Hydrological influence on the variation of the terrestrial gravity field in Manaus, Amazonas, Brazil

Abstract

The influence of groundwater on the hydrological cycle has been studied at large-scale across the Amazon Basin, one of the largest freshwater reservoirs in the world. Considering this Basin's importance in the global hydrological cycle, local studies also are critical. Here, we focus on the local hydrological cycle in Manaus, at the center of the Amazon Basin. We observed the variability of local gravity, fluvial height, soil moisture, precipitation, evapotranspiration and groundwater in the rainy (December–May) and dry (June–November) seasons, during drought (2016) and flood (2017) years. In general, in the beginning of the rainy season, soil moisture is instantaneously influenced by the first precipitation, increasing gravity. After, the soil becomes saturated and the variation in soil moisture achieves maximum values, which are sustained until the end of rainy season. After soil saturation by infiltration, the groundwater starts to be fed by percolation and the water table starts to rise. Thus, variation in local gravity reflects mainly variations in groundwater level. After the dry season starts, the soil loses moisture (soil moisture values decrease), while the gravity signal maintains high values, representing the sustained behavior of the groundwater. Modeling the vertical gravity component influenced by soil moisture and by groundwater results, respectively, in 49.63 nm/s2 and 314.70 nm/s2. In comparison with the gravity measurement of 368.02 nm/s2, we confirm that the groundwater is the main contributor to the gravity signal. We also find a strong correlation between fluvial height and the gravity signal, but demonstrate with 3D modeling that the Negro River height variation does not directly influence the gravity signal. Instead, we explain the similarity in fluvial height and the gravity signal by the connection between fluvial height and groundwater, and the correlation between groundwater and gravity variations.