WEIGHING LYSIMETERS FOR EVAPOTRANSPIRATION RESEARCH IN A HUMID ENVIRONMENT

Three weighing lysimeters were developed for evapotranspiration research at the University of Florida, Institute of Food and Agricultural Sciences, Plant Science Research and Education Unit near Citra, Florida. The lysimeter design followed accepted procedures as well as aspects unique to the study site, including a foundation designed for a perched water table outside the lysimeters, fetch distance, deep drainage, and lightning protection. Each lysimeter has a planted surface area of 2.32 m2 and a soil depth of 1.37 m. The soil in each lysimeter is reconstructed sandy soil originally from the experimental site. The lysimeter facility includes monitoring wells, an automatic pumping system, and additional lightning protection system for load cells and soil moisture sensors. The construction materials and installation cost (excluding labor) were $63,443 for the three lysimeters. Lysimeter on-site maintenance, operation, and performance are discussed. Four load cells with an accuracy of 0.02% (0.12 mm) are used to weigh the average 5.8 Mg lysimeter mass, including the steel lysimeter tank and soil. Initial data show that the three lysimeters provided a consistent hourly evapotranspiration (ETc) measurement over a five-day period in the summer season, although many field activities and precipitation events occurred. An additional 30 days of daily bahiagrass ETc resulted in a 0.82 ratio between the ETc and Penman-Monteith reference evapotranspiration in November 2003.