Lead Investigators:

Thomas Harmon (UC Merced), Jose Saez (Loyola Marymount), Jenny Jay (UCLA), Steve Margulis (UCLA), Deborah Estrin (UCLA)

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
• In addition, we typically introduced CENS-fabricated nitrate microsensors into our deployments in order to better understand the effects of sensor form factor on soil observations.

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 tested these in the Merced “backyard,” Palmdale, and James Reserve (AMARSS) test beds.

Systems/Experiments

1. Backyard Test Bed - In order to test proposed single-layer- and commercial nitrate selective sensors under real environmental conditions before the deployment the sensors were directly deployed in the soil at the research site at UC Merced (Castle Facility). One nitrate selective electrode manufactured by Sentek Ltd and two CENS-fabricated nitrate selective electrodes were place in 30 cm long plastic polyvinyl chloride tubes and connected to a commercial data-logger for continuous data monitoring. In order to provide necessary level of moisture content the tested area was sprinkled with regular tap water and concentrated (5 M) NaNO₃ solution at regulated time intervals (25 minutes of sprinkling every 12 hours).

 

Figure 1: Locations of EC-10s (Moisture nest 1, 2, 3), PR2 probe sensor (DL2), nitrate sensors (Nitrate1, 2), and temperature sensors (Temp nest).

 

2. Palmdale Test Bed - Soil sensor deployment was implemented on May, 2006 in Palmdale, CA with the sensor layout depicted in Figure 1. 

Soil Moisture and Temperature Sensors.  18 soil moisture sensors in all were installed on May 11.  Twelve Decagon (Echo-10) moisture sensors were installed at four depths (20, 40, 60, 80 cm) at each of three locations, referred to as nests. The three nests were located along the path covered by the same pivot nozzles.  In addition, an agronomic moisture-monitoring array (Dynamax, Model DL2) with moisture sensors at 6 depths (10, 20, 30, 40, 60, 100 cm) was installed near moisture nest 1 (see Figure 3) in order to compare the responses of the relatively inexpensive Echo-10 sensors with the DL2 array.  The DL2 operates under the same principles as the Echo-10, sensing moisture by measuring the dielectric constant of the surrounding soil-water matrix.  Four self-logging temperature sensors were deployed as a vertical array on May 13 to provide a long-term record of the soil moisture profile in the top 0.5 m of the soil.  These sensors remain in place and are logging the temperatures every 5 minutes.

Meteorological Sensors.  One meteorological (met) station (Davis Instruments, Pro Plus Weather Station) was installed at the western end of the deployment area next to moisture sensor nest 3.  In addition to the met station, three mini-rain gauges (Figure 2) were deployed in a pattern triangulating about the center Decagon moisture sensors (nest 1) and the 6-sensor agronomic array (DL2).  These sensors were deployed in an effort to observe spatial variability in the irrigation rate provided by the passing pivot.

Nitrate Sensors.  Two nests of 4 nitrate sensors were installed adjacent to moisture nests 2 and 3.  Each nitrate sensor nest was comprised of two commercial nitrate sensors (Sentek Ltd., DirectIon nitrate selective electrodes) and two UCM-fabricated mini-electrodes.    All of the nitrate sensors were deployed through a 1.5 inch PVC pipe that was placed at a 45 degree angle into the ground.  The pipe was open-ended allowing the sensors to protrude into the soil at the desired depth.   The above-ground end of the PVC pipe was sealed to prevent irrigation water from entering the tube.  This deployment strategy is being tested as a convenient means of inserting chemical sensors and periodically removing them for maintenance.

   
Figure 2: EC-10 moisture sensors installed in a pylon and PR2 multilevel soil moisture probe (left); nitrate sensor deployed in diagonal PVC tubing (right); both at the Palmdale small-scale controlled irrigation test.

Accomplishments

For the Merced backyard system, typical results from the testing of the CENS-fabricated nitrate sensors under unsaturated soil conditions are shown in Figure 3.  In Palmdale, the lager-scale deployment was successful in terms of observing nitrate transport in soils under center-pivot irrigation conditions.  When the watering occurs, soil moisture and nitrate responded quickly (Figures 4 and 5).

Figure 3:  Commercial (red) and CENS-fabricated nitrate sensor responses to nitrate transport for observations in unsaturated soil during controlled irrigation experiment in Merced/Castle backyard test bed.

Figure 4:  Soil moisture trend at 20 cm depth when the pivot passes by four different soil moisture sensor locations (PR2, EC-10 nest1, EC-10 nest2, and EC-10 nest3).

 

Figure 5: Responses of Sentek and fabricated sensors to reclaimed water irrigation at two locations during the small-scale controlled irrigation test (reductions in voltage correspond to arrival of nitrate concentration front; shaded windows and dashed lines denote irrigation events).

Water/soil temperature sensors for HOBO U12 Family data Loggers (Onset Comp. Inc.) and HOBO Pendant Temp/Light loggers were used for soil temperature measurements. 4 Pendant HOBO loggers were left on the site after the deployment to perform seasonal temperature changes monitoring (Figure 6).  The data provide a clear depiction of the diurnal and seasonal temperature fluctuations at the site.

Figure 6. Seasonal and diurnal temperature variation in Palmdale soil at 10, 20, and 30 cm depths (L to R).

Future Directions

The results from this limited Palmdale deployment are being used to design a final, large-scale irrigation control demonstration at the 30-acre Palmdale experimental irrigation pivot site. Six to eight pylons equipped with soil moisture, temperature, and nitrate sensors will be installed and interfaced with wireless networking system. A weather station data will be used to estimate surface temperature and evapotranspiration rate. The pylons will be deployed in two clusters, on opposite sides of the pivot irrigation circle. This configuration will probe reasonably average soil conditions at the site on one side, and soils with greater hydraulic conductivity on the other side.

 

People

Faculty:

• Thomas C. Harmon (UC Merced Engineering)
• Jenny Jay (UCLA Civil & Environmental Engineering)
• Jose Saez (Loyola Marymount Civil & Environmental Engineering)

Researchers:

• Alexander A. Rat'ko (UC Merced Environmental Systems)

Staff:

• John Hicks (UCLA CENS)

Graduate Students:

• Nithya Ramanathan, (UCLA Computer Science)
• Tom Schoelhammer (UCLA, Computer Science)
• Yeonjeong Park (UCLA Civil & Environmental Engineering)

Undergraduates:

• Chris Butler (UC Merced Environmental Engineering)

External Research Partnerships
County Sanitation Districts of Los Angeles: Water recycling.  This partnership led to the creation of the Palmdale test bed. The Districts are under constant regulatory scrutiny, yet they need to dispose of water from the Palmdale Water Reclamation Plant. Their current strategy uses groundwater monitoring wells. However, this method allows them only to identify a problem, not avoid it. The Districts are interested in the preventative feedback and system control that a network of soil pylons could deliver. In the current partnership, the Districts supply complete access to the experimental agricultural site, 24-h security, operation and maintenance support on the pylons (weeding, battery changing), and share data on irrigation times, flow rates, and the water quality of the reclaimed water.  As the wireless sensor network envisioned becomes more stable and user-friendly, we anticipate the Districts assisting financially with the scale up in this test bed.

Agriculturalists for Scientific Environmental Research (AFSER).  This partnership, starting in March 2007, will fund one UC Merced graduate student and supplies for test deployments of 3 to 4 soil pylons at local dairy operations.  The goal of the project is to assess the utility of the pylons for monitoring nitrogen releases (via waste streams) to the subsurface, and create a plan for comparing these releases at dairies under a range of conditions (underlying soil types, dairy management practices, etc.).

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