Overall, Spain is not poor in waterways, but the hydrographic network is rather sparse. It is divided into five main rivers: four of them, the Tagus, the Duero, the Guadalquivir and the Guadiana, following the natural inclination towards the W of the Meseta, turn to the Atlantic, all (except Guadalquivir) running their lower stretch in Portuguese territory; the Ebro instead it develops its course between the Iberian System and the Pyrenees, flowing into the Mediterranean Sea. Atlantic rivers generally have a rugged profile, forced as they are to dig deep beds and to descend steeply in steps from the inner plateaus to the coastal plains; they also have a rather limited flow rate, in accordance with the modest rainfall characteristic of large areas of the peninsula and the scarcity of snowpack and glaciers, to which is added the intense evaporation. Only two rivers have an important flow: the Duero and the Guadalquivir; in both cases they are fluvial axes which receive, through a part of their tributaries, notable contributions of water. The Douro basin is the largest in the country (over 98,375 km²) and in fact corresponds almost exactly to the entire northern Meseta; born from the Iberian System, it receives good contributions from the Trás-os-Montes sector(Portugal), the Cantabrian Mountains and the Central System. The Guadalquivir, whose basin roughly corresponds to the depression of the same name, has the contribution of the tributaries of the Sierra Morena and the Betic System, and, thanks to its regular regime, is very important for the effects of irrigation and navigability. Lesser flow and rather irregular regime presents the Tagus (although with its 1007 km of course it is the longest of the Iberian rivers), fed by tributaries above all of the Central System. The least important of the five Iberian rivers is the Guadiana, which has few contributions from the tributaries that descend from arid and not very high ranges and which, like the Tagus, crosses large drought areas of the southern Meseta. On the Mediterranean side the most important river is the Ebro, the maximum entirely Spanish, which, born from the Cantabrian Mountains, collects the waters of the southern and northern Pyrenean slopes of the Iberian System; it runs through the Aragona, irrigating and crossing a steppe territory that largely contributes to limiting its flow, and after having winding through a dense series of meanders, the passage through the Catalan Prelitoraneous System laboriously opens up to flow to the S of Tarragona with a vast delta with a characteristic lanceolate shape. The other tributary rivers of the Mediterranean (Segura, Júcar), characterized by short and tumultuous courses (ramblas) and subject to sudden and ruinous floods, have a limited regional dimension. There are numerous dams built on the Spanish rivers; the major basins are located on the courses of the Duero, Guadiana, Guadalquivir, Tagus.
From the climatic point of view, Spain, given its position between the Atlantic and the Mediterranean, depends fundamentally on the penetration of the humid air masses of Atlantic origin and on the more or less prolonged and tenacious establishment of the anti-cyclonic Mediterranean air masses. According to findjobdescriptions, the former are particularly frequent on the northern facade of the peninsula, which is by far the rainiest; the anticyclonic air masses predominate in the central and Mediterranean part, especially during the summer, which is always drought and very hot: the precipitations throughout Spain are in fact mainly winter and spring. However, although the territory is almost completely surrounded by the sea, due to the arrangement of the reliefs, often directly aligned along the coasts, and the squat shape of the peninsula, the lands of the interior remain on the edge of maritime influences, so we can speak of a continental climate for almost the whole country, in particular for the Meseta, the Iberian depression and the innermost area of the Andalusian one. In the northern part of the country, affected by the frequent passage ofAtlantic cyclones, an average of 1000 mm of rain fall annually, distributed with a certain regularity over the year; on the Cantabrian and Pyrenean slopes exposed to the ocean, rainfall can exceed 1500 and sometimes 2000 mm per year: Galicia for example has a purely Atlantic climate, humid all year round. Central Spain and the Mediterranean region are always dry; in general the rainfall values are lower than 500-600 mm per year, with minimums of 200-300 mm per year, concentrated in the winter period, in the innermost depressions, in particular in the Mancha, in the valleys of the Ebro and Guadalquivir and in the extreme south-eastern part. Also from the thermal point of view there are significant differences passing from the Atlantic coastal areas, where temperatures are mitigated by Atlantic influences both in winter (8-10 ºC) and in summer (18-20 ºC), to inland ones, characterized by the marked temperature variations typical of continental climates: in Madrid from 6 ºC in January it rises to 24.5 ºC in July, with a few days of frost. On the sunny Mediterranean coast, there are hot but not excessive summers, thanks to the presence of the sea, and winters softened by the Mediterranean winds; the Andalusian depression, on the other hand, has climatic characteristics that already herald neighboring Africa.