Hydrography of Spain

Watersheds of the Iberian Peninsula.

Like the hydrography of any other country, the Spanish one is determined by two types of factors: climatic and geological (relief and soil). Most of Spain has a Mediterranean climate, so a first general characteristic is that of rivers with strong droughts and little flow. The rivers of so-called humid Spain are exempt from this characteristic: the north and northwest, and in part, also the large collectors or large rivers, since their tributaries coming from the mountains cushion these effects.

The relief determines the Spanish hydrographic network. On the peninsula, the Central Plateau is the most important relief unit due to the surface it occupies and because the rest of the large units are structured around it, such as its mountain ridges and the depressions and outer ridges. The fact that it is tilted towards the west is the reason that three of the great rivers (Duero, Tagus and Guadiana) pour their waters into the Atlantic. Of the other two large rivers, the Guadalquivir also empties into the Atlantic due to the inclination of the Baetic depression towards that ocean, while the other large river, the Ebro, is the only one of the large collectors that empties into the Mediterranean, given the opening of the Iberian depression towards that sea.

Lakes are not abundant in Spain, although they are of varied origin, since they exist both of endorheic origin (tectonic and volcanic), and of exorheic origin (glaciers, karsts...) and even of mixed origin.

Conditioning of the layout of the fluvial network

The first feature that stands out in the layout of the peninsular river basins is the great dissymmetry between the Atlantic and Mediterranean slopes. Indeed, despite the fact that the length of the coast of both is slightly different, the rivers that flow into the Atlantic drain more than two thirds (69%) of the Peninsula, while those that flow into the Mediterranean drain only less. of a third part (31%) of the peninsular territory. The dividing line between the two basins passes much closer to the Mediterranean coast and describes a great arc with the concavity turned towards the Atlantic, towards which the large peninsular collectors discharge, such as the Duero, Tagus, Guadiana and Guadalquivir rivers, with only the Ebro River, the only great peninsular river that escapes said domain.

The cause of this asymmetry must be found in the structure and geological evolution of the Peninsula since, as a result of the alpine folding, the entire large block of the Plateau was tilted towards the west, while its eastern edges folded to form the Iberian Cordillera. Consequently, from the Iberian ridge the waters slide towards the Atlantic, following the inclined plane of the Meseta. Those of the North Sub-plateau constitute the Duero basin, while those of the South Sub-plateau are distributed, due to the relief of the Montes de Toledo, between the Tagus and Guadiana basins.

The characteristics of the remaining mountainous edges of the Plateau also condition the features of the corresponding peripheral spills. Thus, the high Cantabrian ridge, abrupt and steep and with heights greater than 2,500 m, Torre Cerredo 2,650 m. -Picos de Europa- gives rise to short courses, which have to overcome in routes not exceeding one hundred kilometers, slopes sometimes of more than 2000 m. In addition, these mountains, well fed by the abundant and regular rainfall typical of the Atlantic climate, give rise to mighty rivers with great erosive force due to their slope, which have cut the energetic reliefs of the mountain range into deep and narrow gorges that count between the most magnificent fluvial landscapes of the Peninsula, such as the gorges of the Cares river and those of the Deva river, excavated in the Picos de Europa massif.

Apart from the fluvial device, determined by the features of the central Spanish Plateau, another major feature of the peninsular structure determines the layout and orientation of the rest of the waters of its main collectors: the alignment of the two great alpine mountain ranges: Pyrenees and Cordilleras Béticas, and their respective pre-pits. The Pyrenees, oriented from east to west, will join the northeast edge of the Meseta formed by the Iberian Cordillera, leaving between both mountain systems, differently oriented, the triangle of sunken lands that constitutes the Ebro Depression, open to the Mediterranean; For this reason, said river is the only large Spanish collector that does not discharge into the Atlantic. At the bottom of this depression, the waters collected by the Pyrenean runoff come from the northern mountains, and those from the Iberian edge of the Meseta come together on the south side. This explains why the Ebro is the largest river in Spain.

A similar disposition conforms to the south, between Sierra Morena and the Betic Cordilleras, another great depressed triangle, crossed by the Guadalquivir river and open towards the Atlantic. On the Sierra Morena side it receives contributions from short and indigent rivers, while to the south it receives runoff from the Betic mountains, much less abundant and regular than those of the Pyrenees. The other external and peripheral spills of the Betic Cordilleras give rise to even shorter and more torrential courses, with terrible floods and marked droughts.

Study of Spanish rivers

Main rivers of Spain
	Name	Length1 (km)
01	Rio Tajo	1007
02	Guadiana River (Tablas de Daimiel) + Cigüela	742 + 225 = 967
03	Rio Ebro	910
04	Rio Duero	895
05	Rio Guadalquivir	657
06	Río Garona	569
07	Rio Júcar	498
08	Genil River	337
09	Safe River	325
10	Rio Miño	310
11	Rio Esla	288
Note 1: Total length of the river, including sections that run through Portugal or France.

