Construção, instalação e calibração de lisímetros para a determinação da evapotranspiração de referência na região do litoral de Pernambuco

Abstract

Among the direct methods employed for estimating evapotranspiration lysimeters are more employees, as well as allow its direct determination, serve as instrument calibration and validation of empirical methods. This paper describes the procedures for construction, installation and calibration of two lysimeters in the experimental area in the coastal region of Pernambuco State, with tropical climate. Two lysimeters were built, the first drain and the second hydraulic weighing in an area completely surrounded by bahiagrass (Paspalum notatum) totaling 252 m². Lysimeter both, consist of an internal tank-type glass fiber with a water tank having circular edge area of 1,32 m² and a volume of 1 m³, being planted on the surface of the same program of the surrounding area. This tank is inserted into another, dug in the ground and walls of concrete and masonry. Lysimeters have a free drainage system, which is collected by the water level difference. The weighing system in the second lysimeter consists of three hydraulic pillows constructed with segments of flexible hose of butyl rubber nylon 100 mm in diameter, filled with water and interconnected by flexible tubes made of nylon, ending in the reading system composed of mercury column and sensing the hydrostatic pressure, which automates the readings lysimetric. The calibration of the weighing lysimeter consisted in the addition and removal of standard weights, equivalent to 1 mm layer of water at the same time sequences. This procedure was performed in the center and four quadrants of the lysimeter. The relationship between the difference of standard weights and the reading recorded in both the gauge and the sensor, were highly linear (R² in the range of 0,999) in all quadrants and the center of the weighing lysimeter. The average positional error in the manometer and the pressure sensor was 1,34 and 0,97%. This reflects high stability of readings even in situations of possible side slopes. The accuracy of the weighing lysimeter was 0,135 mm ETo registered by pressure gauge and up to 0,061 mm ETo registered in the pressure sensor, showing that the said sensor sensitive to variations in efficiency of the system mass lysimeter.