Estudo da evaporação nas microrregiões do cariri e sertão da Paraíba.

Abstract

The main objective of this research is to study the influence of the local climate on the evaporation losses of lakes and reservoirs in the semi-arid region of the Northeast. Evaporation in the reservoirs represents the main loss of water, in the semi-arid regions, due to the favorable climatic conditions and it is one of the important meteorological parameters in the management of water resources. Specific information about the evaporation from the lakes and reservoirs is not always available for water balance studies. There is the necessity of estimating the values of evaporation using some methods in many cases the estimate value may be different from the real evaporation.. On this, research three different types of tank evaporimeters were used to determine the more adequate and viable process of measurement and also the best method to estimate the evaporation. The experimental part was conducted in two different climatic regions in the state of Paraiba. The experiments were installed in the cities of Boqueirao (lat. 07° 29'S; long. 36° 07'W; alt. 380m) and Patos (lat. 07° 01'S; long. 37° 17*W; alt. 250m), during the period of April 2002 to February 2003. Automatic data acquisition systems were programmed to collect the data once in a second and store the 30 minutes mean data. The meteorological parameters observed were: temperature and relative humidity of air, temperature of the water in the tank evaporimeters, global radiation, net radiation, rainfall, wind velocity and direction and also the water temperature of the reservoirs of "Epitacio Pessoa" in the city of Boqueirao and "Jatoba" in the city of Patos. Daily observations of tank evaporimeters, rainfall and water level of two reservoirs were also made at 09:00h. The results show that, perhaps, due to its exposure and metallic constitution, the Class A tank was much more influenced by the climatic conditions than the other types of the tanks. It is also observed during study period that the evaporation indicated by the Class A tank super estimated the evaporation of the standard tank. When it is corrected by the traditional factor of 0.70, the Class A tank underestimated the observed evaporation in the standard tank during the entire period of study and this error was increased during the humid and cold periods. On the other hand, it is verified than the use of cement tank made it possible to obtain more precise measurements of evaporation than the Class A tank. Unlike Class A tank, which presented tank coefficients (kp) with significant variations of 0.93 to 0.76, between the cold and warn periods, these tanks showed less variations of the coefficient (1.00 to 0.93). The results also showed that the evaporation percentage in the total loss of the "Epitacio Pessoa" and "Jatoba" reservoirs seemed to be more than the expected evaporation. Even though, the rains in "Boqueirao" were normal during the period of study, the mean evaporation was 56.9% of the total water loss. Particularly, in Boqueirao, in the month of April of 2002, this index represented 62.8% and in the months of december/2002 and january/2003, it was more than 60%. In Patos the mean index for the period April at december/2002 represented 65% of loss. Because of the inauguration of Coremas system, the Jatoba reservoir stopped supplying water for local needs, in the months of January and February of 2003, the evaporation was about 88% of the total loss of water from the Jatoba reservoirs. None of the evaporation estimation models studies presented good performance for daily estimates, except for few days. In terms of evaporation for the whole period of study, the models of Snyder (1992) and Kohler et al. (1955) presented relatively errors of-5.9 and 5.0%, respectively.