The hottest, driest summer since Chinese records began 61 years ago has wilted crops and left reservoirs at half their normal water level.

Factories in Sichuan province were shut down last week to save power for homes as air-conditioning demand surged, with temperatures soaring to 45C, and their closure may be extended for another week.

Agriculture Minister Tang Renjian said the coming 10 days was a "key period of damage resistance" for southern China's rice crop, according to the Global Times newspaper.

Authorities would take emergency steps to "ensure the autumn grain harvest", which was 75 per cent of China's annual total, Tang said.

They would "try to increase rain" by seeding clouds with chemicals and spray crops with a "water-retaining agent" to limit evaporation, the agriculture ministry said on its website. It gave no details of where that will be done.

The disruption adds to challenges for the ruling Communist Party, which is trying to shore up sagging economic growth before a meeting in October or November when President Xi Jinping is expected to try to award himself a third five-year term as leader.

A reduced Chinese grain harvest would have a potential global impact. It would boost demand for imports, adding to upward pressure on inflation in the United States and Europe that is running at multi-decade highs.

The thousands of factories in Sichuan province that make solar panels, processor chips and other industrial goods are awaiting word on whether last week's six-day shutdown will be extended.

The governments of Sichuan and neighbouring Hubei province say thousands of hectares of crops are a total loss and millions more have been damaged.

Hubei's government declared a drought emergency on Saturday and said it would release disaster aid. The Sichuan government said 819,000 people face a shortage of drinking water.

Sichuan has been hardest hit by drought because it gets 80 per cent of its power from hydroelectric dams, and reservoirs are at just half of normal water levels.

Local authorities earlier called on manufacturers to "leave power for the people".

Australian Associated Press