• Monthly and annual flow rate at each power station.
• Total flow rate evacuated by the river during the year.
• Seasonal variations of the average monthly flow over the course of the year, which is what constitutes the river regime, which depends on many factors, but mainly on the seasonal abundance of precipitations collected in the watershed.
• Relation between the flow circulated by the channel and the precipitations received, or balance of the use, which depends on the evaporation and filtration.
• Relative abundance, that is, the flow relative to the extension of the receiving basin, expressed in liters per second and square kilometer of basin.
• Annual and interannual Irregularity, very important in climates such as the Mediterranean, where the abundance of rains varies extraordinarily from year to year, while it is more uniform in the Atlantic climate.

Of all the factors that influence fluvial runoff, the most important is the climate, especially as regards the amount of rainfall and its distribution throughout the year, and for this reason, knowledge is necessary. of the climate before carrying out the study of the rivers. In addition to the climatic factor, there are other notable factors that condition runoff, mainly topography, since the steeper the slope, the faster the water falling down the slopes and the water that circulates through the channels towards the sea runs, while, in In plain rivers, the runoff is slower and more regular. The torrential nature of a river depends, then, in the first place on the slope of its channel.

Secondly, the nature of the rock that the river crosses influences. In permeable rocks such as limestone, a large amount of surface water is lost and instead an intense underground circulation occurs, which can even make the river itself disappear (figure in the illustration), such occurs, for example, with the Ésera river., in the Pyrenees. On the other hand, impermeable rocks such as slates or clays do not hold water well, which immediately runs towards the main channel.

To a lesser degree, the density of the vegetation cover also influences, which, the denser it is, the better it retains water and returns it more slowly, contributing to the regularization of the flow. That is why the basins of the reservoirs are usually replanted with forest. On the other hand, in basins with poor vegetation, the rapid evacuation of rain often has catastrophic effects.

Absolute and relative abundance of Spanish rivers

The largest river in the Peninsula is the Duero, which near its mouth, in Porto, has an average annual flow of 675 m³/s. Similar values are recorded in the most important peninsular rivers as can be seen in the attached Table.

These absolute values do not differ too much from other large European rivers, such as the Seine River, with 480-500 m³/s, because, although those drain basins with less rainy climates, on the other hand they cross higher reliefs and therefore better fed. However, it should be noted that the Mediterranean rivers, except for the Ebro River, are much less plentiful, since they are largely semi-arid regions where there is little rain and evaporation is intense. On the other hand, the Cantabrian rivers are better fed with abundant flows, as corresponds to the Atlantic climate.

But, of course, under equal climatic conditions, rivers are all the more mighty the larger their feeding basin is. Hence, the abundance or relative flow data are much more expressive, a concept also called coefficient or specific module, expressed in liters per second and kilometers of basin. This value, contrary to what happens with the absolute flow, decreases downstream of the valley, since rainfall tends to decrease in the same direction, as altitude decreases.

On average, in a holo-humid area like northern Spain, the annual runoff coefficient is about 20 l/s km². In the Pyrenean rivers, values of 30 L/s/km² are reached, even higher in the headwaters, such as the Caldarés, a tributary of the Gállego, with 46.28 l/s km². The values of the Atlantic region are also high: in Nalón, 25.7, and in Miño, 19.1. On the other hand, in the south, in the semi-arid zone, the relative abundance values are much lower: 1.6 in the Guadiana and 0.6 in the Vinalopó.

Seasonal Variations

In general, the regime of a river that depends solely on rainfall perfectly reflects the maximum and minimum rainfall in its basin, so that the expressive curve of its regime is parallel, in general lines, to that of rainfall. This is the case in the rivers of the low and medium mountains, and in some short Atlantic (Tambre, Ulla, in Galicia) and Mediterranean (Segura, Mijares, Zújar, Cíjara) rivers.

On the other hand, in the rivers that originate in the high mountains, where snowfalls are frequent, and much more so in those fed by glaciers, the maximum flows occur at the end of spring or in summer, in the season melting of snow or ice accumulated during the winter, even when the precipitations then go through the minimum characteristic of the Mediterranean climate.

Regime Types

Nival food: the river Caldarés (in Ibón de los Baños, Huesca)

Nivo-luvial food: the Segre River (in Seo de Urgel, Lérida)

Flood food: the river Guadiela (in Buendía, Cuenca)

Snow regime

Limited to the highest areas of the main mountain ranges, with elevations greater than 2,500 m s. no. m.. In the pure snowy type, as observed, for example, in the Pyrenees in the upper course of the Caldarés River, a tributary of the Gállego, with a unique and very prominent maximum extended between May and July; There is therefore no summer minimum. Many other Pyrenean rivers, such as the Ter in Gerona, the Segre in Puigcerdá, etc., offer a curve in which the somewhat more attenuated influence of the snow factor can be seen, at least in its upper course, but with a marked summer minimum..

Nivo-pluvial regime

Owner of the mountains of 2:000-2,500 masl, variable according to latitude; with a main maximum produced by the melting of snow and other secondary maximums of pluvial origin; as in the Aragón and other Pyrenean and pre-Pyrenean rivers, and also, in general, in the upper course of the rivers of the Cantabrian Mountains (Sella, Nalón, Narcea) and in those of the Central System (Tormes in Barco de Ávila, etc.).

