Caracterização física da sub-bacia do Ribeirão Santa Cruz, Lavras, MG, e propostas de recuperação de suas nascentes

Abstract

The objectives ofthis study were to characterizethe physical aspects of the Santa Cruz Stream's sub-watershed, study the vegetation around the springs and propose recovering models. In order to do that, the following was accomplished: a) the physical characterízation ofthe Santa Cruz Stream; b) the characterízation of the perennial springs and of their recharging áreas; c) the analysis ofthe accomplishment of the environmental legislation in the áreas of permanent preservation of the streamlets, springs and hillsides and in the recharging áreas ofthe perennial springs; d) the floristic survey ofthe arboreal stratum and of the natural regeneration of the springs; and e) the selection of species to be used on the reforestation of degraded and disturbed springs ofthe sub-watersheds. Topographic maps, satellite images, aerial vertical photos and field surveys were used. The gathered data were processed in a GIS using SPRING. The data base was composed by thematic maps of the drainage network, slope, soil, native forest, land use, áreas of permanent preservation (APP), ofconflicted use and of the recharging áreas. The springshad their flows measured and were classified accordingto the type of associated reservoir and to the state ofconservation ofthe vegetation. The floristic and structural surveys were done in twelve springs, with three springs for each one of the following categories: punctual disturbed; diffuse disturbed; punctual degraded and diffuse degraded. In each spring 4 parcels of 50 x lOm were delimited in order to perform the floristic inventory of the arboreal individuais with diameteratbreast height(DBH) > 5cm and 20 parcels of 2 x lOmto perform the floristic inventory of the individuais of the natural regeneration with a DBH inferior to 5cm and height superior to lOcm. Detrended correspondence analysis (DCA) and cluster analysis were used to compare, among the sampled springs, the floristic profile of the arboreal stratum and ofthe natural regeneration. About 50% ofthe subwatershed área and of the springs recharging áreas presented slope above 12%, with wavy to mountainous relief. The predominant types of soil in the subwatershed were Latosol, whereas in the recharging áreas were B-textural and Latosol.Pasture was the predominant land use (62.41%), representing 84.57% of the conflictive use occurring in the sub-watershed and 78.45% ofthe conflictive use in the recharging áreas. The total área under conflictive use in the subwatershed was 11.04% and in the recharging área was 9.3%. The total área of permanent preservation (17.5%) together with the 20% of the área oflegal reserve totalized 37.5%. Yet, the native vegetation occupied only 25.74% ofthe sub-watershed, demanding reforestation of11.75% ofthe área. There were 177 perennial springs located and distributed in 6 categories: punctual preserved (10.17%), diffuse preserved (4.52%), punctual disturbed (34.46%), diffuse disturbed (25.99%), punctual degraded (8.46%) and diffuse degraded (16.38%). The largest recharging áreas did not show correlation with the highest flow values. Yet, the recharging áreas that presented the smallest áreas ofconflictive use had the highest flow values. In the vegetation structural survey, were registered 6851 individuais, distributed in 224 species, 127 genus and 54 botanical families. The diversity index ofShannon-Weaver (H') and the equality index ofPielou (J'), calculated for the set of 12 springs, were, respectively, 3,889 and 0,719. A relative high diversity ofspecies was found in the punctual disturbed (160) and degraded (126) springs, when compared to the diffuse disturbed (70) and degraded (49) springs. There is predominance ofspecies on initial stages at the degraded springs and of more advanced stages in the disturbed ones. The density ofindividuais is superior in the disturbed springs. From the distinction shown by DCA among the most common species in the natural regeneration as a function ofdifferent springscategories, it was possible to indicate the preferential species to be used for reforestation in each state of conservation and environmental conditions (humid soil or well drained).