Avaliação emergética de microbacias hidrográficas do semiárido submetidas a diferentes manejos

Abstract

Discussions about the enviroiunent have been gaining supportin all productive sectors, due to the environmental impact generated by economic activities. The agricultural sector could not remain out of this discussion, since agriculture is present in virtually all types of landscape on the planet, occupying a great part of continental land-surface. Agricultural activity has a large impact on natural resources in order to ensure food supply for the world population, for this it needs to incorporate new areas and eiuiance its techniques to increase production. This mode of operation has been causing degradation mainly in the arid and semi-arid regions of the planet, as these have ecosystenis that, due to seasonal periods of rainfall associated with periods of drought, and shallow soils with low organic-matter content, produce a seasonal pattern of vegetation and a low capacity for development. Seeking alternatives for sustainable management of caatinga-type shmblands, this research aimed to test two types of management systems in the shmblands and compare them to an area of shrubland which has been preserved. In order to evaluate the performance of the management systems, emergetic methodology has been employed and various indicators, developed by using this methodology, were calculated. The research area consisted of three watersheds of the first or second orders, with areas of between 1.1 and 2.8 ha, located in the experimental watershedmanagement area at the Federal Institute of Education, Science and Technology of Ceará on Iguatu campus. In the first area (Bl) the shmbland was thinned out in order to produce herbaceous and arboreal biomass, the second area (B2) was kept unchanged as a control for assessment and in the third area (B3) the shmb was cleared and burned and the area planted with Andropogon gayanus Kunt grass (indiscriminate management being used in the shrublands) in order to produce herbaceous biomass. The catchment areas were monitored monthly during 2010 and the productivity of arboreal and herbaceous biomass, precipitation parameters, soil humidity, surface nmoff and sediment production were evaluated, as well as the physicochemical characteristics of the soils. Watershed Bl yielded 3,555 kg ha-I DM of herbaceous biomass and 7,264 kg ha-I DM of arboreal biomass, B2 yielded 1,115 kg ha-l DM of herbaceous biomass and 6,250 kg ha DM of arboreal biomass, while B3 yielded 8,513 kg ha DM of herbaceous biomass. The runoff coefficients were 1.2, 1.6 and 12% with sediment production of 42, 168 and 3,115 kg ha in the BI, B2 and B3 watersheds respectively. Evaluation of management practices carried out using emergetic methodology showed that the area which had been thinned out (Bl) had the best emergetic results, with a transformity of 12,975 seJ J'1' while the areas B2 and B3 had transformities of 14,477 and 22,062 seJ J'1 respectively. Renewability for the three types of management were 87, 92 and 69% for BI, B2 and B3 respectively, and the indices of sustainability were 43 for Bl and 5 for B3. In the preserved area this index cannot be calculated considering that it uses data from resources which originate in the economy, and this area does not make use of such resources. Emergetic techmque found that shrub-thinning gave the best performance because it had the highest productivity ofbiomass, it reduced both runoffand soil loss and its emergetic indices showed both the best return and good sustainability and as such can be recommended management system for the shrublands.