The Río Grande’s diminishing water supply:
causes and cures

By Jim Earhart

Mathematical calculations indicate that the Río Grande will be dry around the end of this century. Habib Far, Laredo Community College mathematics instructor, made these calculations and the resulting prediction based on analyzing a century of U.S. Geological Survey water flow data. Last year the river ceased to flow into the Gulf of Mexico. The sandy beach that replaced the mouth of the Río Grande is a reminder of Mr. Far’s ominous prediction presented at a mathematics seminar in 1998. If stakeholders in the Río Grande Watershed do not begin serious cooperative efforts to manage water resources, they face a future of political and economic strife over a diminishing water supply.
Drought is a normal component of the arid and semiarid country that comprises the majority of the Río Grande Watershed. The ever-growing human demand on surface and ground water, coupled with the encroachment of water-thirsty alien plants, has resulted in a steadily diminishing water supply. The current low water levels in Amistad, Falcon, and other reservoirs in both the United States and Mexico are further evidence of this trend. On May 20, 2002 water levels in U.S. and Mexican reservoirs published on the International Boundary and Water Commission Website ranged between zero and 44% of their capacities. Amistad Reservoir, near Del Río and the largest of all reservoirs in the watershed, was at 27.5% of its capacity. Amistad stores the water that provides for the needs of Laredo and other downriver communities. Falcon Reservoir, located downriver from Laredo, was at 8.5% of capacity. This lake supplies water to the lower Río Grande valley.
Recent rains and flooding lead many to believe that the drought is over and that we have plenty of water. Not so; Amistad Reservoir, as of October 11, 2002, had only about 30% of its normal capacity. Falcon had about 26% of its capacity.
Another way to look at the effects of recent rain episodes on water storage levels is to consider the average reservoir level in all the major Río Grande reservoirs in both the U. S. and Mexico. Prior to recent rainfall episodes the average capacity of Mexican and U. S. Reservoirs stood at 18.6%. On November 11, 2002 the storage levels were at 32%, still less than a third of normal capacity.
The diminishing water supply in the Río Grande is of paramount concern to the agricultural industry and municipalities, two of the three major reasons for the decline of water availability. Drought and flooding are components of a natural cycle that was operating in the Río Grande Watershed long before people came on the scene. Now, humans are inserting their presence into this natural cycle. The human population, already large, continues to grow and create increasing demand for water. Water intensive agricultural practices consume 80 to 90% of the water taken from the Río Grande for human activities, and invasive water-consuming weed species have been introduced into the watershed.The major weed species resulting in decreased water availability is salt cedar (Tamarix spp.). This invasive tree, introduced to North America as an ornamental and used for erosion control, has a very high evapotranspiration rate. A large salt cedar tree can lose as much as 200 gallons of water to the atmosphere each day. Studies in Arizona have shown that under favorable conditions one acre of salt cedar can use more than nine acre feet of water each year. Salt cedar thrives in both wet and dry conditions, giving it a competitive advantage over other plants that grow in the Río Grande Watershed. If the roots of a salt cedar are inundated with water, the tree can still survive for nearly two years. In drought it can survive indefinitely since its roots can extend over 150 feet into the earth to find water. Underground water is brought to the surface and lost to the atmosphere by evaporation. This explains why forests of salt cedar are capable of drying up the surface water of a flood plain while lowering the underground water table. Salt cedar is one of the culprits causing the diminished water supply that worries city planners and farmers along the Texas-Mexico Border. Not only does salt cedar steal our water, but also its prolific growth can increase the frequency of wild fires and reshape stream channels. A basin wide cooperative approach in dealing with this problem is imperative if river water levels are to be restored.
Human activities such as irrigation, root plowing, and deforestation are increasing the salinity of the Río Grande. Increased salinity favors the growth of salt cedar while discouraging the growth of native trees such as willow and cottonwood. If the Río Grande water supply for the future is to be maintained, salt cedar must be controlled. Small preliminary studies show that an increased flow of surface water may follow the removal of salt cedar. Test projects have shown that cutting trees near ground level and immediately applying herbicide to the cut stumps can successfully remove this invader. The effectiveness of a salt cedar eradication program is currently being tested on a 658-acre test area along the Pecos River. Since the Pecos is a major tributary of the Río Grande, increased contribution of water by that river into Amistad Reservoir would help supply the future water needs of Laredo and other cities along the Río Grande.
