Avaliação dos componentes do balanço de energia e evapotranspiração em região semiárida usando imagens orbitais Landsat T-5 TM e Terra-Modis.

Abstract

The main objective of this work was to evaluate the components of energy balance and the evapotranspiration in semiarid region using SEBAL - Surface Energy Balance Algorithm for Land - (Bastiaanssen, 1995) and satellite images from Landsat-5 TM and Terra/Modis. The area of the study was the irrigated perimeter Senator Nilo Coelho, located in the margins of São Francisco river in Petrolina-PE, Brazil. For attainment of the objectives, orbital images for the both satellites were used, obtained on 24/09/2003 and 12/10/2004, that cover part of the irrigated areas of the perimeter, the Sobradinho lake, a part of São Francisco river and areas with native vegetation. It was determined the surface albedo (a), vegetation indexes (NDVI, SAVI and LAI) and the surface temperature (Ts) for each pixel. The shortwave incident radiation was determined in function of the solar constant, atmospheric transmissivity, solar incidence angle and of the inverse squared relative earthsun distance. The incident longwave radiation was estimated using air temperature measured in the meteorological station and the emitted longwave radiation was estimated in basis of surface temperature. From these variables was determined the net radiation flux (Rn), whose medium values estimated by Landsat-5 TM and Terra/Modis to the day 24/09/2003 was 677.8 W m-2 and 615.8 W m-2, respectively, and to the day 12/10/2004 the medium values estimated was 667.0 W m-2 and 579.0 W m-2, respectively. The soil heat flux (G) estimated in function of Rn, Ts, a and the NDVI, it presented for the day 24/09/2003 the medium value of 118.3 W m-2 for Landsat-5 TM’s image and the value 112.8 W m-2 for the Terra/Modis; however, to the day 12/10/2004, the medium values for the same variables was 113.6 W m-2 and 118.0 W m-2, respectively. The sensible heat flux (H), was estimated using an iterative process in function of the wind speed, the vegetation height of the meteorologic station and were utilized two anchor pixels (hot and cold) to determine the difference between the surface temperature and air temperature. The average values for H on 24/09/2003 were 302.3 W m-2 and 385.5 W m-2, to the satellites Landsat-5 TM and Terra/Modis, respectively. The average values for H on 12/10/2004 were 314.4 W m-2 and 399.1 W m-2, respectively, above to the values of 2003. The latent heat flux (lE), was compute as a residual of surface energy balance. The average values of lE on 24/09/2003 were 225.8 W m-2 and 233.9 W m-2, respectively, and 255.3 W m-2 and 260.4 W m-2 on 12/10/2004. The medium values of lE in 2004 they were also a little superiors to the of 2003, probably due to the beginning of the rainy period in the study area that affected the albedo, the surface temperature and consequently the soil heat flux in no irrigated areas. The evapotranspiration (ET), presented medium values of 2.1 mm day-1 and 1.8 mm day-1 for the 24/09/2003, values those determined by Landsat-5 TM and Terra/Modis, respectively, with maximum values of 4.2 mm day-1 and 4.5 mm day-1. Already for the date of 12/10/2004 the medium values of ET were a little superiors to the 2003; the medium estimated ET in 2004 for Landsat was 2.5 mm day-1, with a maximum value of 5.3 mm day-1. The medium estimated ET value by the Terra/Modis for the same day was 2.8 mm day-1, with a maximum value of 4.9 mm day-1. The net radiation flux (Rn), the sensible heat flux (H), the soil heat flux (G) and the latent heat flux (lE), estimated by SEBAL they overestimated the measured values in field; the average daily ET estimated by SEBAL was 2.3 mm, with a medium maximum value of 5.1 mm while the measured medium value in field was 2.0 mm and the medium maximum value of 4.6 mm.