Christian Aid looked at 66 villages and found that farmers in the worst-affected areas had lost all of their crops after the rains failed during winter and spring.
Their study found that most of the water sources in Herat, Badghis and Ghor provinces had dried up and the wheat harvest was down by 90 to 100 per cent in parts of Faryab.
Ninety to 100 per cent of the rain-dependent harvest in Herat had gone and the harvest, reliant on irrigation, was down by 40 per cent.
There was also a 70 to 90 per cent drop in the harvest in the province of Ghor.
Christian Aid urged international donors to pledge funds for Afghanistan's $76 million emergency drought appeal which was set up by the government and the United Nations.
"People are not dying of starvation yet, but it is very obvious that a great deal of help is needed or the situation will become very serious within a few months," said Sultan Maqsood Fazel, from Christian Aid in Afghanistan.
"Meat is scarce in some areas and people are telling us that their food supplies will not last much longer."
The survey found that farm workers were leaving areas severely affected by drought to find work, placing increased pressure on areas which still have water supplies.
One million affected
Christian Aid estimates that more than one million people in the region were affected by the drought.
Dr Ahmad Zia Shams, from the agency for humanitarian and development assistance in Afghanistan, said: "There is a shortage of water in rivers and wells and the rapidly falling water tables have resulted in an acute shortage of drinking water and water for irrigating farmland."
In July, the UN and the Afghan government said that 2.5 million people were suffering from food shortages, in addition to the 6.5 million Afghans in rural areas who suffer from annual seasonal food shortages.