Transient water balance of the Southern Urucuia aquifer in Bahia

Abstract

Quantification of the temporally and spatially variable water balance components in a catchment is extremely relevant for the management of water resources and the understanding of fluctuations in groundwater levels. The Urucuia aquifer has been monitored by dataloggers installed in dedicated wells and shows a continuous decline in water levels reaching up to 1.1m/year. Isotopic analysis was performed in conjunction with steady-state and transient flow simulation in the southern portion of the Urucuia aquifer (23% of total area) to describe the dynamics of the water balance components between 2005 and 2018. The isotopic analysis were fundamental for the construction of the conceptual model indicating river-aquifer interaction and the effect of evapotranspiration on recharge. Results show that recharge decreased progressively between 2012 and 2017, being strongly influenced by evapotranspiration and diminishing rainfall, causing a decline in groundwater levels and river base flows, also influenced by pumping wells. The numerical model was further used to compute step responses to a 10% drop in recharge, indicating that hydraulic heads and river base flows re-equilibrate after approximately one decade. It is highlighted that continuous hydroclimatological monitoring and withdrawals measurements to observe the dynamics between water demands and availability are important.