Overview

The soil pylon is a vertical array of soil sensors used to delineate mass and energy fluxes in subsurface systems.  Over the past year, our soil pylon technology has improved and was demonstrated in several test beds at Palmdale, CA and in Bangladesh.  Specific objectives are:

• To design, fabricate, and test a robust hardware/software system (referred to as a sensor pylon) for deploying embedded networked sensing technology in the subsurface environment.
• To successfully deploy sensor pylons in a variety of soil and sediment types in support of science and engineering applications, including:
• Observing, forecasting, and controlling the transport of nitrate in agricultural soils in the Mojave Desert in Palmdale, CA
• Observing and understanding the transport and fate of redox active species and their connection to arsenic biogeochemical cycling in Bangladesh

Approach

The soil pylon is a 1D sensor array deployed vertically in the soil to measure moisture and energy fluxes, along with chemical fluxes of interest.  The pylon array sensors include moisture and temperature sensors along with a range of water quality sensors for pH and ionic species (e.g, nitrate).  We have fabricated various versions of the pylon and are testing the pylon system in the laboratories and test beds of UCLA and UC Merced.  Upon gaining sufficient confidence in the hardware/software architecture we moved to field deployments in Palmdale, CA (moisture, temperature, and nitrate) and in Bangladesh (redox-active aquatic species).

Systems/Experiments

Palmdale Test Bed.  The Palmdale test bed is currently instrumented with a suite of 30 sensors (described below) on 4 soil pylons and 1 microclimate station.  The network currently employs ESS version 3.5, with a cellular card (GRPS) on a Stargate to serve as the gateway node.

Figure 1. Wireless Sensor Network System designed for irrigation control in Palmdale

Figure 2. Soil moisture data at 1ft showing the arrival of the irrigation system roughly 7 days after initial deployment.

Soil dielectric constant sensors (ECH2O, Decagon, Pullman, WA) were used to measure moisture content at several soil depths.  Thermistors (Digikey, Thief River Falls, MN) were used to measure soil temperature.  Digital “tipping-bucket” rain gages (Davis Instruments, Davis, CA) were used to sample local irrigation rates.  On each pylon, the MDA300 sensor board is used as an interface between the sensors and the MICA2 motes.

Figure 2 shows the soil moisture data at 1ft depth and Figure 3 shows the temperature data at 1,3,5 feet obtained on July 2005 in Palmdale. Unfortunately we were unable to obtain right moisture value for 3ft and 5ft depth. However, we can see the wetting front from 1ft data so that ECH2O sensor can capture the moisture change very well. As expected temperature data described diurnal change at 1, 3, 5 ft.

Figure 3. Temperature data at 1, 3, and 5 ft demonstrating the pronounced and attenuated diurnal effects at shallow and deeper observation points, respectively.

Bangladesh Test Bed - In a joint collaboration with scientists at the Bangladesh University of Engineering and Technology and MIT, we deployed a sensor network in January of 2006 in a rice field near Dhaka, Bangladesh in order to aid in validating this hypothesis. A full pylon contains 3 complete suites of sensors (soil moisture, temperature, carbonate, calcium, nitrate, chloride, oxidation-reduction potential, ammonium, and pH), each deployed at a different depth (1, 1.5, and 2 meters below ground), and a pressure transducer at the base to monitor water depth. We could not find an off-the-shelf, in-situ arsenic sensor to include in this deployment. Instead, output from a manual arsenic sensor will be combined with the data collected from the sensor network, which is primarily used to get a better understanding of the groundwater chemistry at shallow depths.

We deployed one fully-equipped pylon, and two partially equipped pylons (with one and two depths of sensors) for a total of 47 sensors deployed in the field for a period of 10 days.  In spite of the short deployment, the sensor network captured some interesting phenomena as seen in the figure below.

Figure 4. Schematic of the soil pylon deployment for observing redox active chemical species in the subsurface of a rice paddy near Dhaka, Bangladesh.

Accomplishments

The following are the major accomplishments in this area:

• Initial network deployment in the Palmdale test bed with 4 pylons, 1 microclimate station and a Stargate base station with cellular link (GPRS)
• Successful demonstration of “campaign” style deployment of the sensor pylon technology in Bangladesh