Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 4, 2000
Publication Date: N/A
Interpretive Summary: A multisensor capacitance probe (MCAP) can measure soil water content more easily than gravimetric samples and has fewer regulations than nuclear probes. The MCAP sensors may be affected by salinity and temperature, which would be a problem in irrigated soils. Our objectives were to calibrate and verify the accuracy of MCAP measured water content for two soils, two water salinities, and with cyclic temperature fluctuations in the greenhouse. Soil was packed into 30 gallon barrels, wetted, and measurements obtained for water content using time domain reflectometry (TDR) and gravimetric methods and temperature using thermocouples. The TDR measured water content was similar to gravimetric values but MCAPs required calibration to be accurate for our soil. Adding saline water to the columns elevated MCAP water content above available pore space. Cyclical soil temperature variations caused similar fluctuations in MCAP water content. Accurate water content with MCAPs required soil specific calibration and knowledge of soil temperature and salinity.
Technical Abstract: The multisensor capacitance probe (MCAP) is an alternative to in-field gravimetric or nuclear soil water content measurements. The MCAP measured water content is more convenient than gravimetric methods, is not regulated like nuclear methods, and may be automated. Studies of MCAP sensors noted salinity and temperature effects on water content measurements. Our objectives were to calibrate and verify the accuracy of MCAP measured wate content for two soil materials, two water salinities (1.3 and 11.3 dS/m), and with diurnal temperature variations. The surface and calcic horizons of an Olton soil (thermic Aridic Paleustolls) were packed into triplicate 100-L columns and wetted. We compared water content measured volumetrically to time domain reflectometry (TDR) and MCAP values. The TDR water content varied plus or minus 0.01 m**3 m**-3 from volumetric values for air-dry and saturated soil. Factory calibrated MCAPs indicated water content > 0.20 m**3 m**-3 over volumetric water content. After calibratio against TDR, MCAP water content was within plus or minus 0.03 m**3 m**-3 of volumetric water content. There was little difference in MCAP calibrations due to soil materials; so, we used a pooled calibration. Saline water elevated MCAP water content about 0.20 m**3 m**-3 over available pore space. Cyclical soil temperature fluctuations caused similar water content variations because temperature affects the method used by MCAPs to determine water content.