Runoff Zoning of the Paciência River basin, Ilha do Maranhão - MA/Brasil

The purpose of this study was the development of the potential runoff map from the Paciência River Basin. This basin covers an area of 150 km² and is embedded in the four municipalities of the Metropolitan Region of Greater São Luis at Ilha do Maranhão: São Luís, São José de Ribamar, Paço do Lumiar and Raposa. The fieldwork was carried out in partnership with the Basin Research Laboratory Hydrographic - LEBAC from Federal University of Maranhão. In preparing the runoff map it was employed the Geotechnical Mapping Technique as Zuquette (1993) and Zuquette and Gandolfi (2004), which aimed to evaluate and characterize the variables of the physical environment associated with different land uses. In this technique the geotechnical characteristics related to geology, soils (texture, morphology and permeability), drainage density, slope and features favorable to surface storage, are grouped into classes and specific value are assigned. The final product was the result of physical and man-made resources attributes crossing through the software of use in Geographic Information System environment. From the analysis of the results it was concluded that the identified problems are consequence, first of all, of poor management of public managers evidenced in the unpaved streets and absence of sidewalks in the marginal transportation routes, coverage and maintenance of the sewer system, when present, unplanned urbanization, concession area for the construction of real estate projects without considering the environmental legislation and the physical parameters of the land. The resulting problems such as flooding, affect both the residents of the basin and a portion of the island population during its movement. There were no records of deaths caused by flooding and the consequences generally incur in disturbances in the transport flow, in small trade and material losses. Five runoff potential classes were established: very high, high, medium, low and very low. In a 35% of the basin area, the high potential to runoff was predominant, especially in the upper reaches, where are the largest urban concentrations, which is the main parameter to the observed phenomenon and not the slope. As can be seen in the runoff map and its analysis, the basin had a very favorable runoff scenario, with a 58% of its territory ranging from very high to medium potential. Such range is equivalent of the high and medium-haul basin that are characterized by being more urbanized areas. In the opposite side, the lower course and the interfluve banks represent a 42% of the basin territory ranging from low to very low potential. The very low potential areas, corresponding to the tops of hills and trays in the lower course of the river basin, are areas with predominant sandy soil, vegetated and poorly urbanized. Zoning, in the preservation of these areas, is important not only to avoid the problems resulting from runoff, but also to the water security of the basin. The central hypothesis of the research was confirmed, but it is necessary the realization of similar study in other neighboring basins or even across the Ilha do Maranhão in order to obtain parameters and thus better scale of the runoff impacts. This information can assist in planning the environmental use and cover of the land from this basin, being an important document for future geographic spatial planning projects. (AU)