Uso de tanque de cimento amianto como alternativa para medir evaporação.

Abstract

This research was conducted at the Meteorological Station of the Department of Atmospheric Sciences of CCT of the Federal University of Paraiba - UFPB, Campus II, in the municipal district of Campina Grande - PB (7°13'32 " S; 35° 54*15 " W; alt. 518,34 m), during the period of November 15, 1999 to April 01, 2000, with the main objective of evaluating the performance of pans of cement amianthus as a substitution alternative of the pan Class A, to measure evaporation. The experiment was accomplished in four different stages, using four pans, two Class A pans being operated in agreement with the standards of WMO and two of cement amianthus (water pans of 500 liters), where still wells were built with pipe of PVC of lOOmm, to have the same conditions of the Class A pan. In the first three phases of the experiment the pans were operated at the same levei in relation to the soil: one of Class A (Cl) and one of cement amianthus (Al) in the area cevened with grass and the other two pans C2 and A2 in an area almost without vegetation. The pan A2 in each phase was operated in a different way, in the first phase with a layer of water of 60cm, in the second phase with a layer of 40cm of water and 20cm of sand, in the third phase with a layer of 20cm of water and 40cm of sand, already in the last phase, the same was transferred to the area covered with grass, buried 45cm and operated with a layer of 60cm. Also during the three experimental phases air temperature and humidity, net radiation, speed and direction the wind, as well as the temperature of the water in the four pans were measured. For obtaining these measurements a Micrologger 21X was used and is programmed to make readings to every second and to make averages every ten minutes, twenty-four hours a day. When comparing the values obtained in the two types of pans, it was verified that there was larger evaporation in the pans of cement amianthus in the first three phases and that in the first phase this index the attained 16%. Already in the last phase there was an evaporation a little larger in the pan Class A. It was also noticed that with the increase of humidity there was a tendency of increased evaporation. The temperature of the Class A pans was always warmer than the pans of cement amianthus during the day, while in the night period it happened the inverse, the pans of cement amianthus got heated up more than the Class A pans. It was observed that the Class A pan cooled rapidly and also became warm very fast, and that on the average its water was slightly warmer than that of the pan of cement amianthus for a period of seven hours during the day. The evaporation estimates by the Penman method superestimated the values measured in the three experimental phases, presented a larger lag in relation to the pan Class A, 32,2; 37,4 and 38,9%, respectively.