WRRCTR No.13 Pilot Evapotranspiration Study: Lysimeter Design

Abstract

The marine sub-tropic climate of Hawaii is particularly suited to the use of semi-contained hydraulic lysimeters. Field experiences in the use of these lysimeters reveal a number of problems among which were factors related to the unusual tropical crops of pineapple and sugarcane. This project investigated the modification of the lysimeters to minimize some of these problems. Modular planting schemes in 1.52 meter rows or beds and tillage depths of 0.3 to 0.6 meters dictated minimal lysimeter size. The great height and lodging of the cane required a large lysimeter area to help in the definition of the effective transpiring area. Polyester resin fiberglass reinforced tanks, 3 x 3 x 1.52 m deep with paper honeycomb strengthened bottoms were designed to meet the need for a large and relatively deep container of minimal weight. The prevalence of high relative humidities necessitated the use of engineering graph paper to reduce distortion of recording traces from shrinkage and swelling of the paper. Despite the limited diurnal and annual temperature variations, careful insulation of the exposed portions of the manometer was necessary. The regular diurnal changes in wind velocity imposed fluctuations on the open-end manometer used for recording the pressure changes in the hydraulic load cells that were offset by venting near ground level. Nylon reinforced butyl rubber irrigation hose with the ends clamped was used to form the bolsters for the load cells. A silicone rubber sealant was used to ensure water tight fittings with standard copper tubing connectors to the bolsters. Stability against rolling was achieved by placing some of the bolsters at right angles to the others. Large tensiometers of perforated polyvinyl pipe covered with porous Porvic membrane were designed to induce suction at the base of the soil column. The water release curves for the Latosols suggested a 0.1 bar suction for an approximation of field capacity.