Khat
Khat is very popular in Yemen.  Much of the country's farming resources are used for it because farmers get more money than for other cash crops.  It is estimated that 40% of the country's water supply goes towards irrigating it.  Its production is said to increase by about 10% to 15% every year.  It uses so much water that groundwater levels in the Sana'a basin are getting lower.  For this reason, government officials have proposed to move large portions of the population of Sana'a to the coast of the Red Sea.