Uso de sensoriamento remoto na caracterização espacial e ambiental da aluvião do riacho São José na bacia sedimentar do Cariri cearense

Abstract

The Brazilian northeastern region is characterized by an intense water deficit, limiting the viability of agricultural practices and limitung the land capacity to support the rural populations. In this context, the alluvial areas appear as a source of water that can be used in the drier times of the year to preserve the sustainability of the area. However, for these alluvial areas to be sustainably used, it is necessary to know some characteristics like: delimitation, storage capacity and restrictions on their use and occupation. This study was conducted at the riacho São José Basin (ou Bacia Hidrográfica do riacho São José – BHSJ) , located on the Cariri region of the Ceará state, and was mainly aimed at characterizing the alluvium of the stream, classifying the BHSJ as to its vulnerability to groundwater pollution and determining the agricultural zoning of the basin. The work was divided into four steps: (1) The mapping of the alluvial areas, (2) The determination of the water-supplying potential of the area where the alluvial stream is perennial, (3) the mapping of the classes of vulnerability to aquifer pollution, and (4) The use of the DRASTIC and pesticide DRASTIC indexes and the agricultural zoning of the BHSJ area. At all stages of work development were used the SPRING, ArcGIS and Global Mapper softwares. The most successful alluvium area mapping method was that which considered the buffer width as a logarithmic function of the upstream sediment contributing basin area. The sediment contributing areas exceeding one million square meters with average slope greater than 5% were restrictive conditions to the formation of alluvium. In the alluvium section where the stream is perennial, predominate the medium textured and sandy sediment, with a maximum depth of 20 m and an average effective porosity of 8.3% and water storage capacity of about 498,000 m³. The DRASTIC index divided the basin into four classes of vulnerability: negligible, low, very low and moderate. The insignificant class corresponds to about 78% of the area, the moderate class corresponds to only 3% of the area in the alluvium section where the stream is perennial and where it is needed greater control over the use and occupation of land. The pesticide DRASTIC index divided the basin into six classes of vulnerability: negligible, very low, low, moderate, high and very high. The very low amounts to about 41% of the area, whereas the moderate, high and very high, respectively correspond to 7, 6 and 3%. These last three classes correspond to the entire alluvium and controlling pesticide use is very important there. From the pesticide DRASTIC index and from the stream APP(Permanent Protection Areas) area delimitation, BHSJ was mapped as to the agricultural planning, defining three zones: (a) Permanent protection zone, (b)Restricted pesticide use zone and (c) less restricted pesticide use zone.