Integrated hydrogeological study of San Cristobal Island (Galapagos)

The understanding of the hydrogeological functioning in regions where surface water is limited is fundamental for a suitable management of its resources. San Cristobal is the only island in the Galapagos Archipelago with permanent freshwater resources, in which water is supplied for the inhabitants...

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Detalles Bibliográficos
Autor Principal: Dom?nguez Gonzalez, Christian Gonzalo
Otros Autores: Bruijnzeel, Sampurno L.A.
Formato: Tesis de Doctorado
Lenguaje:fra
Publicado: Paris / Universidad Pierre y Marie Curie 2016
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Acceso en línea:http://repositorio.educacionsuperior.gob.ec/handle/28000/2517
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Sumario:The understanding of the hydrogeological functioning in regions where surface water is limited is fundamental for a suitable management of its resources. San Cristobal is the only island in the Galapagos Archipelago with permanent freshwater resources, in which water is supplied for the inhabitants from water catchments in the streams located in the southern windward side of the island. To date, these water resources are poorly managed as a consequence of the lack of knowledge surrounding its occurrence. In this context, this work is the first study of the hydrogeological functioning of high-level aquifers in the windward side of San Cristobal using a multi-disciplinary approach at watershed scale, based in the implementation of an experimental site, hydro-climatic monitoring, field surveys, indirect methods and numerical modeling. For this purpose a hydrological network was installed in Cerro Gato and surrounding watersheds. Two hydrological years of records from hydro-climatic stations (2013-2014 and 2014-2015), show a median annual rainfall of ca. 1000 mm (300 m a.s.l.) with an orographic rainfall gradient of 270 mm/100 m of elevation. Cloud water interception (CWI) by the endemic vegetation is estimated using a Rutter-type canopy model, which shows that CWI at the windward highlands (600 m a.s.l.) is not negligible. It represents ca. 28% of the water input during the cool seasons, and it reaches 23% of the total water input for a representative year. Deep percolation (DP) is estimated using the joint modeling of a physically based canopy and soil water transfer model. DP rates are markedly different depending of the altitude, land cover and seasonality. It is mainly affected by altitude in mid-elevation watersheds, whereas land cover is the main controlling factor at high-elevation watersheds because of the additional input of fog interception in forests. A water balance, performed in three gauged watersheds using estimates of recharge and measured streamflow, reveals that recharge is significantly higher than discharge in high-elevation watersheds, while it is lower than discharge in mid-elevation watersheds. Streamflow is simulated in the three watersheds using a lumped hydrological model (GR4J/GR7J) to characterize the hydrodynamic behavior of these watersheds. Results show that losses from the high-elevation basins (above 400 m of elevation) become groundwater inputs in the mid-elevation basins (below 400 m of elevation), while other mid-elevation basins have inputs from basins at the same altitude, such as Cerro Gato, where its hydrogeological watershed represents 140% of the catchment surface and its extension is more likely in the same elevation of the catchment. The detailed geometry of the hydrogeological watershed of Cerro Gato is obtained from a 19-layer resistivity model, which was developed from the dataset of a high resolution AEM SkyTEM survey conducted in San Cristobal. Two types of geometry are defined with a small and a large threshold of resistivity, which is associated with saturated fractured basalt. The two cases were tested with a hydrogeological approach, using a 2D saturated/unsaturated groundwater flow model, in which the groundwater flow of springs fed by a perched aquifer in fractured basalt suspended by an impervious thin palaeo-soil layer is simulated. Hydrodynamic parameters of the model were assumed either: from the literature, calibrated in the model, estimated from a seismic refraction survey or measured in the field, depending of the concerned lithology. The model is constrained with the spring discharge measurements. Results from the groundwater model conform the plausibility of the existence of a perched aquifer with an extensive and smaller length and reveals the influence of the sub-surface weathered cover in the hydrodynamic behavior of the aquifer. Results of this thesis help to improve the general knowledge of the hydrogeology of basaltic islands, especially of high-level perched aquifers. Also, this thesis provides scientific basis for the effective water management, which could be used by the local institutions to offer solutions to the increased water demand and to provide guidelines to the conservation of freshwater ecosystems.