Engineer Kim Chang Ho monitors the daily ebb and flow of river water into a Japanese colonial-era dam. But there's one variable he can't fathom: North Korea, which lies upstream.
Democratic South Korea and communist North Korea are split by a sealed border and animosity that dates from the 1950-53 Korean War. But they share a major tributary of the Han River, which cuts through the heart of Seoul, the South Korean capital.
The cross-border tributary is vital to both, which funnel their water from dams to hydroelectric plants, providing power and potable water to millions of people. Downstream, the South has long been uneasy about the North's ability to regulate the flow.
"They shut and open their dam according to their own convenience," Kim said at the hydroelectric plant near Hwachon Dam, 15 miles south of the border.
It's hard to predict the river flow, he said, "unless they notify us, which they're not doing."
The predicament took on extra urgency in recent years with North Korea's virtual completion of a giant dam near Diamond Mountain, a scenic area which the North has opened to tourists from the South in exchange for desperately needed cash.
Energy-starved North Korea is now diverting huge amounts of water from the Han tributary to hydroelectric facilities on the east coast. Tens of thousands of Northern soldiers burrowed through mountains to reroute the water.
The amount of water flowing annually into Hwachon Dam--one of half a dozen Southern dams on the Han tributary--has dropped by 12% to 2.6-billion tons since 1996, according to the South's Ministry of Construction and Transportation.
Experts said drought in the North may also have contributed to the water loss. However, the ministry said the loss has not affected electricity and drinking-water supplies to areas as far south as Seoul.
Park Eung Kyuk, a public policy professor at Seoul's Hanyang University, believes the issue could escalate.
"We cannot live without water resources," Park said. "This is one of the most urgent problems between the South and the North since the Korean War."
At the height of a reconciliation process last year, the two Koreas embarked on a host of projects, including family reunions, the reopening of border liaison offices and construction of a cross-border railway.
The two sides discussed building a dam to control flooding on the Imjin River, which flows across the western sector of the border. No progress was made, and government contacts stalled this year amid tension between Washington and Pyongyang.
This month, the South said the North was building two small dams on the Imjin River, but had yet to divert any water.
South Korean jitters over the larger Han River reached a peak in 1986, when North Korea started the dam near Diamond Mountain, or Kumgangsan in Korean.
Former South Korean President Chun Doo Hwan feared the North might use the dam as a weapon, unleashing stored water and flooding parts of the South.
So Chun ordered the construction of a dam to block any onrush of water, and millions of South Koreans eagerly donated to the project. Critics dismissed the South's $122-million Peace Dam project as a wasteful symptom of anti-communist hysteria.
Today, the dam sits idle, its four gates open to the current from the North. A nearby museum displays the wetsuit, binoculars and rucksack of a North Korean infiltrator.