SOIL TEMPERATURE AND WATER EVAPORATION OF SMALL STEEL AND PLASTIC LYSIMETERS REPLACED DAILY

Soil water evaporation (E) measured by small, weighable lysimeters is affected by their size, construction materials, and replacement frequency. Steel and plastic are commonly used materials, but they have different thermal characteristics. Our objective was to investigate how wall material affects E and soil temperature of small lysimeters filled daily with undisturbed soil. Research was conducted during 5 days in August 1995 at Bushland, Texas, where the soil is a Pullman silty clay loam (fine, mixed, thermic Torrertic Paleustoll, 30% clay, 53% silt). Plastic and steel lysimeters 76 mm long, with inside diameters of 82 and 86 mm, respectively, were filled each morning by pressing them into undisturbed soil, and E was determined by mass change. Soil temperatures inside additional steel and plastic lysimeters were measured by thermocouples. No significant differences in E due to wall material were measured. For lysimeters of both wall materials, daily E ranged from 2 to 5 mm, daytime and nighttime E averaged 2.7 mm and 0.5 mm, respectively, and total cumulative E was 15.5 mm. Evaporation from small lysimeters was within 5% of E from a nearby large, precision, weighing lysimeter. Steel lysimeters were warmer at night near the surface, with significant differences from 0.5 to 0.9 °C, and warmer during the day at the bottom, with significant differences from 0.5 to 2.8 °C. Plastic lysimeters had greater vertical soil temperature differences than steel lysimeters. Significant differences inside plastic lysimeters ranged from about 2.0 to 3.5 °C greater than those of steel lysimeters during the daytime, and 0.5 to 1.0 °C greater during the nighttime. Measured temperature differences were consistent with greater thermal conductivity and enhanced heat transfer in steel sidewalls compared with plastic. Wall material affected temperature distribution, but not evaporation, of small lysimeters that were replaced daily.