AUSTIN, Texas — A group of Rio Grande Valley irrigators and farmers is seeking $500 million from Mexico for crop loss and other damages the group says were caused by that country's failure to comply with a water-sharing treaty.

Seventeen irrigation districts, a water supply company, and 29 individual farmers notified Mexican officials recently of the claim under the North American Free Trade Agreement (NAFTA), signed among the United States, Canada, and Mexico and took effect in 1994.

"We want to be a good neighbor to those people. It's hard to do when you're losing money," said citrus farmer Jimmie Steidinger, who estimated he's lost about $200,000 over the last decade and can't farm all his land because of a lack of water.

The 1944 water-sharing treaty requires Mexico to send the United States an average of 350,000 acre-feet of water annually from six Rio Grande tributaries. The United States in return must send Mexico 1.5 million acre feet from the Colorado River. An acre-foot is 326,000 gallons, enough water to flood an acre of land a foot deep.

Mexico has owed water for the past decade. By 2002, Rio Grande farmers were going out of business and municipal water supplies were running low while Mexican produce flowed into the state.

While Mexico agreed to several partial payments and has reduced some of its water debt, those taking the legal action think the government hasn't pushed enough. The farmers want up to $500 million for damages of lost crops and sales incurred from 1992 to 2002. The claim was calculated by a Texas A&M University economist.

"I've seen the abandoned fields and the reduced crop yields," said Jo Jo White, a spokesman for the farmers and water districts. "This was man-made. Not an act of Mother Nature."

Attorneys for the group said they sent paperwork Friday to Mexican officials, giving them 90 days to pay the claim or submit to arbitration.

The Mexican government sharply criticized the claim, saying it could cause problems for bilateral cooperation on the issue.

"Any claim that there could have been some damage to U.S. farmers is unfounded, given that (Mexico's) average water payments have been greater than those called for in the treaty," said Juan Bosco Marti, the Foreign Relations Ministry's director for North American Affairs.

Abundant rains in 2003 and 2004 largely replenished South Texas' two Rio Grande reservoirs and allowed Mexico to reduce its water debt from 1.5 million acre-feet to less than 800,000 acre-feet.

Nancie Marzulla, an attorney representing the Rio Grande Valley water interests, said the claim falls under a NAFTA provision requiring countries to compensate for taken property and forbids discriminatory treatment of foreign investors. She said the same NAFTA provision has been used for damage claims two dozen times in the history of the trade agreement, with claims against Mexico, the United States, and Canada.