The influence of conceptual model uncertainty on management decisions for a groundwater-dependent ecosystem in karst

Abstract

Summary Groundwater management in karst is often based on limited hydrologic understanding of the aquifer. The geologic heterogeneities controlling the water flow are often insufficiently mapped. As karst aquifers are very vulnerable to pollution, groundwater protection and land use management are crucial to preserve water resources and maintain ecosystem services. Multiple Model Simulation highlights the impact of model structure uncertainty on management decisions using several plausible conceptual models. Multiple Model Simulation was used for this purpose on the Yucatan Peninsula, which is one of the world’s largest karstic aquifers. The aquifer is the only available fresh water source for human users and ecosystems on the Peninsula. One of Mexico’s largest protected areas, the groundwater-dependent Sian Ka’an Biosphere Reserve (5280 km 2 ) is fed by the aquifer’s thin freshwater lens. Increasing groundwater abstractions and pollution threatens the fresh water resource, and consequently the ecosystem integrity of both Sian Ka’an and the adjacent coastal environment. Seven different catchment-scale conceptual models were implemented in a distributed hydrological modelling approach. Equivalent porous medium conceptualizations with uniform and heterogeneous distributions of hydraulic conductivities were used. The models demonstrated that Sian Ka’an’s wetlands are indeed groundwater-fed. The water quantities in the wetlands and the flooding dynamics are determined by the larger groundwater catchment. The overall water balance for the model domain showed that recharge constitutes 4400 ± 700 million m 3 /year. Of this, 4–12% exits as overland flow, and 88–96% exits as groundwater flow. Net groundwater outflow from the model domain to the north via the Holbox fracture zone appears as an important cross-basin transfer between regions of the Peninsula. Probability maps of Sian Ka’an’s catchment were obtained through automatic calibration and stochastic modelling. Groundwater travel time zones were calculated based on different calibrated effective porosities. The spatial modelling results highlight the impact of regional-scale structures on the flow field and transport times.