DIURNAL-NOCTURNAL FLUCTUATION OF SOIL WATER MATRIC POTENTIAL AND SOIL WATER TOTAL POTENTIAL GRADIENT

As the water matric potential is the most important component of the water total potential in unsaturated soils, it must be measured accurately. The aim of this study was to evaluate the effect of soil temperature on water matric potential and water total potential gradient over the period of water redistribution in a Latossolo Vermelho-Amarelo (Oxisol/Hapludox) located in the municipality of Piracicaba, SP, Brazil. In a circular plot of 3 m diameter, 10 tensiometers with pressure transducers (model SWT3 from Delta-T Devices) were set up at depths of 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50 and 0.55 m in order to determine water matric potential, F m, at those depths and to calculate the water total potential gradient, (Delta Phi m/ Delta z)+ 1, at the depth of 0.25 m with Delta z of 0.10; 0.20 and 0.30 m (using tensiometers set up at 0.20 and 0.30; 0.15 and 0.35, and 0.10 and 0.40 m for Delta Phi m, respectively). In the same plot, thermocouples (type T) were set up at the soil surface and at the depths of 0.025, 0.05 and 0.075 m. Tensiometer and thermocouple readings were made simultaneously and stored in a Datalogger (model DL2, Delta-T Devices). Measured values of soil water matric potential exhibit fluctuations throughout the day and differential effects for tensiometers at different depths, also causing fluctuations in the soil water total potential gradient. The best time period for performing tensiometer readings is in the first seven hours of the day or after 6 pm.