May 28, 2003

As the peak of the growing season approaches in northern Mexico, some trends are becoming evident regarding the disposition of Mexican water resources. This is the third report for the 2003 irrigation season and water-year. The current report includes the latest information about irrigation activities and environmental conditions in the Rio Grande Basin of northern Mexico based upon satellite observations and information reported by the Mexican Section of the International Boundary and Water Commission.

The most significant development during the past month has been the extraordinarily large volume of water delivered by Mexico for irrigation in Tamaulipas. Since April 1st, Mexico has released 355,326 acre-feet from their storage in Falcon Reservoir, leaving only 36,000 acre-feet under Mexican ownership in the International Reservoir as of May 17th. During that same interval, Mexico released 197,640 acre-feet from Presa Marte Gomez in the San Juan Basin. The combined total of 552,966 acre-feet has been used to irrigate farmland in northern Tamaulipas. Satellite imagery of the region collected during the 8-week period shows a rapid decline in the pool elevations of Falcon and Gomez reservoirs along with the greening of fields in areas of Tamaulipas that do not appear to have produced crops during 2002.

Since March 1, 2003, the release of water from Mexican storage now totals 804,238 acre-feet with the Conchos and San Juan basins delivering 234,209 and 214,703 acre-feet, respectively, and the remainder supplied from storage in Falcon Reservoir on the Rio Grande. Under the current pattern of operation, Mexico will release over one million acre-feet for irrigation by the first week of July.

Table 1:  The most recent figures for storage in the Mexican reservoirs

Figure 1: Landsat 7 images collected in 2002 and 2003 highlight changing irrigation practices in Chihuahua.

Changes in irrigation practices can be seen in the comparison of satellite images acquired in 2002 and 2003 for the Delicias District of central Chihuahua. A clear Landsat 7 image collected on April 25, 2003, shows new center-pivot irrigation systems in an area south of Cd. Delicias. The new center-pivot systems are marked “A”, “B”, “C” and “D” on the April 2003 image. Note that “A”, “C” and “D” replace rectangular fields of conventional furrow irrigation that are visible in the May 2002 image. “B” is a new field that expands irrigation into an area not farmed before. A likely explanation for the increased use of center-pivot irrigation is a shift towards growing alfalfa and sorghum to supply dairy farms and cattle feedlots in this area.

Figure 2: A Landsat 7 thermal difference image displays changes in the intensity of irrigation

The intensity of irrigation can also be compared between conditions observed in 2002 and 2003 using thermal data from satellite sensors that measure ground temperatures recorded on the two dates. A composite satellite image displays temperature differences that are directly related to the presence of water on the surface. Fields that are displayed in shades of blue were substantially cooler and wetter on April 25, 2003, than on May 8, 2002, indicating greater irrigation during the 2003 growing season. By contrast, yellow fields indicate more irrigation during May 2002. Green areas, including the very dark green fields, were actively irrigated in both April 2003 and May 2002. Note that the number of blue fields indicates considerably more irrigation is underway in the central and northern areas of the Delicias District during the 2003 growing season. This observation is consistent with the reported operation of the Boquilla and Madero dams that are currently delivering 104 and 160 percent, respectively, of their average daily release in May 2002. The much larger surface area of the Madero reservoir is shown in blue in contrast to the relatively low pool elevation last year displayed in black in the composite thermal image. In a few areas of the Delicias District, fields that appear to have been inactive for several years are once again receiving irrigation water in 2003, but their total acreage is relatively small, amounting to less than 5000 acres.

Coinciding with the large release of irrigation water for districts in Tamaulipas and the increased use of irrigation in central Chihuahua, the amount of water reaching the Rio Grande from tributaries in northern Mexico is approaching historically low levels. The Rio Grande channel runs dry in portions of the canyons through Big Bend, as noted by recent reports in the news media. For the past month, the Rio Grande stream gauge below the confluence of the Rio Conchos at Ojinaga has recorded a discharge of less than 20 acre-feet per day. Historically, the flow in this river segment averages 1714 acre-feet per day during May, according to IBWC records for 1968-2000. With only about 1 percent of the river’s historical discharge available, the riparian environment along this portion of the Rio Grande is seriously threatened by loss of wildlife habitat and natural vegetation due to drought and increased competition from invasive plants.

For the greater part, the discharge of the Rio Grande in the Big Bend region and the flow of the lower Rio Conchos are controlled by the release of water from the Luis L. Leon Dam in northern Chihuahua. Although the Leon Dam has released 24,857 acre-feet since March 1, virtually all of the water has been diverted for irrigation in areas south of Ojinaga, where the Rio Conchos joins the Rio Grande. No deliberate release of water to the Rio Grande has occurred from the Leon Dam since mid-February 2001. The Leon reservoir currently stores 100,528 acre-feet, approximately 37 percent of its conservation capacity.

Seasonal rains are beginning to move northward across Mexico and have recently reached the latitude of Mexico City. During the summer and fall of 2002, northern Mexico benefited from increased rainfall triggered by El Niño conditions in the eastern Pacific. Storage behind Mexican dams increased from 1.2 to 3.2 million acre-feet from early-July 2002 through mid-January 2003. The rainfall prospects for 2003 are less optimistic. Most long-range forecast models predict the onset of La Niña conditions in the eastern Pacific perhaps as soon as July or August. If this occurs, the beginning of the La Niña episode would coincide with the normal peak of the rainy season in northern Mexico. Historically, La Niña events correlate with a reduction in regional precipitation and inflows to the reservoirs. As a consequence, the timing of a forthcoming La Niña could strongly influence whether the storage lost during this year’s irrigation season will be replenished during the coming months.

The next report compiled from our sources will highlight conditions at the beginning of the summer rainy season in northern Mexico.

Table 2: Status of Irrigation Systems along Mexican Tributaries to the Rio Grande

Latest clear satellite image:
May 20, 2003 - Eastern Conchos Basin and irrigation districts in Chihuahua

Imagery shows greening of agricultural fields in the Delicias District on the Rio Conchos and along the Rio Florido in southeastern Chihuahua.

Next opportunity to collect satellite imagery:
June 6, 2003