WESLACO, Texas – After months of gradual increases from beneficial rainfall, lake levels at Falcon and Amistad Dams are beginning to drop, not from use by municipalities and farmers, but from evaporation.

Lakes at the two dams on the Rio Grande in South Texas are the major sources of water for the Lower Rio Grande Valley. For years, their levels had been perilously low.

“Increased evaporation is the price you pay for having lots of water,” said Carlos Rubinstein, watermaster with the Texas Commission on Environmental Quality in Harlingen. “The more water you have, the more surface area you have and the more you lose to evaporation. But don’t get me wrong; I’d rather have the water.”

Rubinstein said losses to evaporation in May totaled 40,000 acre-feet. Projected losses for June are between 60,000 and 65,000 acre-feet of water, roughly the amount of water used in a three-and-a-half month period by all municipalities between Del Rio and Brownsville. An acre-foot is 325,581 gallons of water, or enough to cover an acre of land one foot deep.

“These evaporative losses were anticipated. With higher temperatures and larger surface areas of water, evaporation will increase. It’s a normal part of operating this system. But we do have reserves, called operating reserves, to cover these losses,” he said.

Rubinstein’s office tracks evaporation losses on a weekly basis.

“Losses at Amistad are greater now because 70 percent of the water we own is at Amistad,” Rubinstein said.

Evaporation reversed what had been a steady increase in lake levels the past several months due to above-normal rainfall that began last fall.

The current combined U.S. ownership of water at both dams stands at almost 70 percent. Reserves were roughly half that a year ago.

Evaporation losses are an unavoidable facet of water management here, according to Juan Enciso, a water engineering specialist at the Texas A&M Agricultural Research and Extension Center at Weslaco.

“Evaporation is a loss that is accepted,” Enciso said. “It’s something that can’t be avoided. The only recourse is to cover bodies of water and, of course, it’s impossible to cover a lake.”

Water losses due to evaporation in the Valley’s vast and aging irrigation system are also substantial but largely unavoidable.

“In an irrigation system, you can only control run-off, spills and deep percolation losses,” he said. “Water is either piped or sent through open-air canals. Piping deters evaporation but piping has its economic and physical limitations.”

Without sufficient slope in terrain, transferring high-flow volumes becomes an economic issue.

“That’s where economists come into play,” Enciso said. “In situations of high volume and not enough gradient, economists have to evaluate and prioritize those financial investments. Pumps can be used to speed up water flow and reduce pipe diameter, but for agriculture here and now, in many cases, it’s not justifiable because the investment just won’t pay for itself.”

Enciso said the situation here could change in the future, as it has in Europe, but for now such investments are not feasible.

“In France, for example, practically all water is moved with pumps and pipes. But France made that investment because water there is so expensive. And the costs of that infrastructure are paid largely by water fees in urban areas, not by rural agricultural interests,” he said.

Rod Santa Ana III is a writer for Texas A&M University.

e-mail: r-santaana@tamu.edu