Potential and effective recharge of the Federal District shallow aquifers, Brazilian savanna region - coupled analysis of GLDAS-2, Water Table Fluctuation (WTF) and rainfall pattern

Abstract

Aquifer recharge is a key parameter for water management. Rainfall pattern shifts combined with external factors such as vadose zone properties and evapotranspiration determine groundwater fluctuation. Different combinations of these variables within aquifer time response might cause a disequilibrium in water budget, threatening both surface and groundwater availability. An important approach to understanding groundwater behavior is the evaluation of potential and effective recharge considering multiple methodologies. This study provides an analysis of the porous aquifer of the Federal District, Brazil. The area is relevant for national and international water production, as it hosts the headwaters of three hydrographic regions: Amazonas, Tocantins-Araguaia, and Paraná/Paraguay. Potential recharge estimations and delineation of deficit periods were compiled from the National Institute of Meteorology (INMET) climatic network and have been found to be a suitable database to support rainfall distribution analysis. Effective recharge was estimated based on the water table fluctuation - WTF method applied to GLDAS-2/GRACE groundwater storage model (GWS) and water level time series of five monitoring wells. According to the satellite data, 22–40% of the corresponding hydrological year rainfall is converted into saturated zone recharge. Direct observations estimations are 22–48%. Spearman's correlation coefficients for GWS and WL are up to 0.90, so that modeled time series are reliable for trend analysis. A coupled analysis of climatic and water table data has shown that groundwater position is well described by moving average rainfall. Ideal windows were found to be between 150 and 270 days and dependent on groundwater depth, with the latter being the most representative. The study of Federal District porous aquifer for management purposes could consider a minimum of 2-year rainfall time series. Maintenance of total volume but atypical or unevenly distribution along this window has the potential to alter hydraulic head by the end of the dry season and the subsequent water table ascension period. Future work at the Cerrado Biome can consider the methods herein presented to corroborate research findings and expand the variety of geomorphological and pedological locations evaluated.