Monitoramento hidrológico e determinação dos atributos físico-hídricos do solo de uma microbacia do Rio São Lourenço

Abstract

The study of the hydrological cycle at the catchment scale and an improved understanding of hydrological processes are important for environmental planning, appropriate land use, flood control, hydraulic structures and erosion control, among others. This study aimed to characterize the physical attributes of soil and water from a watershed in the upper São Lourenço River basin. The basin is circular in shape with flat terrain, a slope of 1.94%, and a low drainage density. The predominant land use in the study area is cotton and soybean cultivation. In this context, a monitoring scheme was established, including data collection of rainfall events, discharge and soil moisture, as well as climatic parameters, in order to determine the coefficient of runoff for the study area. In addition, disturbed and undisturbed soil samples were collected for the investigation of their physical attributes. Some rain events were separated according to the criterion of erosive rainfall for a water balance study and the determination of the runoff coefficient. Watershed characterization, including shape, terrain, and geomorphology, was performed using Geographic Information Systems (GIS). Among the analyzed rainfall events, maximum values of rainfall intensity with 39.6 mm/h and minimum values with 3.4 mm/h were observed. The average flow coefficient was 0.02, which can be related to the predominant soil type Oxisol. It is characterized by good soil stability with strong aggregates and a high porosity favoring infiltration. The results support the following conclusions: a high time of concentration due to the high permeability of soils in the basin, creates a low risk of flooding, even though the observed values for morphometric characteristics (Kc and Kf) would appear to favor inundation. Furthermore, although the study area is intensively cultivated, it shows a relatively low runoff coefficient (Ces) showing that current management and use practices strongly promote retardation of water at the soil surface. However, the runoff coefficient depends more on soil moisture conditions than on the characteristics of rainfall events. Land use (pasture, riparian forest, soybean and cotton) significantly influenced the content of organic carbon, the mean particle size diameter, and the index of soil aggregate

stability. The heterogeneity of soils in the watershed was the main factor responsible for observed variation in macroporosity, microporosity, total porosity, and soil bulk density.