A Comparison of Soil Water Potential and Soil Water Content Sensors

Malheur Experiment Station

Information for Sustainable Agriculture

A Comparison of Soil Water Potential and Soil

Water Content Sensors

Clint Shock, Ali Akin, Levent Unlenen, Erik Feibert,

Kindra Nelson, and Autumn Tschida

Malheur Experiment Station

Oregon State University

Ontario, OR, 2002

Introduction

Six soil moisture sensors were compared by their performance in producing soil moisture data in a drip-irrigated potato crop. The sensors were: Aquaflex, Gro Point, Moisture Point, Neutron Probe, Tensiometer, and Watermark. The Watermark sensor was tested as read automatically by a datalogger and read manually with a hand-held meter.

Materials and Methods

This sensor comparison study was done in a drip-irrigated potato field at the Malheur Experiment Station in Ontario, Oregon. Potato seed of cultivar 'Mazama' was planted on April 26, 2002 in rows spaced 36 inches apart. The potato seed pieces were spaced 9 inches apart in the row. The soil was an Owyhee silt loam with a pH of 8.1 and 2 percent organic matter. Drip tape (T-tape, T-systems International, San Diego, CA) was laid at 4-inch depth between two potato rows. The drip tape had emitters spaced 12 inches apart and a flow rate of 0.22 gal/min/100 ft. The crop was irrigated daily to replace the previous day's evapotranspiration. Potato evapotranspiration (Et_c) was calculated with a modified Penman equation (Wright 1982) using data collected at the Malheur Experiment Station by an AgriMet weather station. From July 15 to July 25 and again from July 30 to August 7, the crop was not irrigated to evaluate sensor performance under variable soil moisture, both wetting and drying conditions.

In mid-June the sensor study was installed along one of the potato rows. Six types of sensors were installed between the drip tape and the potato row. The sensors were installed 8 inches from the drip tape and 10 inches from the potato row. The sensors were centered at 9-inch depth. The experimental design was a randomized complete block design with four replicates.

The sensors tested were: Aquaflex sensor (Streat Instruments, Christchurch, New Zealand), Watermark sensor (Watermark Soil Moisture Sensors model 200SS, Irrometer Co., Riverside, CA) read automatically every 8 hours by an AM400 Soil Moisture Data Logger (M.K. Hansen Co., East Wenatchee, WA), Watermark sensor read manually with a 30 KTCD-NL meter (Irrometer Co.), Tensiometer (Moisture Indicator, Irrometer Co.), Gro Point (Environmental Sensors Inc., Escondido, CA), Moisture Point (Environmental Sensors Inc.), and Neutron Probe model 503 DR hydroprobe (Boart Longyear, Martinez, CA).

The tensiometers and Watermark sensors measure soil water potential (SWP). The Watermark was previously calibrated to SWP (Shock et al. 1998). The other sensors used 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 was relatively compact and easy to bury. The neutron probe required the installation of PVC access tubes for each location to be monitored. The Moisture Point used a 3-ft probe permanently installed at each location to be monitored. The Moisture Point probe was installed into a hole made with a metal rod provided by the company. The Aquaflex sensor was a flat ribbon 9.8 ft long. The two Aquaflex sensors were connected to an Aquaflex two-channel datalogger (Streat Instruments). The neutron probe and Moisture Point allowed measurement of soil moisture at different depths at each location.

The neutron probe required calibration. One undisturbed core soil sample was taken in each of the four sensor replications during sensor installation. The soil samples were immediately placed in tin cans and weighed, then oven dried at 100°C for 48 hours and weighed again. Volumetric soil moisture content was calculated for the soil samples using the gravimetric method. The neutron probe was read at the same time as the soil samples were taken. The neutron probe was read as counts during 32 seconds. Four standard counts were also made. The volumetric soil water content determined from the soil samples was regressed against the ratio of the neutron probe counts to the standard count. The regression equation transforming the neutron probe readings to volumetric water content was Y = -33.6 + 97.7X, where X is the ratio of the neutron probe soil count to the standard neutron probe count and Y is the percent volumetric soil water content (Fig. 1). The coefficient of determination (r²) for the neutron probe calibration equation was 0.91 at P = 0.01 (Fig. 1). The average soil moisture data from the neutron probe were regressed against the average soil moisture data for each of the other sensors.

Results and Discussion

The tensiometer, Watermark sensor, and neutron probe responded to the wetting and drying cycles of the soil (Fig. 2). The Gro Point responded, but the amplitude of the response was less than the neutron probe. The Moisture Point was the least responsive to the wetting and drying cycles of the soil compared to the other sensors, probably due to the soil pulling away from the sides of the probe. For undetermined reasons, the Aquaflex datalogger only collected 3 days of data, this did not allow for a graphic display.

Both the AM400 datalogger and the 30 KTCD meter showed correlations to the tensiometer (Fig. 3). The AM400 and the 30 KTCD were correlated to each other; both instruments use the same equation to convert electrical resistance to SWP (Shock et al. 2001).

All sensors showed correlations (r² > 0.6) to the neutron probe except the Moisture Point sensor (Fig. 4). The Aquaflex and Gro Point estimates of soil water were often lower than the neutron probe (Fig. 4). The Moisture Point estimates of soil water were substantially lower than the neutron probe, Aquaflex, and Gro Point.

References

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

Shock, C.C., A. Corn, S. Jaderholm, L. Jensen, and C.A. Shock. 2001. Evaluation of the AM400 soil moisture datalogger to aid irrigation scheduling. Pages 111-116 in Irrigation Association, 2001 Proceedings of the International Irrigation Show.

Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108:57-74.

Figure 1. Regressions for calibration of the neutron probe to volumetric soil water content determined gravimetrically. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 2. Soil moisture over time for five types of soil moisture sensor. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 3. Regressions of soil water potential (SWP) measured by three instruments. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.

Figure 4. Volumetric soil water content measured by a neutron probe (X axis) regressed against soil moisture data (Y axis) measured by 6 types of soil moisture sensor. Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.