Sensitivity of an integrated groundwater flow model to model parameters-application to vulnerability assessment of karst aquifers

The sustainable use and management of groundwater can benefit from improvements to both qualitative and quantitative groundwater vulnerability methods for a variety of hydrogeologic systems. Vulnerability methods in karst aquifers rely on key factors assigned different weights according to their estimated influence on groundwater resilience to contamination. Because of the duality of infiltration and flow in karst aquifers, the assessment of vulnerability may be best validated by a process-based approach involving a selection of key parameters and their coefficients. In this study, an integrated validated numerical groundwater model of a karst catchment area (Lebanon) is used to evaluate the impact of model parameters on recharge and spring discharge and identify important parameters that control the intrinsic system vulnerability. The model sensitivity is evaluated using an automatic calibration tool for local sensitivity analysis in addition to a variance-based local sensitivity assessment of model output time-series to various model parameters for two consecutive years (2016-2017). The influence of each parameter was normalized to estimate standardized weights for each of the process-based key-controlling parameters. Findings indicate greatest model sensitivity to 1) soil (moisture content), 2) fast infiltration through dolines (simulated using a bypass function), 3) climatic parameters (precipitation, melting temperature, and degree-day coefficient), and 4) aquifer hydraulic properties (conduit, matrix), which have been assigned weights according to their impacts on spring hydrographs and recession. Other parameters play less important roles. Based on the results, this method proposes to reevaluate the weighting coefficients of key vulnerability factors in the conventional methods based on a numerical model for the proper delineation of protection zones and the establishment of water protection guidelines. The conclusions and methods from this study can be upscaled to different karst systems and climatic settings and can assist decision makers toward sustainable groundwater development and protection policies based on scientific evidence.