3 Water footprints:

Green water footprint is water from precipitation that is stored in the root zone of the soil and evaporated, transpired or incorporated by plants. It is particularly relevant for agricultural, horticultural and forestry products.

Blue water footprint is water that has been sourced from surface or groundwater resources and is either evaporated, incorporated into a product or taken from one body of water and returned to another, or returned at a different time.

Grey water footprint is the amount of fresh water required to assimilate pollutants to meet specific water quality standards. The grey water footprint considers point-source pollution discharged to a freshwater resource directly through a pipe or indirectly through runoff or leaching from the soil, impervious surfaces, or other diffuse sources. 

Fruits and Legumes vs Meat Green, Blue and Grey Water Footprint Comparison

Water footprint per ton (m3 / ton) and per unit of nutritional value for fruits, pulses (legumes like beans, peanuts) and bovine meat*:   

* This table made by the author of the site Fruitarians.net (Lena), based on THE GREEN, BLUE AND GREY WATER FOOTPRINT OF FARM ANIMALS AND ANIMAL PRODUCTS, VOLUME 1: MAIN REPORT, M.M. MEKONNEN, A.Y. HOEKSTRA, DECEMBER 2010, VALUE OF WATER RESEARCH REPORT SERIES NO. 48, the link to PDF is provided on the article page.

Study Conclusions 

(UNESCO-IHE, Institute for Water Education, 2010)

As a general picture we find that animal products have a larger water footprint per ton of product than crop products.

... The global average water footprint per ton of crop increases from sugar crops (roughly 200 m3 /ton) and vegetables (~300 m3 /ton) to pulses [legumes] (~4000 m3 /ton) and nuts (~9000 m3 /ton). For animal products, the water footprint increases from milk (~1000 m3 /ton) and egg (~3300 m3 /ton) to beef (~15400 m3 /ton).

Also when viewed from a caloric standpoint, the water footprint of animal products is larger than for crop products. The average water footprint per calorie for beef is twenty times larger than for cereals and starchy roots.

... The water footprint per gram of protein for milk, eggs and chicken meat is about 1.5 times larger than for pulses. For beef, the water footprint per gram of protein is 6 times larger than for pulses.

... The general conclusion is that from a freshwater resource perspective, it is more efficient to obtain calories, protein and fat through crop products than animal products. 

... A vegetarian diet compared with the average current per capita food intake in the USA can reduce the water footprint of an individual by as much as 58%. 

A Few Facts

The production of one kilogramme of beef requires ~ 15000 litres of water (93% green, 4% blue, 3% grey water footprint). There is a huge variation around this global average. 

The water footprint of a 150-gramme soy burger produced in the Netherlands is ~ 160 litres. A beef burger from the same country costs on average ~ 1000 litres.

The water footprint of US citizens is 2840 cubic meter per year per capita. About 20% of this water footprint is external. The largest external water footprint of US consumption lies in the Yangtze River Basin, China.

The global water footprint of humanity in the period 1996-2005 was 9087 billions of cubic meters per year (74% green, 11% blue, 15% grey). Agricultural production contributes 92% to this total footprint. 

Water scarcity affects over 2.7 billion people for at least one month each year.