After sustained growth over the past three months, computing power on the bitcoin network has seen a fallback as the summer rainy season trails off in China.

According to data from Poolin, the world’s largest bitcoin mining pool by real time hash rate, bitcoin’s seven-day average computing (or hashing) power has dropped to around 90 exahashes per second (EH/s) since Oct. 24, signaling that some miners have been unplugging from the network. It had previously been estimated that the hash rate would go above the 100 EH/s threshold by the end of 2019

As a result of the power drop, data from mining pool service BTC.com estimates that bitcoin’s difficulty – a measure of how hard it is to compete for mining rewards on the world’s top cryptocurrency by market value – will decrease by 1.5 percent when it’s set to adjust in about seven days.

Bitcoin’s mining difficulty had reached an all-time-high at 13.69 trillion on Oct. 24, following a 38 percent increase since early August. The climb resulted mainly from an increase in miners’ hashing power made possible by the abundant and cheap hydroelectricity in China’s southwestern provinces.

Mining difficulty is designed to adjust itself to go up or down about every 14 days, based on whether the hashing power on the network in the two-week cycle increases or declines, respectively. The Oct. 24 difficulty record followed a jump in the 14-day average hash rate to an all-time-high at 97.90 EH/s.

Poolin’s co-founder Chris Zhu said in a recent WeChat post that one main reason for the fallback over the last week is the gradual end of this year’s rainy season in China. As a result, some hydropower stations in China’s Sichuan province – estimated to account for 50 percent of bitcoin’s global computing power – no longer have the capacity to generate enough energy to support mining activities.

Miners without sufficient hydropower supply would have to shut down their operations or relocate to other provinces like Xinjiang or Inner Mongolio, where mining farms have a more stable, but more expensive, power supply generated from fossil fuel plants.

Xun Zheng, CEO of Hashage, which owns mining facilities in China’s southwestern Sichuan province, echoed Zhu’s comments, adding that even if some may still be able to find a hydropower resource, the cost has gone up from $0.04 per kilowatt-hour (kWh) in the summer to around $0.05.

Further, bitcoin’s sudden price drop on Oct. 23 to below $7,500 could have resulted in a large scale of shutdown of older but widely used mining models like the AntMiner S9 made by Bitmain. The S9’s profit break-even price point is between $7,000 to $7,500. However, the price’s significant rebound since last weekend may have stopped that panic.

The fact remains, though, that the profitability of the S9 and other similar modes made by Bitmain’s rival miner makers, is a critical issue for their utility lifespan. And that may soon be affected by the higher winter cost of electricity in China, as well as the scheduled halving of bitcoin mining rewards in May 2020 – before next year’s rainy season.

According to a miner profitability index provided by Poolin and its rival F2pool, at bitcoin’s current price and an electricity cost of $0.05 kWh, the mining profit margin of models like S9 is about 30 percent.

Some, like INBTC, a sister company of Poolin, are currently making efforts to extend the life of the S9 miner by merging two units into one in an attempt to generate a higher ratio of hashing power over electricity consumption. That would allow a higher daily profit margin than would be achieved using two individual units, though it remains to be seen if such a method can be proven to work and adopted on a large scale.

Mining farm image via CoinDesk archives