Design, installation and calibration of a weighing lysimeter for crop evapotranspiration studies

Lysimeters are tanks filled with soil commonly used in studies of crop ET and movement of water and chemicals in the soil profile. The objective of this work is to describe the procedures for construction, installation and calibration of a precision weighing lysimeter for water demand studies of Sumatra type tobacco (Nicotiana tabacum L.) in Bahia, Brazil. The lysimeter was 1.60 m long, 1.10 m wide and 0.60 deep, with an internal soil surface of 1.76 m(2). The lysimeter was installed in an area covered by a 30% shading screen used specifically for production of this type of tobacco. The inner box of the lysimeter was made of steel plates and the outer box was built on site with burnt-clay bricks bonded with cement mass. The weighing platform comprised four 1,000 kg capacity load cells placed underneath the inner tank and a datalogger for data recording. The lysimeter was calibrated in the field using standard loads of known mass. Five cycles of loading and unloading were performed (47 points) in the range of o to no kg (62.5 mm). The system showed a strong linear relationship between standard mass and load cells output (r(2) > 0.99 and S-y/x = 0.033 mm). The load cell accuracy, which combines the effects of hysteresis, linearity and repeatability errors, was found to be of the order of +/- 0.06 mm and the datalogger was found to be able to resolve mass changes less than 0.1 mm. These uncertainty parameters combined with the ability of the lysimeter to respond to relatively small mass changes suggest that the weighing lysimeter herein described is adequate to measure tobacco ET over time periods of one hour or longer. The investment was around US$ 5,100.00, including hardware and field work for installation.