groundnuts) from 9,200 to 16,100.
The authors have concluded: 
“The water footprint of any animal product is larger than the water footprint of crop products with equivalent nutritional value.”
“Replacing all meat by an equivalent amount of crop products such as pulses and nuts will result in a 30% reduction of the food-related water footprint of the average American citizen.”
“Managing the demand for animal products by promoting a dietary shift away from a meat-rich diet will be an inevitable component in the environmental policy of governments.”
Despite the variation in research findings, it is clear that non-animal food options are generally far more water efficient than meat from cattle and sheep.
The distinction between non-animal products and other animal-based products in terms of water use is less significant, meaning that other factors such as impacts on climate change, biodiversity loss and the animals themselves may be more valid considerations when reviewing the various alternatives.
Additional figures and related amendments included on 13 January and 24 February 2019.
 Meyer, W. 1997 “Water for Food – The Continuing Debate”, https://www.researchgate.net/publication/269396797_Water_for_Food_-_the_continuing_debate
 Pimentel, D & Pimentel, M, “Sustainability of meat-based and plant-based diets and the environment”, American Journal of Clinical Nutrition 2003; 78 (suppl): 660S-3S, http://www.ajcn.org/cgi/content/abstract/78/3/660S
 Thomas, G.W., Water: critical and evasive resource on semi-arid lands. In: Jordan W.R., ed. Water and water policy in world food supplies. College Station, TX: Texas A&M University Press, 1987:83–90, cited in Pimentel, D. & Pimentel, M., ibid.
 Pimentel D, Berger B, Filiberto D, Newton M, Wolfe B, Karabinakis E, Clark S, Poon E, Abbett E, Nandaopal S. 2004. Water Resources, Agriculture, and the Environment. Ithaca (NY): New York State College of Agriculture and Life Sciences, Cornell University. Environmental Biology Report 04-1
 Meyer, W, “Water and meat producers”, Nov 2007 and updated Dec 2007 and Jun 2008
 Hoekstra, A.Y. & Chapagain, A.K. “Water footprints of nations: Water use by people as a function of their consumption pattern”, Water Resource Management, 2006, DO1 10.1007/s11269-006-9039-x (Tables 1 & 2), http://www.waterfootprint.org/Reports/Hoekstra_and_Chapagain_2006.pdf
 The Water Footprint Network, http://waterfootprint.org/en/
 Hoekstra, A, Email correspondence 9 Sep, 2009.
 Mekonnen, M.M. and Hoekstra, A.Y., “The green, blue and grey water footprint of crops and derived crop products”, Hydrol. Earth Syst. Sci., 15, 1577–1600, 2011, 25 May 2011, www.hydrol-earth-syst-sci.net/15/1577/2011/, doi:10.5194/hess-15-1577-2011
 Mekonnen, M.M. and Hoekstra, A.Y., “A Global Assessment of the Water Footprint of Farm Animal Products”, Ecosystems (2012) 15: 401–415, DOI: 10.1007/s10021-011-9517-8, https://link.springer.com/content/pdf/10.1007%2Fs10021-011-9517-8.pdf
Sofiaworld, Cow and calves with clouds in the back, Shutterstock ID 37168933