Design and comparison of a small, monolithic weighing lysimeter to a large lysimeter and to the penman and penman-monteith equations

The design, construction, installation, and evaluation of a small, monolithic, weighing lysimeter (<1 m²) is described. The small lysimeter is composed of three components: an outer container, a soil column assembly (soil container and soil column), and a weighing mechanism. The circular lysimeter has a surface area of 0.072 m² and an undisturbed column depth of 0.61 m constructed using 12” PVC (polyvinyl chloride) pipe. The weighing device is comprised of an aluminum frame and an electronic load cell that only weighs the “live load” or change in water content of the lysimeter. This is made possible by counterweighting the “dead load” or dry soil and container. It was designed to be easily installed in a single circular pit by two or three persons, with minimal disturbance to the surrounding soil and crop. The weighing device was constructed and tested in an environmentally controlled chamber for temperature effects and sensitivity. After concluding that the temperature effects were negligible and the sensitivity was sufficient, the soul column assembly was constructed and the lysimeter was installed in the field. It was installed at the Plateau Experiment Station in Crossville, TN near an existing large weighted lysimeter (4 m²) and a weather station. The lysimeter was operated for 90 days and the ET measurements were compared to ET measurements from the existing large lysimeter and ET values calculated from on-site weather data using two ET estimation equations. This field test revealed that the small lysimeter stayed within 1.5 mm of the large lysimeter and had an accuracy of 0.680 mm when the lysimeter soil area. The small lysimeter ET measurements almost always exceed the large lysimeter. This difference could not be strongly correlated to any of the weather parameters. When using the total lysimeter area, the small lysimeter had an accuracy if 0.290 mm, which was very similar to the accuracy of the two estimation equations (0.342 mm and 0.272 mm). The results indicate that by using the lysimeter soil area, the small lysimeter did not meet the specified accuracy. However, using the total lysimeter area, the small lysimeter could accurately measure ET in a humid climate.