A simple calibration of Hobo 4 channel analogue dataloggers for use with Watermark 253–L soil water potential sensors

Nkosinathi D Kaptein,Louis W Titshall

doi:10.4314/wsa.v45i2.01

Abstract

The combination of granular matrix sensors with a Hobo 4 analogue channel datalogger provides a relatively inexpensive continuous soil water monitoring system. However, the datalogger excites all 4 channels concomitantly as it reads the sensor measurement of each channel in sequence. This results in localised electrolysis causing measurement bias error in Channels 2, 3 and 4. To evaluate this channel bias, Watermark granular matrix sensors were connected to Hobo 4 channel dataloggers to measure electrical conductance of the soil. This study formed part of a larger study aimed at understanding water use by Eucalyptus plantations at different soil depths. The measured soil conductivity was calibrated against the gravimetric method in soil derived from Natal Group Sandstone and Dwyka Tillite that occur in southern KwaZulu-Natal. The channels of a Hobo 4 channel datalogger were successfully calibrated against the gravimetric method for both soil types (R2 > 0.92). The voltage measurements of each channel increased in the order 1 < 2 < 3 < 4 for both soil types at a soil water content range of 12 to 44% and 6 to 46% for Dwyka Tillite and Natal Group Sandstone soils, respectively. Channel measurements were similar at soil water content ranges below 12 and 6% for tillite and sandstone soils, respectively. The study showed that Channels 2, 3 and 4 of the Hobo 4 channel datalogger are affected by electrolysis. If this analogue datalogger is used with these types of sensors, these channels need to be calibrated back to Channel 1.

Highlights

South Africa is generally classified as a water-scarce country with an average annual rainfall of 495 mm, compared to the global average of 860 mm (Schulze and Lynch, 2007)
The SWP is the preferred method since it is a measure of the suction energy that drives water movement in the soil and gives an indication of the energy required by plants to extract water from the soil
Soil samples were collected from the orthic A horizons in two KwaZulu-Natal soil types; the first derived from Natal Group Sandstone (30°16′7.72′′S 30°15′35.39′′E) and the second from Dwyka Tillite (30°12′12.02′′S 30°16′7.28′′E) at a depth of 0–20 cm

Summary

Introduction

South Africa is generally classified as a water-scarce country with an average annual rainfall of 495 mm, compared to the global average of 860 mm (Schulze and Lynch, 2007) This rainfall is often not evenly distributed leading to regular and localised drought conditions (Phillips, 2016). The SWP is the preferred method since it is a measure of the suction energy (or matric potential) that drives water movement in the soil and gives an indication of the energy required by plants to extract water from the soil. This is considered a more direct indicator of the potential for plant water stress conditions (Jones, 2008)

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