Logo: University of Southern California

The Water Beneath the Surface

Mahta Moghaddam and her team monitor soil moisture using wireless sensors and radars

April 18, 2014 —

“Water content of the soil is one of the most important environmental variables for ecologists and hydrologists because it’s a first order determinant of the water balance globally.”

Mahta Moghaddam, USC Viterbi professor in the Ming Hsieh Department of Electrical Engineering, and her team are using a combination of tools to monitor soil moisture levels. The implications are serious. The effects of climate change are happening now, and we need to know how different environments are changing, by how much, and when.

Variation in soil moisture can be observed at multiple scales from meters to 100s of kilometers. To capture this, Moghaddam and her team are using a number of novel technologies.

To monitor plot level variations in soil moisture, a network of custom made soil moisture sensing nodes are being installed, as part of the SoilSCAPE project. Currently there are over 100 sensors set up across central California, with additional sites in Oklahoma and Michigan. Multiple sensors, strategically positioned across the landscape, are required to capture local variations. The deepest sensors are placed up to 90 cm (36 inches) under the ground to get accurate readings of soil moisture at the root level. “It involves a lot of digging,” said Daniel Clewley, a postdoctoral researcher in Moghaddam’s lab.

Ph.D. students (left to right) Pratik Shah, Richard Chen and Agnelo Silva installing wireless soil moisture sensors.

Traditionally a small number of soil moisture sensors are connected to a large solar panel, and measurements are stored to a data logger, from which they need to be physically retrieved. Therefore, installing large numbers of sensors is costly, and there is often a large gap between collecting and receiving data. For SoilSCAPE, a completely different approach was considered. Wireless technology was used to allow a large number of sensors to be installed per site. “The idea for this was to have them wirelessly send the data to one central unit,” said Moghaddam. “Then that sends it back to us.”

All of the wireless devices are custom designed and built. "We've done a lot of work on energy efficiency, allowing the non-rechargeable batteries in each sensor to last over two years, collecting and transmitting data every 20 minutes," said Ph.D. student Agnelo Silva. These advances mean that it is possible to get real-time data from field sites hundreds of miles away, and to monitor increases in soil moisture as it rains. These data are made immediately available to the scientific community and general public through the project website (http://soilscape.usc.edu).

As well as providing very detailed information over a small area, the soil moisture sensors are used to help validate estimates of soil moisture over larger areas produced from airborne and spaceborne instruments. Moghaddam is leading the AirMOSS (http://airmoss.jpl.nasa.gov/) mission, an airborne campaign to estimate soil moisture for representative biomes across North America. Through the use of physics-based models of vegetation and the ground surface, Moghaddam and her team are able to produce maps of soil moisture covering areas of 2500 km2 at a resolution of approximately 100 m. For every 100 m x 100 m pixel soil moisture is estimated at a range of depths from 0 – 100 cm. The maps help understand spatial variations in soil moisture at the surface and root zone levels and are being used by collaborators at various other universities and government labs as input into hydrology and carbon models. The coverage and resolution of the soil moisture maps produced from AirMOSS are allowing them to produce better predictions of net ecosystem carbon exchange, and gain a better understanding into the natural carbon cycle.

Soil moisture over the Metolius site in Oregon at a depth of 20 cm derived from AirMOSS data.

Both the ground and airborne measurements of soil moisture will be used to help validate spaceborne radar and radiometer estimates of soil moisture from NASA's Soil Moisture Active-Passive (SMAP) mission, due to be launched later this year.

Changes in soil moisture are a harbinger of climate change and all the complications that arise with it: drought, vegetation changes, vanishing habitats and ecosystems in peril. Monitoring and understanding these effects will be ever-increasingly vital.

“The environmental scientists have been after global information about soil moisture, and there are [multiple] satellite systems right now either operating or being planned to be launched that study soil moisture. We are engineers, but we help scientists to make these measurements,” said Moghaddam.