Example:
"A new method to grow rice could save hundreds of billions of cubic meters of water while increasing food security, according to a study by the World Wildlife Foundation"

So from a global warming point of view this could prevent hundreds of billions of tons greenhouse gas from getting into the atmosphere. It's not normally viewed that way, but rather seen as saving water.

The above article continues:
"With a focus on India – a country which faces a major water crisis, yet has the world’s largest rice cultivated area – the study found that the system of rice intensification (SRI) method has helped increase yields by over 30% — four to five tonnes per hectare instead of three tonnes per hectare, while using 40% less water than conventional methods. Rice is the main source of directly consumed calories for about half the world’s population and 90 per cent of it is produced and consumed in Asia. Contrary to popular belief, rice is not an aquatic plant and the main reason it is submerged in water is for controlling weeds."
blog.thesietch

Now, I'm simply looking for a number for amount of water used in irrigation globally, but according to this:
"Since it takes 1,000 tons of water to produce one ton of grain"
http://www.earth-policy.org/Books/Seg/PB2ch03_ss2.htm

I could simply look up amount grain produced in the world- though granted some significant percentage of the water will come from rain.

"World coarse grain production reached a record 907 million tons in 1996-97 as the result of very large crops in the European Union, China, Canada, Brazil, and Argentina which more than offset a poor crop in the former Soviet Union. World production exceeded consumption by about 30 million tons that year. World production declined to 876 million tons in 1999-2000 and to 860 million tons in 2000-01. Over the past three years, including the forecast for the current year, production has ranged from 869 to 892 million tons. "
http://www.farmdoc.uiuc.edu/marketing/weekly/html/111703.html

So, if times by 1000, one has 869 to 892 billion tons water used to make this grain per year

So, roughly humans probably put about a trillion tonnes of water vapor in the atmosphere per year.