SMALL SPATIAL SCALE SOIL-WATER CONTENT MEASUREMENT WITH TIME-DOMAIN REFLECTOMETRY

Plants growing on stored soil water often exhibit symptoms of water deficit even though a relatively large quantity of soil water is available for uptake where roots are present in the subsoil. To explore the possibility that this less effective absorption phenomenon was related to nonuniform root distribution or root clumping around soil peds, we examined the suitability of time-domain reflectometry for measuring soil water content distribution on a small spatial scale. Measurements were made on clay loam and sandy clay soils with 21-mm-long parallel balanced transmission lines and compared with gravimetric water content measurement. Water content values ranged from oven dry to saturation. To quantify the error in propagation time measurement using short transmission lines, measurements were made in air using rod lengths ranging from 10 to 150 mm. The coefficient of variation was quite high (2.8-7.3%) for times shorter than 100 ps for air-dry soil. In this case, the technique proved less reliable. For longer times, corresponding to higher water contents, the coefficient of variation was < 3%. A few samples of clay loam soil with water contents >0.29 m3 m-3 showed excessively high values of dielectric constant. Care should be taken in data interpretation at high water content for these media. Time-domain reflectometry proved effective in measuring water content with the tested transmission line for values >0.07 m3 m-3. Thus, a tool is provided for in situ measurement of spatial variability or at a spatial scale compatible with many problems ranging from the water uptake of clustered roots to seed germination in the field.