Aplicação do algoritmo SEBAL e imagens Landsat para estimativa da demanda hídrica do algodoeiro irrigado no Oeste da Bahia

Abstract

This research aimed at estimating by means of SEBAL algorithm and Landsat 5 TM images the radiation and energy balance components and evapotranspiration of center pivot-irrigated cotton fields. The selected area is in the Busato Farm (13,25o S; 43,42o W; 436 m asl), 30 km west of the Bom Jesus da Lapa town, western of Bahia. Six images corresponding to distinct dates over the crop season (first semester of 2007) were used for implementation of SEBAL routines. In each image a reduced portion was selected which included the whole perimeter of the Busato Farm with its 25 center pivots all used to irrigate cotton. The net radiation (Rn), the sensible heat flux (H), the soil heat flux (G) and the latent heat flux (LE) were determined by the SEBAL routines. Irradiation measure by the seven reflective bands of TM – Landsat and weather data from a nearby weather station were the main inputs for all calculations. Results shown that the net radiation Rn varied from 430 W m-2 (DOY 148) to 700 W m-2 (DOY 84). The 550-600 W-m-2 range for Rn was the most frequent and occurred in the period of maximum crop development. In the initial stage of the crop, G reached 113 W m-2 and H reached 350 W m-2. In the intermediate stage of the crop (reproductive phase), when LAI reached its maximum, G and H reached the lowest values, i.e., 60 and 16 W m-2, respectively. On the other hand, LE varied from 450 to 600 W m- 2 in the same stage, which corresponded to 90% of Rn estimated by SEBAL. Few days after planting, LE was 330 W m-2 and 350 W m-2 in the harvest time. On the average, cotton actual ET estimated by SEBAL was 3.5 mm day-1 few weeks after planting, reaching 6.6 mm day-1 in the maximum crop development stage, and then moving down to 4.2 mm day-1 during harvest period. The results found here are in agreement to other studies with irrigated cotton in arid e semiarid regions of the world.