The Vega de Granada is an important Mediterranean aquifer located in an alluvial plain surrounded
From: Dragoni, W. & Sukhija, B. S. (eds) Climate Change and Groundwater. Geological Society, London, Special Publications, 288, 53 -62.
DOI: 10.1144/SP288.5 0305-8719/08/$15.00 © The Geological Society of London 2008.
by mountains in southern Spain, within the province of Granada (Fig. 1). The aquifer presents a multilayer structure, with the superposition of sedimentary materials showing a varied range of permeabilities. The surface of the aquifer has a smooth topography between the altitudes of 530 and 760 m above sea level and it is the receptor of a drainage basin of 2900 km2. The annual mean rainfall over the aquifer is 450 mm, although it can reach 1000 mm above some points of the drainage basin such as the Sierra Nevada range. The aquifer has a surface of around 200 km2. Its Quaternary alluvial sediments reach a thickness of 250 m in the middle and diminish towards the northern and southern borders to 50 m. The sediments are mainly gravel, sands, silts and clay, with frequent spatial changes of lithofacies, resulting in a multilayer unconfined aquifer. There is an increase of the silt and clay fraction from the central axis of the aquifer (defined by the Genil river) towards the northern and southern ends (PNUD/ FAO 1972). Figure 2 shows a cross-section along the main river, where lithofacies and permeabilities were interpreted from drilling data. As seen, the changes in permeability in the aquifer are considerable, which may explain the spatial variations in the spectra calculated; however, the role of the distance to sources and to the aquifer borders would appear to be more important (Luque-Espinar 2001).
The mean transmissivity of the aquifer is around 4000 m2/day with a wide range of variation from 40 000m2/day in some central sectors, to 100 m2/ day at the border when clay materials are more frequent. The mean effective porosity is estimated at 6%, with most values ranging from 1 to 10% (PNUD/FAO 1972). The mean groundwater flow direction is from east to west, with the steepest gradients in the NE and eastern sectors (Fig. 1).
The main inputs into the aquifer come from the infiltration of superficial runoff and return from irrigation water, plus infiltration of rainfall water. An important part of the superficial runoff comes from snow melting in the Sierra Nevada range (with altitudes up to 3000 m). Total runoff water is estimated at 400 hm3/year, and the renewable aquifer resources are estimated at 180 to 230 hm3/year (PNUD/FAO 1972; ITGE 1989). The output from the aquifer is mainly natural drainage towards rivers, while well pumping represents around 30 hm3/year (ITGE 1989).
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