Hydrological Influence on the Variation of Terrestrial Gravity Field in Manaus, Amazonas, Brazil

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 Basins 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 observe the variability of local gravitational field, fluvial height, soil moisture, precipitation, and groundwater in the rainy (December-May) and dry (June-November) seasons, during drought (2016) and flood (2017) years. In general, when the rainy season starts, gravitational field, fluvial and soil moisture are at minimum values. The soil moisture is instantaneously influenced by the first precipitation. After the first rains, an increase in gravitational field coincides with an increase in soil moisture. The soil becomes saturated and achieves a maximum value, which is sustained until the end of rainy season. Soil saturation was sustained for 53 days in 2016 and 76 days in 2017. The first maximum in soil moisture and precipitation precedes the maximum of gravitational variation by 3 months in 2016 and by 5 months in 2017. Due to the relationship between soil moisture and precipitation, the maximum of gravitational variation are delayed from both by the same amount. After soil saturation, the groundwater starts to percolate and the water table begins to rise. Thus, gravitational variation reflects mainly the groundwater. Later, the soil loses moisture, but high gravity values are sustained, again reflecting the sustained behavior of groundwater. Due a deficit in water storage in the soil and the low water table in the preceding drought years, the variation in the groundwater level is at a maximum in 2017. The estimated groundwater level using the gravitational variation further reveal a connection between groundwater variation and surface water, such as rivers.