Conceptual and numerical modeling approach of the Guarani Aquifer System

Abstract

Abstract. In large aquifers, relevant for their considerable size, regional groundwater modeling remains challenging given geologic complexity and data scarcity in space and time. Yet, it may be conjectured that regional scale groundwater flow models can help in understanding the flow system functioning and the relative magnitude of water budget components, which are important for aquifer management. The Guaraní Aquifer System is the largest transboundary aquifer in South America. It contains an enormous volume of water; however, it is not well known, being difficult to assess the impact of exploitation currently used to supply over 25 million inhabitants. This is a sensitive issue because the aquifer is shared by four countries. Moreover, an integrated groundwater model, and therefore a global water balance, were not available. In this work, a transient regional scale model for the entire aquifer based upon five simplified, equally plausible conceptual models represented by different hydraulic conductivity parametrizations is used to analyze the flow system and water balance components. Combining an increasing number of hydraulic conductivity zones and an appropriate set of boundary conditions, the hypothesis of a continuous sedimentary unit yielded errors within the calibration target in a regional sense. The magnitude of the water budget terms resulted very similar for all parametrizations. Recharge and stream/aquifer fluxes were the dominant components representing, on average, 84.2 % of total inflows and 61.4 % of total outflows, respectively. However, leakage was small compared to stream discharges of main rivers. For instance, the simulated average leakage for the Uruguay River was 8 m 3 s −1 while the observed absolute minimum discharge was 382 m 3 s −1 . Streams located in heavily pumped regions switched from a gaining condition in early years to a losing condition over time. Water is discharged through the aquifer boundaries, except at the eastern boundary. On average, pumping represented 16.2 % of inflows while aquifer storage experienced a small overall increment. The model water balance indicates that the current rate of groundwater withdrawals does not exceed the rate of recharge in a regional sense.

Karst water resources in a changing world: Review of hydrological modeling approaches

“…The equivalent porous medium approach (EPM) assumes that hydraulic heterogeneities can be represented by “average” properties, i.e., an equivalent porous medium (Figure b). Previous studies showed that at regional scales this assumption can be used for karst water resource estimation and predictions [e.g., Loaiciga et al ., ; Rodríguez et al ., ; Scanlon et al ., ], since local influences of karst conduits may average out over larger areas [ Abusaada and Sauter , ]. However, since the EPM approach does not consider rapid flow in the karst conduits, it loses realism in systems with a high degree of karstification [ Worthington , ].…”

Combining Experimental Methods and Modeling to Quantify the Complex Recharge Behavior of Karst Aquifers

“…Advances in the knowledge of carbonate aquifers have been accompanied by the simultaneous development of modeling approaches, both conceptual (schematic understanding of the hydrogeological system) and particularly mathematical procedures (Hartmann, Goldscheider, et al, ; Kiraly, ; Kovacs & Sauter, ). In the latter category, there is an increasing number of numerical models (distributed, lumped, and hybrid) performed for simulating karst aquifers with a broad range of purposes: from the integrated hydrodynamic analysis (Kovacs & Sauter, ; Mazzilli et al, ) to the simulation of karst hydrology and the prediction of water resources under the impact of the global climate change (Hartmann et al, ; Rodríguez et al, ; Scanlon et al, ). Nevertheless, the hydrogeological heterogeneity and the high complexity of karst media resist simplifications when flow and transport processes are numerically simulated.…”

Recharge estimates for various land uses in the Guarani Aquifer System outcrop area

“…The GAS has an estimated water volume of about 25 000-37 000 km 3 (OAS/GEF 2009). More than 25 million people live in the area where the GAS is increasingly exploited for urban water use, as well as for industrial and agricultural uses (Rodríguez et al 2013). Outcropping sandstone areas of the GAS are supposed to be responsible for almost all groundwater recharge to the flow system.…”

“…Moreover, agricultural use accounts for 75% of the total land use/cover of the GAS outcropping in São Paulo State (IPT 2011). Although the GAS is still not considered overexploited (Rodríguez et al 2013), the hydrological effects of increasing agricultural activity in its outcrop areas are already an issue.…”