A pier sits high above above the shoreline of a lake where an evaporation-suppressing treatment was tested last summer. Credit: Larry Smith/EPA/Corbis

In the southwestern United States, where years of drought are leading water managers to consider drastic water-provision measures such as desalination and cloud seeding, entrepreneurs have suggested reviving a water-saving technique that was tried and abandoned half a century ago. They propose to stretch dwindling water supplies by slowing down evaporation from reservoirs by means of a surface barrier of cheap, non-toxic, biodegradable chemicals just one molecule thick — two-millionths of a millimetre. The technology is far from proven, but it showed some potential in field tests in Texas last year.

Worldwide, more water evaporates from reservoirs than is consumed, and the losses are especially acute in hot, dry regions. The idea of a coating to slow evaporation has been floated for decades, and government researchers in the United States and Australia have investigated the concept. The approach, which generally uses chemicals derived from coconut or palm oil, is already used on small bodies of water such as golf-course ponds and swimming pools. But it has not been practical for larger bodies because wind tends to break the layer apart, says Moshe Alamaro, an engineer at the Massachusetts Institute of Technology in Cambridge.

Last summer, a field test in Texas attempted to overcome that problem. The US$325,000 test ran from July to October on Lake Arrowhead, a 21-square-kilometre reservoir that serves the city of Wichita Falls. Flexible Solutions International, a company in Victoria, Canada, that makes evaporation-reducing coatings for small water bodies, programmed a boat to run on autopilot and make a grid pattern across Lake Arrowhead spreading the coating behind it. An analysis published in January by the Texas Water Development Board suggested that the results were promising, but not conclusive. Compared with a similar reservoir nearby, the treated reservoir lost an estimated 15% less water to evaporation. But the analysis could not attribute all of that difference to the coating, because it did not account for variables such as stream inflows and seepage outflows. The coating probably helped, says Mark Wentzel, a hydrologist for the Texas Water Development Board and co-author of the report, but “I wouldn’t stake my life on it.” Liquid market Alamaro suggests that a more aggressive technological approach is needed. Radar instruments carried on a blimp or drone could reveal where the reservoir’s coating has broken up, he says, by sensing the way that it dampens ripples on the water. More coating can be added when and where it is needed, potentially cutting evaporation by 70%, he estimates. Alamaro has founded a company, More Aqua in Cambridge, Massachusetts, that is trying to develop a system that would use diffusers and skimmers to keep a body of water thoroughly and continuously covered. More Aqua is planning its own pilot test near Palo Alto, California, this summer using its own coating. It plans to offer its services in exchange for owning the water that it saves, and to sell that on the open market in the state, where the cost of water for irrigation can exceed$1,000 per acre-foot (1,233 cubic metres).

Assuming evaporation savings of 15%, says Daniel O’Brien, president of Flexible Solutions, the $160 cost of saving one acre-foot of water with the coating compares favourably with a Texas market price of$345–700 per acre-foot.

Water consultant William Mullican of Lubbock, Texas, who is retired from the Texas water board, says that although the results from Lake Arrowhead were unclear, this is also often the case in field tests of other water-sparing techniques such as cloud seeding and cutting back the brush to keep it from sucking moisture out of the soil.

Given the Texas test’s promising result and the fact that the ongoing drought in the state is severe, he says, there is every reason to try the technique out again, “unless it starts raining”.