TIME-DOMAIN REFLECTOMETRY MEASUREMENTS OF SURFACE SOIL-WATER CONTENT

Surface soil water content is an important parameter influencing such processes as residue decomposition, nutrient cycling, microbial activity, and weed seed germination and emergence. This experiment was conducted to determine if time-domain reflectometry (TDR) could be used to measure water content in the 0- to 50-mm soil layer. Wave guides were installed horizontally at a depth of 25 mm below the soil surface in held plots differing in current crop-tillage history. The TDR system was interrogated hourly by a data logger. Daily average soil water contents from TDR were computed and compared with water contents obtained from gravimetric soil sampling. The TDR values were linearly related (r(2) = 0.84) with gravimetric samples of soil water content when wave guides were at a depth of 25 mm, but not when wave guides were at a depth of 13 mm. Continuous measurements of surface soil water content can be obtained from an automated TDR system.