In its native range from Southern China to Northern Africa 115 insect and four mite species attack salt cedar. It may be that some of these species can be introduced to exert biological control. That action, however, carries a risk of creating other ecological problems. A number of biological control studies are now in progress. The removal of salt cedar will obviously be a major undertaking, but with basin wide cooperation, planning and effort it must be done.
Another major culprit in the decreased water flow of the Río Grande is the use of water wasting agricultural irrigation methods. The flooding of field crops and orchards results in the loss of tremendous quantities of water to evaporation. Newer irrigation techniques exist that provide more efficient use of water. Drip irrigation was employed very successfully a decade ago at the Texas-Israeli Exchange Farm on the Laredo Community College Campus. This irrigation technique can save large volumes of water by carefully dispensing the necessary quantity of water to each plant. This technique, of course, requires added expense for setting up the water delivery system. Farmers could recoup costs of changing to more efficient irrigation systems by selling saved water to municipalities.
A simple example demonstrates the potential advantage of establishing a coalition between farmers and cities. Out of 100 gallons of water taken from the Río Grande, agricultural irrigation uses 80 gallons and cities use 20. Let’s say a farmer, using a more efficient irrigation technique, reduces his water usage by 10%. He can now do with 72 gallons of water an irrigation job that previously required 80. A city purchasing the eight gallons of water saved by the farmer will now have 28 gallons or 40% more than it was getting prior to its partnership with the farmer. Compare this increase in water yield to the publicized estimate of a 10% water increase for Laredo by tapping the Carrizo-Wilcox Aquifer in northwest Webb County. The agricultural savings of water would be a preferable "secondary source" because, unlike the Carrizo-Wilcox Aquifer, its quantity and quality are both known and it does not have to be pumped from great depths and transported by pipeline to the city. The water yield of the simple scenario described above is probably very conservative. An Israeli study comparing drip irrigation to conventional irrigation showed a reduction in demand of water by 60% in growing potatoes, apples, and bananas and about 30% for avocados and cotton.
It has been publicly stated that Laredo, unlike San Antonio, has no aquifer and that unused water will drain into the Gulf of Mexico and be wasted. Not so! As of late the Río Grande doesn’t even reach the Gulf of Mexico, and besides Laredo does have a good place to store its water -- Amistad Reservoir near Del Rio. Lake Amistad, with its deep canyons, is a relatively efficient storage facility and its water is released on a demand basis. If downriver users conserve water, fewer releases will be made and water can be saved for longer periods of time.
Although U. S. cities in the Río Grande Watershed use much less water than is consumed by agriculture, they can still reduce usage and extend water availability by using good conservation techniques. Laredo uses 220 gallons per person per day, whereas El Paso and Albuquerque use 170 gallons and 200 gallons per person per day respectively. Mexican cities in the watershed consume less than 100 gallons of water per person per day.
San Antonio has developed a model water conservation program, using xeriscape, water saving devices in homes, rain harvesting, and water recycling to reduce demand on the Edwards Aquifer. Use of various water conservation techniques has reduced San Antonio’s per capita water consumption from 200 gallons to 140 gallons per day. Cities in the Río Grande Watershed should follow San Antonio’s example.
Presently municipalities can afford to pay more for a gallon of water than can an irrigating farmer. A gallon of water in an urban setting generates more jobs and money than it does on the farm, but we must not forget that the production of food is vital in supporting our society. We must work with our watershed neighbors up, down, and across the Río Grande to control invasive plant species, to improve irrigation practices, and to implement more efficient water conservation techniques. We must reach some kind of intelligent compromise in conserving and equitably dividing our limited water resources if we are to improve and maintain our present standards of living.

(Dr. Jim Earhart is executive director of the Río Grande International Study Center. He may be reached at (956) 721-5392 or rgisc@laredo.edu.)