Spain has an area of approximately 505,000 km2. The average precipitation is 700 mm/year. However, this average has considerable spatial deviation, ranging from 100 mm/year in some islands in the Canary Archipelago to more than 2000 mm/year in the humid north. The average annual temperature is 14°C. The average potential evapotranspiration is about 700 mm/year. In most of Spain potential evapotranspiration is higher than precipitation. The average stream flow is about 110 km3/year (220 mm/year). From this amount, about 80 km3/year (160 mm/year) is surface runoff and about 30 km3/year (60 mm/year) is groundwater recharge. At least one-third of Spain is endowed with good aquifers. These aquifers may be detrital, calcareous or volcanic (Fig. 13.1). The Water Administration has formally identified 411 aquifer systems or hydrogeological units (see Table 4.1 in Llamas et al., 2001), which cover an area of approximately 180,000 km2. The estimated natural recharge of these aquifers averages 30 km3/year but varies with weather conditions between 20 and 40 km3/year (Fig. 13.2).
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