Hamir Soomrot points to vast stretches of water. "That's where my land used to be," he says. For as far as the eye travels, there is little sight of the soil his family has cultivated for decades. Only the very tips of the rice stalks can be seen. In the distance, there are the remains of crumbling brickhouses, abandoned when the waters crept in a fortnight ago.

"All that's left is a nice sunset," Soomro says, staring across his family's 1,200 acres, which grew rice in the summer and wheat in the winter. The water shimmers brightly as a full orange sun dips in the distance.

Beyond the death and human misery that has struck Pakistan, the unprecedented floods now threaten to devastate its agricultural economy. Millions of acres have been submerged. Crops have been ruined. The people who worked on the land have flocked to relief camps in nearby towns. They will remain without work for months to come.

Two days ago, the president of the World Bank Robert Zoellick said that the floods may have destroyed crops worth around $1bn. About a quarter of Pakistan's economy is derived from agriculture, and nearly half of its workforce are employed by the agricultural sector. The damage is on top of the cost of 1,600 lives, 2m people driven from their homes, and the lives of 20m – fully 12 per cent of the population – disrupted across the country. Six million still need food, shelter, and water, and yesterday the first dreaded case of cholera was confirmed in Mingora, the main town in the northwest's Swat Valley.

UN spokesman Maurizio Giuliano said other cases were suspected and aid workers are now responding to all those exhibiting acute watery diarrhoea as if it was cholera.

Cholera can lead to severe dehydration and death without prompt treatment, and containing cholera outbreaks is considered a high priority following floods.

Meanwhile, Pakistani officials say that the economic cost may be higher than the $1bn the World Bank estimates.

Javed Saleem, the former head of Pakistan's Crop Protection Association, has said that more than 17m acres of agricultural land have been lost to the floods. Over 100,000 animals have been killed. On the road to Rahimabad, the carcasses of buffaloes lie on the side of the road, with wild dogs ravaging them under a cloud of flies. Saleem estimates that the cost to the cotton growing economy alone is $2bn. On Soomro's family lands, on the edge of Shi­karpur, there is no pros­pect of farming returning to normal for at least another year. "Some 90 per cent of the land is under water," he says, gesturing to the expansive waters that surround him.

"The rice crop is gone. I don't have any wheat seed left, because that's under water. Then there's also equipment damage."

Shikarpur district is one of the worst hit areas in Sindh province. "More than three-quarters of the agricultural lands have been flooded," says Aftab Shaban Mirani, the local parliamentarian. "Nearly 300,000 people have been affected. We have 27,000 people living in relief camps now."

At the moment, his energies are concentrated on mounting a relief effort. Travelling from the capital Karachi, he has brought with him a team of volunteers and a container filled with food and water. "I have to do this," Soomro says. "I'll be doing this every week from now on." The government is nowhere to be seen, he laments. "If we don't help these people, they will start to riot."

The Soomro lands employ 3,000 people. The town of Rahimabad is populated by a further 7,000, who depend on the land for their subsistence. Half the yield from the crop is shared by the peasants, the other half is taken by the landowners. In previous years, they have suffered poor crops. "It hasn't been so good because of the water shortages," says Soomro. The heat means that much of the water is lost before it even reaches the roots. Other crops, such as sugarcane, can't be grown, because they drain too much water.

There is no coordination when it comes to either the relief effort or protecting agricultural land. "The problem is that everyone is trying to save their own lands," says Soomro. "The politicians in the area are diverting the water away from their lands and on to those of others."

A widespread allegation doing the rounds is that Khurshid Shah, a minister from the ruling Pakistan People's Party, used his clout to get the water diverted towards the populated areas near the town of Sukkur. Traditionally, floods in Sindh have been mitigated by cutting a breach on the left side of the embankment. The water is then supposed to flow towards the desert. On this occasion, the embankment was cut on the right side.

"Even in this tragedy," says Soomro, "people are still playing politics."