The water storage rate of Obong Reservoir, the water source for Gangneung, Gangwon Province, which is experiencing an "extreme drought," fell to 11.6% at 9 a.m. on the 12th. If the storage rate falls below 10%, water will be rationed from 10 p.m. to 5 a.m. the next day, and if the situation worsens, rationing will occur every other day.
But Gangneung Mayor Kim Hong-gyu said on the 1st, "In the worst-case scenario where the reservoir runs dry, we can hold out by using the (Obong Reservoir) dead storage (死水量)." What does this mean? Does it mean there is more usable water even if the storage rate drops to 0%?
The storage rate and dead storage are separate concepts. By analogy to a building, the water stored on the above-ground floors is the live storage, and the water in the basement is the dead storage. The water on the above-ground floors naturally flows out through the first-floor sluice gate using the head difference. Once transferred to the water treatment plant and purified, this water is supplied to households as domestic water.
However, the water in the basement below the first-floor sluice gate remains standing. In normal times, it is not used as water for supply. This water is called dead storage. Even if the storage rate drops to 0%, water exists below that level for exactly this reason.
The Ministry of the Interior and Safety and the city of Gangneung are preparing to pump up this dead storage and send it to the water treatment plant for use. Mobilizing dead storage during a drought is unprecedented. For this reason, the Ministry of Environment at one point expressed reservations, saying, "If all the water dries up, creatures such as fish cannot survive," and the Ministry of Agriculture, Food and Rural Affairs noted, "Next year's farming could be disrupted." A Ministry of the Interior and Safety official said, "There was brief opposition, but since people's lives come first, there was a consensus forming in a mood of acceptance."
The scale of dead storage at Obong Reservoir is currently estimated at about 800,000 tons (t). In practice, only about two-thirds of this (about 530,000 t) can be pumped up for use.
This is enough to hold out for a little over a week upon reaching a 0% storage rate. Gangneung currently uses about 70,000 t per day as domestic water. However, since the pump can draw up only up to 5 t of water per day, there will likely be constraints on actual water use by residents.
The government is making every effort to slow the pace of decline in Obong Reservoir's storage rate. It is mobilizing fire and military units and private water trucks and cutting a channel from the river above the reservoir to draw a total of 35,000 t of water per day. As a result, the storage rate, which used to drop by 0.4 to 0.5 percentage points (p) per day, is now decreasing by 0.2 p per day.
Ultimately, rainfall is the fundamental solution. The amount of rain forecast for the Gangneung area of Gangwon on the 12th to 13th over two days is 20 to 60 millimeters. Typically, 100 millimeters of rain raises Obong Reservoir's storage rate by 20 percentage points. If up to 60 millimeters falls as forecast, the storage rate can be raised by 12 percentage points, but if only 20 millimeters falls, it will be replenished by just 4 percentage points.
A Ministry of the Interior and Safety official said, "For now, all we can do is hope that as much rain as possible falls," adding, "We hope not to reach the stage of mobilizing dead storage."