A COMPARISON OF SIX SOIL MOISTURE SENSORS

Malheur Experiment Station

Information for Sustainable Agriculture

A COMPARISON OF SIX SOIL MOISTURE SENSORS

Clint Shock, Erik Feibert, and Scott Jaderholm

Malheur Experiment Station

Oregon State University

Ontario, OR

Introduction

Six soil moisture sensors were compared as to their performance in producing soil moisture data in an irrigated hybrid poplar plantation. The sensors were: Aquaflex, Gro Point, Moisture Point, Neutron Probe, Tensiometer, and Watermark.

Materials and Methods

The sensor comparison study was done in a microsprinkler irrigated hybrid poplar tree field at the Malheur Experiment Station in Ontario, Oregon. The trees were planted in April 1997 on silt loam soil on a 14 foot by 14 foot spacing. The tree rows are oriented to the northwest. The trees are irrigated using a microsprinkler system (R-5, Nelson Irrigation, Walla Walla, WA) with the risers placed between trees along the tree row at 14-ft spacing. The sprinklers delivered water at the rate of 0.14 inches/hour at 25 psi and a radius of 14 ft. Two inches of water were applied whenever the soil water potential at 8-in depth reached -50 kPa. Soil water potential was measured by two Watermark sensors (Watermark Soil Moisture Sensors model 200SS; Irrometer Co., Riverside, CA); at 8-inch depth. The GMS were installed along the middle row and between the riser and the tree. The GMS were previously calibrated (Shock et al., 1998) and were read at 8:00 a.m. daily with a 30 KTCD-NL meter (Irrometer Co.).

Two Aquaflex sensors (Streat Instruments, Christchurch, New Zealand) were installed on September 14, 2000. The Aquaflex sensor is a flat ribbon 9.8 feet long. Each sensor was installed at 8-in depth along the tree row and between two trees. The two Aquaflex sensors were connected to an Aquaflex 2-channel datalogger (Streat Instruments). On July 23, 2001, six types of soil moisture sensors were added to the study. One sensor of each type was installed in four groups adjacent to the existing Aquaflex sensors. The position of each sensor was randomized between groups. The sensors in each group were installed in a line parallel to and approximately 8 in from the Aquaflex sensors. The sensors were installed at 8-in depth. Each Aquaflex sensor had a group of sensors on each side. The sensors added to the study were: Tensiometer (Irrometer Co., Riverside, CA), Watermark sensor model 200SS (Irrometer Co.), Neutron Probe model 503 DR hydroprobe (Boart Longyear, Martinez, CA), Moisture Point (Environmental Sensors Inc., Escondido, CA), Gro Point (Environmental Sensors Inc., Escondido, CA), and Gopher (Cooroy, Queensland, Australia). The four Gro Point sensors were connected to two Gro Point 3 channel dataloggers (Environmental Sensors Inc.) The Watermark sensors were connected to an AM400 Soil Moisture Data Logger (M.K. Hansen Co., East Wenatchee, WA). All other sensors were read manually at 9:00 A.M. from Monday through Friday. The tensiometers and Watermark sensors measure soil water potential. The other sensors use various techniques to measure volumetric soil water content.

The tensiometer and Watermark sensors were installed with a 7/8-inch soil auger. The tensiometers required regular resetting due to the column of water breaking suction around -60 to -70 kPa. The Gro Point sensor is relatively compact and was easy to bury. The neutron probe and the gopher require the installation of PVC access tubes for each location to be monitored. The Moisture Point uses a 3-ft probe permanently installed at each location to be monitored. The Moisture Point probe is installed into a hole made with a metal rod provided by the company. The neutron probe, Gopher, and Moisture Point allow measurement of soil moisture at different depths at each location. The Aquaflex is installed horizontally, requiring a 10 foot trench dug to the depth of installation.

The neutron probe and gopher require calibration. One undisturbed core soil sample was taken in each sensor group during sensor installation. The soil samples were immediately placed in tin cans and weighed, then oven dried at 100 ⁰C for 48 hours and weighed again. Volumetric soil moisture content was calculated for the soil samples using the gravimetric method. After the sensors were installed 2 inches of water was applied. On July 25, another set of soil samples was taken and volumetric soil moisture content was determined as before. The sensors were read at the same time as the soil samples were taken. The neutron probe was read as counts during 32 seconds. The volumetric soil water content determined from the soil samples was regressed against the neutron probe and gopher readings. The regression equation transforming the neutron probe readings to volumetric water content was Y = -30.2 + 0.0088X, where X is the neutron probe count and Y is the % volumetric soil water content (Figure 1). The coefficient of determination (r²) for the neutron probe calibration equation was 0.93 at P = 0.01. A calibration for the Gopher sensor was not possible due to a lack of correlation between the gopher readings and the volumetric soil water content determined from the soil samples. The average soil moisture data from the neutron probe and from the tensiometers was regressed against the average soil moisture data for each of the other sensors.

Results and Discussion

All sensors showed correlations (r² > 0.7) to the neutron probe and correlations (r² > 0.5) to the tensiometer except the Moisture Point sensor (Figures 2 and 3). The Moisture Point estimates of soil water were substantially lower than the neutron probe data (Figures 2 and 3). The tensiometer and Watermark sensor responded to the wetting and drying cycles of the soil (Figure 3). The neutron probe, Gro Point, and Aquaflex sensors responded to the soil wetting after irrigations. The neutron probe and Aquaflex sensors were less responsive to the soil drying between irrigations than the Gro point sensor.

Literature cited

Shock, C.C., J.M. Barnum, and M. Seddigh. 1998. Calibration of Watermark Soil Moisture Sensors for irrigation management. p. 139-146 in Proceedings of the International Irrigation Show, Irrigation Association, San Diego, CA.

Figure 1. Regressions for calibration of the neutron probe and Gopher sensor to volumetric soil water content determined gravimetrically. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 2001.

Figure 2. Volumetric soil water content measured by a neutron probe (X axis) regressed against soil moisture data (Y axis) measured by 5 types of soil moisture sensors. Data points for the Aquaflex sensor are the average of two sensors. Data points for the other sensors are the average of four sensors. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 2001.

Figure 3. Soil water potential measured by tensiometers (X axis) regressed against soil moisture data (Y axis) measured by 5 types of soil moisture sensors. Data points for the Aquaflex sensor are the average of two sensors. Data points for the other sensors are the average of four sensors. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 2001.

Figure 4. Soil moisture data over time for five types of soil moisture sensor. Arrows denote irrigations with approximately 2 inches of water applied. The Moisture Point sensor was not available during this time due to repairs being made. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 2001.