Estimativa da produtividade primária bruta em áreas agrícolas e de vegetação primária no cerrado por sensoriamento remoto.

Abstract

The Gross primary productivity - GPP corresponds to the rate at which the atmospheric CO2 is converted by photosynthesis in organic substances. This study aimed to determine the GPP, which basically uses the efficiency use of light model proposed by Monteith . Subsets were made in the Programa de Assentamento Dirigido do Distrito Federal (PAD-DF) and surrounding areas with high agricultural potential region. The daily hotosynthetically active radiation was obtained based on the daily global solar radiation measured in an automatic weather station of INMET, located in Luiziânia - GO. The fraction of photosynthetically active radiation absorbed by vegetation was determined as a function of NDVI and the water determinate factor in the formation of GPP was considered to be equal to the evaporative fraction - EF, obtained through SEBAL. TM - Landsat 5 images and additional meteorological data, collected between June and September 2011 at Luiziânia , GO were selected for the study . The GPP obtainded according to SEBAL were compared with the one extracted from MOD17A2, which is based on the same principle but employing meteorological reanalysis data and MODIS images with a spatial resolution of 1 km. Results obtained in cuttings and selected areas of PAD, showed distinct patterns between native vegetation and irrigated areas, but there is clear influence of rainfall patterns over the variables investigated. Comparisons between GPP Landsat and MOD17A2 product reveal large discrepancies between them , possibly resulting from inaccuracies related to meteorological variables used in MOD17A2 , but also becausee of the value that is refereed to as the maximum efficiency of use of light in Cerrado. The values of Landsat GPP reached 17 gC m-2, while the MOD17A2 did not exceed 7 gC m-2. The pivots with tomatoes and corn showed that the NDVI values followed GPP for both selected areas, with the highest values of GPP in pivots with corn were between 13 and 16 gC m-2day-1, while the highest values in pivots with tomato plantation varied between 14 and 16 gC m-2 day-1. It is concluded that irrigated areas have high rates of carbon fixation, compared with the native vegetation, and the MOD17A2 product does not adequately represented the values of GPP in the areas of native vegetation and also in irrigated areas .