"A vegetarian diet would also greatly reduce water consumption; around 70% of global freshwater consumption is used in agriculture."

No problem. Water doesn't dissapear. Rain will bring it back.

"It takes 15,000 litres to make a kilogram of beef, 6,000 L for pork and 4,000 L for chicken"

This is nonsens wen you breed the animals naturally they do cost little water sinds the animals eat gras they don't drink much. Further, they can drink rainwater. Ofcourse you should not give animals cereals.

" - Compare this with 1,600 L for cereal crops, 900 L for fruit and 300 L for garden vegetables."

Nutritional value of vegetables and fruits are not comparable to that of meat.

"Of course a kilogram of meat packs many more calories than an equivalent weight of fruit, but even if you compare the ratio of water per calorie of available food energy, beef is still 5 times more water intensive than fruit, 7 times more than veggies and 20 times more than cereal."

Unfortunately, your body is unable to extract the same amount of energy from vegetables as from meat. You should eat a wheelbarrow vegetable to get the same amount of food as a kilo of meat. In addition, vegetable products are very poorly digestible to humans.