The Tirino river basin is only 150 km <sup>2</sup> wide; the river is 13 km <sup>2</sup> long and has a mean base flow discharge of 15 m <sup>3</sup> /s. Overland run-off has negligible values all over the year. The base flow is supported by the contribution of many springs fed by the Gran Sasso-Sirente carbonate aquifer, which extends over an area of 1000 km <sup>2</sup> . Tirino valley is located near the south eastern margin of the Gran Sasso structural unit, where it overthrusts the northern margin of the Morrone structural unit (Vezzani & Ghisetti, 1998; Centamore et alii, 1992; BIGI et alii, 1995). The local stratigraphic sequence consists of meso-cenozoic detrital limestones. The Gran Sasso eastern thrust cuts the Tirino valley near Bussi and goes on to the eastern margin of the Popoli plain, where the large springs of Pescara river are located; this thrust acts as a hydraulic barrier. The Tirino valley is partially filled by pleistocenic lacustrine deposits, which overlap the carbonate slopes up to an altitude of 500 m a.s.l. (Giuliani & Sposato, 1995). The lacustrine marly and clay impervious sediments confine the lower carbonate aquifer in artesian conditions. The upper springs (Capestrano-Presciano and Capo d'Acqua) are located at 340 m a.s.l. on both sides of the valley. Between 335 and 310 m of altitude the river flow is fed by "linear springs" (Boni et alii, 1986). At 250 m a.s.l., the main spring of the Tirino river is located in the river bed; it is named "Sorgente del Basso Tirino" (Lower Tirino spring). The upper springs are located where the terrace of impervious lacustrine deposits is eroded down to an altitude of 340 m a.s.l.. At this level the carbonate aquifer, which feeds the springs on both sides of the valley, is in a free condition, or partially confined. At a lower altitude (325-310 m) the carbonate aquifer, due to his artesian condition, feeds "linear springs" by "drainance" through lacustrine deposits. Around the Basso Tirino spring the piezometric head of the carbonate aquifer is located more than 15 m above ground level. This spring has a very regular discharge regime ranging from 7 to 5 m <sup>3</sup> /s. The cumulated discharge of all springs located upstream Madonnina hydrometric station (310 m) is very regular over the year, but has been affected by cyclic variations during the last century. Between 1932 and 1943 the mean discharge gradually rose from 10 to 14 m <sup>3</sup> /s; between 1971 and 1992 the mean discharge decreased from 10 to 6 m <sup>3</sup> /s. The mean discharge at Madonnina station has been evaluated 9 m <sup>3</sup> /s. Cyclic variations of the spring discharges have been referred to cyclic variations of precipitation.

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