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In-Soil and Down-Hole Soil Water Sensors: Characteristics for Irrigation Management

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  5th National Decennial Irrigation Conference Proceedings, 5-8 December 2010, Phoenix Convention Center, Phoenix, Arizona USA  IRR10-8346.(doi:10.13031/2013.35844)
Authors:   Steven R Evett, Robert C Schwartz, Robert J Lascano, Mathew G Pelletier
Keywords:   Soil water content, Irrigation scheduling, Irrigation Management, Sensors, Temperature

The past use of soil water sensors for irrigation management was variously hampered by high cost, onerous regulations in the case of the neutron probe (NP), difficulty of installation or maintenance, and poor accuracy. Although many sensors are now available, questions of their utility still abound. This study examined down-hole (access tube type) and insertion or burial type sensors for their ability to deliver volumetric water content data accurately enough for effective irrigation scheduling by the management allowed depletion (MAD) method. Down-hole sensors were compared with data from gravimetric sampling and field-calibrated neutron probe measurements. Insertion and burial type sensors were compared with a time domain reflectometry (TDR) system that was calibrated specifically for the soil; and temperature and bulk electrical conductivity measurements were also made to help elucidate sensor problems. The capacitance type down-hole sensors were inaccurate using factory calibrations, and soil-specific calibrations were not useful in a Central Valley California soil and a Great Plains soil. In both soils, these sensors exhibited spatial variability that did not exist at the scale of gravimetric and NP measurements or of irrigation management, resulting in errors too large for the MAD approach. Except for one, the point sensors that could be buried or inserted into the soil gave water contents larger than saturation using factory calibrations. The exception was also the least temperature sensitive, the others exhibiting daily water content variations due to temperature of >= 0.05 m3 m-3 water content. Errors were related to bulk electrical conductivity of this non-saline but clayey soil.

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