Soil Water Sensors for Agriculture – Theory and Issues
Soil water sensors have been used for irrigation and water management in agriculture for many years, but with limited success in many cases. Nonetheless, the use of soil water sensors in increasing as water scarcity increases and, conversely, problems associated with over irrigation also increase. Common problems with soil water sensing included sensor failure, problems with wiring, lack of or failure of data telemetry, inaccurate data, lack of timely data, too laborious and interference from dynamic soil temperature and bulk electrical conductivity changes. There are many sensors available, but only four main technologies: neutron thermalization, resistance blocks, capacitance sensing (frequency domain sensing), and travel time sensing (time domain reflectometry and time domain transmission modes). Understanding the theory of these offers insight into what a user can expect from each technology in terms of accuracy, stability and representativeness of the readings. The presentation will cover the types of sensors available, the operational theory of each sensor type, and explanations, with examples, of how the physical theory of operation dictates the limits of sensor calibration and performance, and of sensor representativeness in given soils.
Steven R. Evett is a Senior Research Soil Scientist and Lead Scientist with the USDA Agricultural Research Service, Conservation and Production Research Laboratory, Bushland, Texas. Dr. Evett uses field measurements, electronic sensing and automation systems and energy and water balance models to study irrigated crop water use, irrigation methods and automation as they affect crop water productivity, as well as water content sensing methods used to control irrigation and to quantify crop water use. In addition to research locations in the USA, he has had research projects in Egypt, the Middle East and Uzbekistan on crop water use, irrigation scheduling and soil water measurement; and he has worked in China, Egypt, Jordan and the USA to build and use weighing lysimeters to measure crop water use. Since 2003, Dr. Evett has been the ARS research coordinator for the Middle East Regional Irrigation Management Information Systems (MERIMIS) Project, which has research and extension partners in Israel, Jordan and the Palestinian Authority (http://www.merimis.org/index.html). He is a graduate of the University of Idaho and the University of Arizona, and was raised on an irrigated dairy farm in Southern Idaho. Dr. Evett is a Fellow of the Soil Science Society of America and of the American Society of Agronomy; and he has received the Soil Science Society of America (SSSA) Don and Betty Kirkham Soil Physics Award, the SSSA Applied Soil Science Research Award, the U.S. Dept. of Energy Federal Energy and Water Management Award and the USDA-ARS Technology Transfer Award (twice: 1999 for ET work and 2012 for soil water sensor work). He is a past President of the Texas Council of Chapters of the Soil and Water Conservation Society, and past associate editor of Agronomy Journal and of the Vadose Zone Journal, and he currently is on the Editorial Board of Agricultural Water Management. He is author/coauthor of 277 publications, including 25 book chapters