Simplified integrated modeling and potential groundwater management benefits of the unsaturated zone in the Urucuia aquifer system, Brazil

An integrated perspective of the terrestrial part of the hydrological cycle from precipitation over soil and aquifer storage to river discharge is fundamental for a sound hydro(geo)logical understanding and, hence, a sustainable groundwater management. Simplified (or reduced-order) approaches can be an efficient tool to focus on the most relevant drivers, processes and responses of a system based on a parsimonious set of parameters and a relatively simple computational implementation. The Urucuia sandstone aquifer system in north-eastern Brazil is a high-plain region with mostly deep unconfined water tables and intensive groundwater pumping for agricultural irrigation causing social and political conflict. Here we develop a coupled reduced-order model of the high-plain and valley aquifer portions in the southern part (approximately 10000 km²) of the Urucuia aquifer, considering the soil zone (partitioning rainfall into evapotranspiration and deep percolation), thick vadose zones (up to approximately 100 m deep with unsaturated vertical moisture transport), and the saturated zone (generally over 100 m thick) providing river discharge. Data for model input (precipitation and potential evapotranspiration) and validation (river discharge) are obtained from the CAMELS-BR database as time series over approximately 40 years. Additional but shorter time series of groundwater levels in high-plain and valley regions are obtained from the RIMAS well database. We show that the much lower seasonality in water table fluctuations observed in the high-plain regions can be explained by the deeper unsaturated zone. While seasonality in river discharge may be attributed to the base flow from the valley aquifer portion, discharge recessions during the dry (practically zero rainfall) months of the year are sustained by the much larger aquifer portions underlying the high-plains. Groundwater pumping is considered as abstraction from storage in the saturated zone and its impact on groundwater levels and river discharges are evaluated with respect to climatic oscillations and long-term trends. The fact that the unsaturated moisture transport through the thick vadose zones under the extensive high-plains may take several years offers an interesting opportunity (in term of lead time) for groundwater management, if deep percolation leaving the soil zone is adequately estimated or measured over time.