Processos hidrossedimentológicos e conectividade em bacia semiárida: modelagem distribuída e validação em diferentes escalas

Abstract

Water deficit in semiarid regions, such as the Northeastern Brazil, is a major limiting factor to economic development, with serious social consequences. To ensure the conservation of the available water resources, it is essential to control the human activities that impact negatively on water availability. For instance, uncontrolled soil use and improper agricultural practices, tend to intensify erosion and sediment transport processes, with direct impacts on water availability. Reservoirs siltation causes a reduction of the storage capacity and, hence, of the ability to supply water with high reliability. Sediment input to water bodies also contributes to water quality degradation, by increasing turbidity, which results in a reduction of the concentration of dissolved oxygen, and nutrient enrichment. In addition, soil erosion is a major factor of environmental degradation, causing the loss of nutrients of the top soil layers. Therefore, an accurate assessment of sediment fluxes along its pathway is essential for the identification of potentially erosive areas and regions where deposition processes predominate. For such estimation, a good understanding of the conditions of sediment transport through and between the different catchment compartments is necessary. The potential for movement of the soil particles within the system, denominated connectivity in sediment transport, stands out as a determining factor for the understanding and estimation of the flow of solid mass. In order to evaluate the effect of connectivity on sediment yield in the semiarid region of Brazil, a hydrosedimentological simulation of the Benguê meso-scale catchment (Ceará, Brazil) was carried out with the WASA-SED model. It was found that, in general, water balance in the soil matrix determinates the conditions for runoff generation in the study area: although there is a predominance of surface runoff initiation by Hortonian process, in the catchment scale, excess precipitation is observed only in events of great magnitude or sequences of rain events, when hydrological connectivity is high. Thus, the transport conditions limit sediment yield, generating very similar spatial patterns of runoff and sediment yield. At the catchment scale, the spatial pattern of connectivity was strongly related to soil hydraulic properties, while topographic characteristics played a more important role in sediment transport at the finer scale. The simulation of specific features of semiarid regions by the WASA-SED model, such as Hortonian-type flow, downstream re-infiltration of surface runoff, lateral flow redistribution and dense network of reservoirs, allowed a satisfactory estimate of both water and sediments fluxes in the Benguê catchment (933 km ²), as well as in the Aiuaba sub-basin (12 km ²). A more rigorous test was performed with the technique of Cesium-137 as a tracer of soil erosion/deposition. A comparison between modelled and measured (with 137Cs) values in a representative hillslope showed good correlation between the values. Even though it is a local result, this comparison indicates that the WASA-SED model must be able to simulate sediment fluxes satisfactorily also in the hillslope scale