Relação entre o grau de conservação da cobertura vegetal e os veranicos agrícolas na bacia do alto Jaguaribe - Ceará

Abstract

The Brazilian semi-arid region is characterized by water deficiency and fragile natural resources, presenting a great geographical and climatic diversity and exploration potential. In the State of Ceará, where more than 85% of its total area is within the semiarid region, rainfed agriculture represents a significant part of the economy, subject to irregularities in rainfall distribution and dryspells, which causes large uncertainties in local and smallholder agricultural production. In addition, there is a constant pressure on natural resources, especially deforestation, which leads to land degradation. Research initiatives in the last decades highlight that changes in land cover impacts the local climate, especially on air temperature, soil and air humidity, heat flows, atmospheric circulation and rainfall, with different results, depending on the study scale and the methodology applied. The purpose of this study was to study the spatial and temporal characteristics of rainfall in the rainy season for ten meteorological stations in the Alto Jaguaribe basin, semi-arid region of Ceará, with emphasis on dryspell duration, its relation to the surrounding vegetation cover and impacts on rainfed agriculture of the region. The rainfall data ranged from 1995 and 2014 years with variable available data between stations. The meteorological stations were divided into two study profiles: East profile and west profile, which crosses the Aiuaba Ecological Station (ESEC) with different land use and vegetation conditions, as well as topography in an area with dimensions 30 km x 60 km. The vegetation pattern surrounding each station during the rainy season was obtained from remote sensing with two indexes: NDVI (Normalized Difference Vegetation Index) and EVI (Improved Vegetation Index) with 5.6 km global resolution. The impacts of vegetation cover on dryspell duration and frequency were evaluated through statistical correlation. Additionally, the Doorenbos and Kassan (1979) agro meteorological model that integrates data of plant physiology, precipitation and soil water storage to estimate crop failure and rainfed production through the CROPWAT version 8.0 was calibrated and validated for maize (Zea Mays L.) yield prediction. The results showed a large inter-annual variability for dryspell length and also vegetation indexes during the rainy season (EC) between the months of January and May. There was a high and significant negative correlation between the average dryspell lengths and EVI, a high negative correlation between longer dryspells and EVI for all stations and profiles, with statistical differences for the common years of data between stations as comparing to total years, suggesting a mesoscale influence. The model simulation was applied for two stations with contrasting land cover densities for a 26-year period of rain data and similar soil and technology used. Results showed significant difference in maize production with higher yield levels for the station surrounded by denser vegetation cover in the south. In this environment, smallholder agriculture incomes were 52% higher compared to the less vegetated areas.