Why has 1944 water pact with US sparked Mexico unrest?

Protests over a decades-old water-sharing treaty have shaken northern Mexico, where farmers seized a dam to try to prevent the country from supplying the neighbouring United States.

Demonstrators have occupied the La Boquilla dam in the border state of Chihuahua since September 8, saying they fear a drought will ruin their crops.

Seventeen soldiers were detained for investigation after a woman was shot dead in the unrest. The National Guard called her death an "unfortunate accident."

The government says that the hydroelectric dam has suffered damage amounting to $4.7 million.

Under the pact dating back to 1944, the neighbouring countries share water from two major rivers flowing from the southwestern United States to Mexico.

The agreement obliges the United States to deliver 1.85 billion cubic meters of water a year from the Colorado River.

In return, Mexico must supply an average of 432 million cubic meters annually over a five-year cycle from the Rio Grande, which forms part of the border between the two countries.

The United States complains that its neighbour owes almost a year's worth of water that it must supply by October 24.

Authorities in the US state of Texas say the water is vital for crop irrigation, municipal water supplies and industry.

Texas Governor Greg Abbott has written to US Secretary of State Mike Pompeo to ask him to intervene to ensure Mexico complies with its side of the deal.

Mexican President Andres Manuel Lopez Obrador says that his country has already delivered 82 per cent of its obligation and will supply the remainder on time.

He has ruled out trying to renegotiate the pact, saying: "I don't think there's a better deal."

Although Lopez Obrador has warned that Washington could raise tariffs if the agreement is broken, geographer Gonzalo Hatch Kuri does not expect any such retaliation.

"The United States has never waged war on us for not paying it" in the past, said the professor at the National Autonomous University of Mexico.

"It has not gone beyond diplomatic friction."

But the water debt is becoming a political issue in the United States as Abbott presses Pompeo.

"Mexico needs to end the cycle without a debt. Mexico ended the last cycle, as well as several previous cycles, in a debt. This trend cannot continue," he wrote in a letter dated September 15.

With US President Donald Trump seeking re-election on November 3, Hatch Kuri does not rule out the possibility of water becoming a campaign issue.

But as Lopez Obrador has sought to maintain cordial relations with Trump, the Mexican leader is anxious to meet its obligations under the pact, he added.

Lopez Obrador has assured Mexican farmers they will have enough water for the agricultural season that ends in October.

But the farmers, backed by Chihuahua Governor Javier Corral, say that taking water out of the reservoirs will threaten the next season's crops.

Parched Chihuahua has registered temperatures of 45 degrees Celsius (113 degrees Fahrenheit) this year.

On Sunday farmers massed in the northern city of Delicias in protest against the supply of water to the US and the death of their fellow demonstrator.

Lopez Obrador has said that he is willing to talk with Trump to make the deal more flexible.

But he criticizes the protesters for being intransigent and suggests that there are political motivations behind the unrest ahead of next year's gubernatorial elections.

Corral, who is allied with Lopez Obrador's political opponents, accuses the government of failing to cooperate on the issue.