Study area and climate

Formerly called Albula (after the Latin albus = white referring to the light colour of its blond waters), or Thybris (from the Etruscans) or Rumon (linked to the ancient Etruscan-Latin name of Rome), the Tiber is the main river of central and peninsular Italy. With 405 km of course, it is the third longest Italian river (after the Po, 652 km, and the Adige, 410 km). The source of the Tiber river is on the slopes of Mount Fumaiolo (1268 m a.s.l.), on the side turning towards Perugia (upper Tiber River Basin, Fig. 7), near Balze, a village of Verghereto (in the province of Forlì-Cesena). The Tiber River Basin is rich in tributaries and sub-tributaries, but the river receives most of its water from the left bank, where its main adductors are the Chiascio-Topino system, the Nera and the Aniene. The tributaries of the right bank are the Nestore with Caina and Fersinone, the Paglia (with the Chiani) and the Treja, between the provinces of Rome and Viterbo. The Tiber River also passes in the vicinity of Perugia, Marsciano, Deruta and Todi, while Tivoli and Subiaco are crossed by the Aniene tributary, at east (Fig. 7).

Perspective view of Tiber River Basin (watershed limits in black) from the North (Mount Fumaiolo, 43°47′N, 12°04′E). The Tiber River (in blue) winds through the landscape, passing near the city of Perugia (43°06′N, 12°23′E), after leaving its source, then near Todi (42°46′N, 12°24′E), and flow towards the mouth through the city of Rome (41°53′N, 12°30′E). Its tributary Aniene flows westward past Subiaco (41°56 ′N, 13°06′E) and Tivoli (41°57′N, 12°47′E). The map with the cities and geographical names overlaid is an output image created by the authors from Arm of Carabineers website (http://www.carabinieri.it/editoria/natura/la-rivista/home/tematiche/ambiente/progetti-sul-tevere).

The river was used for many centuries as a means of communication: in Roman times merchant shipping could go directly to Rome, to the Emporium at the foot of the Aventine, while smaller vessels suitable for river navigation transported goods and agricultural products from Umbria, through a capillary navigation system that penetrated the region also through the tributaries, in particular Chiascio and Topino.

From a climatic point of view, the TRB can be divided into three sub-regions (Fig. 7): the flat sub-region, which includes the area surrounding the city of Rome and the rural areas northeast of the city (areas coloured in white tending towards yellow and green); the sub-region of the Apennine valley along the main branch of the Tiber, which also includes the surrounding mountainous territories, such as Todi and Perugia (areas coloured in green); and the third sub-region at east, mainly mountainous, which includes the provinces of Terni and Rieti (areas coloured in ocher brown). The climate of the first sub-region is temperate, with a hot summer and an annual rainfall of 800–1000 mm. The second and third sub-regions are the most interesting from a hydrological point of view, as they are the main source of deluges and floods, which play an important role in driving the most important erosive storms of the year. The climate is mainly continental, with moderately hot summers in the valley bottoms (roughly province of Rieti), and not very hot summers in the hills (roughly province of Perugia). The amount of annual rainfall is about 800 mm at Perugia, and over 1000 mm at Rieti. The rainfall is distributed over 80–100 days per year, with two peaks: the main peak in autumn and the second in late spring. Summer and autumn storms are common in all three sub-regions.