Abstract
This work focuses on the modeling and evaluation of water resources in complex aquifer systems and the use of scarce data. The modeling work is developed based on the GLUE (Generalized Likelihood Uncertainty Estimation) method. This method is still little used in hydrogeology, although its applications in other disciplines such as hydrology proved quite efficient. The study site, located in the Republic of Djibouti (Horn of Africa), is represented by the volcanic massif of Goda. The hydraulic properties of this massif are highly heterogeneous since they are associated with fracturing and weathering of the geological formations. The data are too few to enable a conventional modeling approach of this volcanic system. The implementation of the GLUE method in a numerical groundwater flow model allowed developing a stochastic analysis of the spatial distribution of the hydraulic conductivity and the recharge modalities of this complex volcanic system. The hydraulic conductivities range from 10-6 to 10-8 m·s-1 for the basalt and the rhyolite formations (values are yet generally lower for rhyolites) and are higher than 5 × 10-7 for the sedimentary formations. In addition, considering diffuse recharge as the main mechanism by which the precipitation reaches the aquifer results in more consistent groundwater head simulations than considering only indirect recharge. The average recharge amount estimated for the Goda aquifer system is 28 mm·yr-1. The results led to a numerical representation of this system, with the least uncertainty. This model was able to estimate the available water resources of this system. This result is important because the Goda system supplies water to the city of Tadjourah. Assessment of available resources is vital for the future development of this city. From a methodological point of view, the GLUE method proved very promising for water resources assessment in complex hydrogeological systems for which little data are available.
Highlights
Understanding the hydrodynamic functioning of volcanic aquifer systems is confronted with the difficulty of dealing with their geometrical complexity and the high spatial heterogeneity of the recharge processes and flow parameters distributions [1] [2] [3] [4]
The implementation of the GLUE method in a numerical groundwater flow model allowed developing a stochastic analysis of the spatial distribution of the hydraulic conductivity and the recharge modalities of this complex volcanic system
The probability densities obtained for K indicate that volcanic formations are less permeable than sedimentary formations
Summary
Understanding the hydrodynamic functioning of volcanic aquifer systems is confronted with the difficulty of dealing with their geometrical complexity and the high spatial heterogeneity of the recharge processes and flow parameters distributions [1] [2] [3] [4]. Volcanic aquifers have been studied quite extensively in developed countries [5] [6], but few studies have been undertaken in developing countries [7], where they are very often the only available water resources [8] [9] Given their strong heterogeneity, studying these hydrogeological systems require a high density and quality of data, which explains why such studies have been mainly carried out for systems equipped with well-developed monitoring networks. Strategies to better account for uncertainties should be implemented such as the GLUE (Generalized Likelihood Uncertainty Estimation) method [17] This stochastic method consists in testing a large number of parameter sets and keeping all those which allow to simulate the observed hydraulic heads taking account of their uncertainties
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