Baltimore Ecosystem Study

Wireless sensor networks for soil monitoring

Figure 1. A MICAz mote with data acquisition board in an urban forest patch.

Having accurate environmental data with a good spatial and temporal resolution is simply not possible using the traditional manual techniques. Inexpensive wireless sensors provide a completely new way to attack these problems: our experiment measures soil moisture and temperature at several depths at a few meters spatial resolution every 10 minutes, without disturbing the environment that we want to study. Combined with other meteorological data like temperature, rainfall, humidity and topographic information the system has an extremely detailed picture of the conditions in the soil at the relevant spatial scales.

In January 2005 we started to build a wireless sensor network for soil monitoring, and in summer we deployed the first system (Fig. 1). In April 2006 the second system was deployed in Leakin park one of the BES permanent forest plots (Fig 2).. This second system provides background data for a field project in collaboration with Chris Swan, BES COPI. The objective of this project is to study interactive effects N-deposition and soil fauna activity on decomposition of speciose leaf litter. The two systems (each operated for a year) collected over 18M datapoints on air and soil temperature, soil moisture (from two different depths), light intensity, and battery voltage.

Figure 2. JHU undergraduate Allison Smykel deploying soil sensors in Leakin Park

Building the system was a very useful experience. First, it became obvious that the custom programming of the wireless “motes” was a non-trivial problem, and it required more computer science expertise than most biologists have. Also, it soon became clear that the road from raw data to a calibrated spatio-temporal data set suitable for the final scientific analysis is long and cumbersome. Third, it became clear that the traditional analysis data analysis tools and processes were not adequate to these data volumes (over 8 M datapoints!). Fourth, it became clear that we need to redesign the hardware, which also meant new software development.

Figure 3. The second generation soil monitoring node with ECH₂O-5 moisture sensors

Our second generation system is based on the Telos SkyMote platform, which has lower power consumption and better sleep mode properties (Figure 3). We have designed our own 4-channel analog interface board with its own stabilized reference voltage. We designed a new watertight enclosure with a lithium battery that has a lifetime of about 2 years. Sensors are attached through a waterproof multipole connector, making field replacements much easier. The humidity inside the box is also monitored with its own sensor. . The embedded software on the motes can now upgraded remotely, using the radio, without opening the box. The USB port of the mote is also accessible from the outside, should a total code upgrade be necessary. In order to connect the dense patches of sensor deployments to one another across larger separations, we have started experiments with long range radio transmitters that use the 900MHz band, and have a range of several miles. We are currently testing the software for repeater nodes that use the long range radio.

We switched from Watermark soil moisture probes to Decagon Echo-5 probes. These probes, although more expensive, produce very consistent results, do not “crosstalk” and do not have temperature dependence. To cut costs (which is considerable for a 200 mote network), we have built our own precision temperature probes which will have a consistency of better than 0.2 °C. We also purchased a Vaisala WXT510 weather transmitter that enables us to directly correlate our soil moisture data to precipitation events. (Currently we are harvesting BWI weather data) This equipment will be deployed in a forested patch of the JHU campus during spring 2007.

References:

Szlavecz, K, A. Terzis, S. Ozer, R. Musäloiu-E., J. Cogan, S. Small, R. Burns, J. Gray, A. Szalay. Life Under Your Feet: An End-to-End Soil Ecology Sensor Network, Database, Web Server, and Analysis Service. http://research.microsoft.com/research/pubs/view.aspx?msr_tr_id=MSR-TR-2006-90

Stuart Ozer; Alex Szalay; Katalin Szlavecz; Andreas Terzis; Razvan Musäloiu-E.; Joshua Cogan 2006 Using Data-Cubes in Science: an Example from Environmental Monitoring of the Soil Ecosystem ftp://ftp.research.microsoft.com/pub/tr/TR-2006-134.pdf

R. Musaloiu-E., A. Terzis , K. Szlavecz , A. Szalay, J. Cogan, J. Gray. Life Under your Feet: A Wireless Soil Ecology Sensor Network. EmNets 2006, Cambridge, May 2006.

For more information on the sensor project and online data please go to: http://www.lifeunderyourfeet.org/

Contact: Katalin Szlavecz: szlavecz@jhu.edu

Collaborators:

Andreas Terzis, Randal Burns: JHU Dept. of Computer Science
Alex Szalay JHU Dept. of Physics and Astronomy
Jim Grey, Stuart Ozer: Microsoft Research
Gordon Heisler, John Hom, Richard Pouyat, UDSA Forest Service
Claire Welty, UMBC CUERE

Students:

Lijun Xia: JHU Dept. of Earth and Planetary Sciences
Razvan Musäloiu-Elefteri, Chie-Jang (Mike) Liang, Jayant
Gupchup:JHU Dept. of Computer Science
Josh Cogan: Dept. of Physics and Astronomy