By Linda Duffin
Business Reporter, BBC World Service, Calgary, Canada
One country has abundant fresh water, far more than it needs. Across the border there is simply not enough and it has yet to find a solution to the problem.
This is the situation Canada and the US find themselves in.
The Canadian Rockies is a source of fresh water
Canada has, by some estimates, up to 20% of the world's fresh water supplies and only 0.5% of the world's population.
You would think there would be enough to go around, perhaps even a little left over to share. But this is not the case.
In the south-western corner of the United States, drought has meant that Lake Mead, which supplies the Las Vegas valley, is shrinking fast.
The Colorado River, a critical source of drinking water for southern California and Arizona, and the feeder of the Hoover Dam, has seen its flow cut by half in the past few years.
Now the US has turned its attention to its northerly neighbour Canada in hoping to find a solution to its water shortage, but it is not looking easy.
The former US ambassador to Canada, Paul Cellucci, says it makes no sense for Canada to refuse to exploit one more of its natural resources.
Lake Mead in the USA is running dry
"The Canadians sell us an awful lot of oil, more oil than Saudi Arabia," he says.
"Eighty-six per cent of our natural gas imports come from Canada. We get uranium, we get all sorts of resources. Interestingly all these resources are finite."
"Water, on the other hand, as long as it keeps raining, is renewable, it is replenishable," he says.
"I always found it odd that the Canadians will sell us oil and gas, but would not even talk about the possibility of selling fresh water."
But rain or no rain, much of Canada's water is a finite resource.
In the snow capped Canadian Rockies giant glaciers inch down the mountainsides, feeding icy turquoise lakes that lay at its feet and down into the rivers that irrigate Canada's vast prairies.
However, climate change means the glaciers are fast melting and once they are gone, they are gone.
The former premier of the province of Alberta, Peter Lougheed, predicts that the US will be aggressively coming after Canada's water in the next three to five years.
And he wants Canadians to say no.
"It would be foolish for us to sell it simply because we have a surplus of it now," he says.
"It's a very, very hot issue. There is not any issue I've come across in public life that people get more emotional about than fresh water.
"At some stage, a US senator is going to say 'we have a free trade agreement with Canada so why don't we exercise that right?'
"The reason I speak is to forewarn Canadians about it, prepare for it and reject it."
Canadians admit they need to clean up their act.
They are some of the world's most profligate users of waters, consuming twice as much as people in France and four times as much as the Swedes.
Canadians are likely to come under increasing pressure to share their water wealth.
But a thirsty US could have an uphill struggle.
One poll suggests 70% of Canadians agree with Peter Lougheed and are vehemently opposed to bulk water exports.