May 6, 2015

A new network of sensors in the bone-dry Texas hill country will produce detailed data on soil moisture -- when there is any. While verifying the measurements of NASA's recently launched Soil Moisture Active Passive satellite (SMAP), the Texas Soil Observation Network (TxSON) will give state water agencies critical information for managing the Lone Star State's limited water.

"The drought has decreased soil moisture so intensely that if it rains, reservoirs don't fill up," said Todd Caldwell, a research associate who leads the TxSON project at the University of Texas at Austin's Bureau of Economic Geology. "We haven't been able to overcome this major depletion of soil water."

The Texas hill country, west and north of San Antonio and Austin, is no stranger to drought, but 2011 was the worst year in recent memory. "It was the hottest and driest summer in American history," recalled Carol Ann Sayle, who owns a farm in Austin. "We spent that whole summer trying to irrigate the soil back to life, but everything crisped to death."

After a year like that, a few rains cannot restore crops, grazing lands or reservoirs to their pre-drought condition. "We've been through times where two feet down, the soil is dry as dust," Sayle said. Before plants can grow and water can run off into reservoirs, the soil must soak up a considerable amount of moisture. Water planners currently can only estimate what that amount is. With SMAP and TxSON monitoring soil moisture, they will soon have measurements to help them plan more realistically.

The TxSON network consists of more than 40 stations in a 500-square-mile area (1,300 square kilometers) near Fredericksburg, Texas. Each station includes a moisture sensor that extends about 20 inches (50 centimeters) into the ground, and a rain gauge, along with supporting hardware and solar panels. The sensors take measurements every five minutes and send their data to the University of Texas once an hour. From there, the data are relayed to NASA's Jet Propulsion Laboratory in Pasadena, California, which manages the SMAP mission.

Satellite data need to be compared against measurements on the ground to assure that the spaceborne instruments are accurate and remain so throughout the mission. This is part of the process known as calibration and validation, or cal/val for short. The SMAP mission is using several research networks worldwide for the process. Because the TxSON network was installed after SMAP's cal/val requirements were established, it is one of the best matched of these networks to the scales of SMAP's data collection. "Our goal was to set up the best possible calibration point that SMAP could have," Caldwell said.

Equally, however, Caldwell's goal is to help state water agencies serve the citizens of Texas. "If we have some idea of the current state of the soil moisture, we can predict a lot, from ranchers' rangeland productivity to regions where rain is more likely to fall or not to fall," he noted.

Caldwell has been meeting with river authorities to show them the potential of the soil moisture measurements to improve their planning models. "We show them the lag between rain and any response, either in wells or reservoirs downstream. They're interested in what role soil moisture might play in that lag time," he said. "The drought has shown several flaws in our conceptual model of the water cycle in Texas. With good soil moisture measurements, we hope to fix those flaws."

Image: Todd Caldwell checking one of the sensors in the Texas Soil Observation Network.