Recarga de águas subterrâneas em ambiente de cerrado: Estudo com base em modelagem numérica e simulação hidrológica em uma bacia experimental

Abstract

Groundwater recharge is a key hydrological process for groundwater and surface water reserves maintenance: accounts for the replenishment of aquifers and the surface water availability in drought periods. In some regions, this process overcomes political and administrative boundaries, as in the Brazilian Cerrado biome, in which particular characteristics make it an important continental recharge zone, responsible for the baseflow of some main Brazilian rivers. Despite this importance, the understanding level about the process in that area is still poor, thus justifying efforts to overcome the lack of knowledge. In this sense, the present work aimed to study and characterize the groundwater recharge process for phreatic aquifers in the Cerrado Biome, through the study in an experimental watershed. For two years, field works were conducted, that included the monitoring of hydrological variables, such as rain, runoff, and water table level, and execution of tests to characterize the hydrodynamics pattern of the phreatic aquifer, as slug tests and pumping tests. This effort allowed the acquisition of the basic data, which enabled the application of five groundwater recharge study methods: numerical modeling of the saturated zone processes; hydrologic modeling of vadose zone processes; baseflow separation method; water table fluctuation method; and integrated numerical modeling of the vadose and saturated zone processes. The results showed some limitations, such as high level of uncertainty, inferred by the observed differences between results from different methods and poor performance of the models in simulating measured hydrologic variables, as piezometric heads and discharges. However, important information was generated, as the recharge zones areas, average recharge rates estimates and recharge rates ranges of several possible combinations of different types of soil, land user/land cover, slope and geology, as well the most important data and parameters identification. It was concluded that the average recharge rate in study area exceeds the 20% rate, in relation to the precipitation, commonly adopted for Cerrado Biome regions, and that the spatial pattern of the process is strongly regulated by soil type and evapotranspiration. Among the applied methods, distributed hydrologic model of the vadose zone processes showed the best predictive capability. The numerical model of the saturated zone processes was not able to satisfactorily reproduce the behavior of the aquifer, because the lack of hydrogeological data hampered the establishment of a more realiable conceptual model. In the case of numerical model simulation of integrated processes of vadose and saturated zone, the shortcomings were the result of the accumulation of errors from the coupled models and they can be minimized if the individual limitations of the two models are overcome. The water table fluctuation and baseflow separation methods are recommended only in preliminary studies and in case of unavailability of data for physically based models aplication, because, although they allow obtaining rapid estimates from limited data, they characterize the groundwater recharge at point or local scale, not allowing the understood of hydrological processes.