Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2001
Publication Date: 8/20/2001
Citation: Seyfried, M.S., and Murdock, M.D. 2001. Response of a new soil water sensor to variable soil, water content, and temperature. Soil Science Society of America Journal 65:28-34.
Interpretive Summary: As electronics have developed a number of different sensors have been designed to measure soil water content that use these new developments. To date, the development of new sensors has outpaced their testing. Most of these sensors must be calibrated to soil water content to determine the relationship between the measured electrical response and actual soil water content. We tested the effects of varying soil properties and temperatures on the calibration of the Water Content Reflectometer (WCR), which has been widely used by a number of researchers and may have considerable commercial application. We found that the response of the WCR sensors was quite different for the same water content in the different soils we tested. Only in sand was the factory calibration suitable. However, we also found that we could develop a very accurate calibration for the sensors for all the soils. For three of the four soils we tested, the WCR senors were highly sensitive to soil temperature and that the degree of sensitivity depends on the initial water content of the soil. It appears, judging from the nature of the response, that the sensors are especially sensitive to the electrical conductivity of the soil. Thus, soil electrical conductivity and temperature must be accounted for when using these sensors in the soils we tested.
Technical Abstract: The success of time domain reflectometry (TDR) has led to the development of other instruments that use the soil dielectric constant as the basis for determining volumetric soil water content. An example is the Water Content Reflectometer (WCR), which is much less expensive than TDR and is used widely, although little has been published concerning its applicability to soil water content monitoring. The primary objectives of this study were to determine the WCR-soil water calibration for different soils and to investigate how it is affected by changing temperature. We found the individual sensors to be very precise (CVó0.05) under the controlled laboratory conditions of this study. Variability among sensors, determined in air and ethanol, indicated significant (à=0.05) sensor differences that were largely accounted for with a simple additive correction. Sensor soil water calibration was investigated in four soils under varying water contents over a 40 øC temperature range. We found that: (i) soil water calibration was significantly (à=0.05) different for each soil tested, (ii) there was a significant (à=0.05) temperature response for all soils; and (iii) the effect of temperature varied with soil water content and soil type. Both the soil type and temperature sensitivities we observed were probably due to the relatively high electrical conductivity of the soils tested.