Design and evaluation of a small weighing lysimeter

The purpose of this project was to design and test a small, 12 inch in diameter, lysimeter to measure evapotranspiration (ET) in the field. An initial lysimeter design using load cells was constructed and tested in a laboratory setting. Temperature tests indicated that this design lacked the sensitivity desired for accurate ET measurement in the field. A second design was developed that incorporated the use of a counterbalance system to remove non-water weight from the system. This allowed an increased sensitivity and, when tested in the laboratory, met desired requirements for a field scale test. This lysimeter was installed in the field and tested over several weeks. ET measurements from the small lysimeter were compared to ET measurements taken from a large monolithic lysimeter installed at the test site and against ET values calculated from on site weather data. These field tests indicated that the small lysimeter tracked the large lysimeter to within 0.5 mm of water for daily ET readings. These field tests demonstrated that it is feasable to design a small lysimeter of less than one meter square that can overcome thermal and surface area effects and effectively measure ET rates in a humid climate.