Pluvio-nival regime

With the snowy influence even more attenuated because they are lower altitudes, between 1,600-1,800 m and increase the influence of purely rainwater contributions from the lower parts of the basin; They register maximums from April to March, summer low waters and an autumnal recovery. As a sample of medium mountain, we can mention sections of rivers of the Pre-Pyrenees (Arba, Llobregat, Fluviá, Condoner, etc.), of the Central System (Henares, Jarama, Tiétar, Alagón, Tormes, Adaja, Eresma, Alberche, etc.), of the Iberian Cordillera (Najerilla, Iregua, Cidacos, Arlanza, Guadalupe, Júcar, Turia, etc.), the Cantabrian Cordillera (Pisuerga, Esla, Cea, etc.), or the Béticas (Guadalfeo)

Rain regime

With a seasonal rhythm traced from the precipitation curve. Within this type it is possible to distinguish, according to the seasonal distribution of rainfall, the following subtypes:

• Levante Mediterranean rainfall, with two maximums, one of spring very sustained or better dedoblated in two peaks, from February to March and from May to June, the second most accused than the first; the estival minimum, so characteristic of the Mediterranean climate is prolonged the months of July and above all August below coefficient 1, and a second otoñal maximum, more or less accused according to the sector, but that in the Levante coasts is usually the main coincidental. Examples: The Segura, the Jalon in Calatayud, the Guadalope in Alcañiz, the Mijares near its mouth in Villarreal, etc.

• Mediterranean rainfall, which distinguishes itself from the previous one by presenting in spring a single maximum, centered at the beginning of the season, a low winter minimum and instead the stival much stronger and prolonged, of three to four months, such as in the Guadiaro in Colmenar and the Guadalhorce in the Chorro.

• Subtropical rainfall or Mediterranean transition, typical of the rivers of much of the Meseta, with a rhythm similar to that of the Levantine Mediterranean but with a more accentuated summer drought of three to four months; the main maximum is in autumn. Examples: the Zújar (Puebla de Alcocer), Cigüela (Villarrubia de los Ojos), etc.

• Atlantic Rain, typical of the regions of northern Spain, subject to the Atlantic climate, but in low altitude basins. Possibly some Galician rivers such as the Tambre and the Ulla reflect this arrangement in their curves, since in most of the others in the area, because they have their sources in elevated areas, the nival influence in their headers is more or less clearly accused.

Complexity of large rivers

The Tagus River in its high course (Guadalajara).

The Tagus River in its low course (Portugal).

The large Spanish rivers that are fed by tributaries from basins with different climates have a complex regime.

Of all the major Spanish arteries, the simplest case is that of the Guadiana river, as can be seen by comparing the curves of the Cíjara station with that of Badajoz (Puente de Palmas) some 300 km distant, because all Its tributaries born in relatively low mountains, with little or no snow influence, participate in the type of regime that has been called subtropical pluvial.

Somewhat more complex, with mixed regimes along its course, are the other large rivers of the Meseta. The Tagus, born in the Sierra de Albarracín, with altitudes close to 1,800 m, benefits from winter snow, which is why its upper course is clearly rainy-snow, as recognized in the Sacedón station (Guadalajara). Then it receives the contributions of the spills of the Central System, such as the Tajuña, Henares, Jarama, Alberche, Tiétar, Alagón, in which the pluvial character increasingly dominates, which is why the river progressively accentuates, as it progresses. through the interior of the Plateau, its pluvial character, which finally dominates at the Alcántara station, close to the Portuguese border, and with the same character it reaches near the mouth, in Vila Velha de Ródão. Thus, throughout its course, it has changed its character, from a pluvio-snowy river to a purely pluvial river. Something similar occurs with the Duero, which from its sources, at the foot of the Picos de Urbión, is purely rainy-snowy, but as it crosses the Upper Plateau it transforms that character into that of rainwater, which is what has near the mouth.

The most complex case of all the main peninsular arteries is undoubtedly that of the Ebro. Born in the Cantabrian Mountains, near the karstic lake of Fontibre (Reinosa), it collects abundant runoff from the elevated mountains of this sector, therefore which in the Arroyo station, not far from the sources, presents the pluvio-nival character. But, as it enters the Ebro Depression, it is replaced by the Mediterranean rainwater, due to the dominant contributions of the tributaries of the sector crossed. But, later on, when it receives the great Pyrenean contributions from Aragón, Gallego and Segre with its tributaries Cinca and Noguera, born on the highest peaks of the Pyrenees, it recovers its pluvio-nival character again, which it preserves right up to its mouth. Thus, in Tortosa, the Ebro offers a regime not very different from that of the great Pyrenean rivers on the northern slope, such as the Garona in its lower course (in Agen and Langon).

The Guadalquivir also has a complex regime. In its upper part, near the confluence with the Guadiana Menor, it has a subtropical rainfall regime, since the heights of the mountain ranges of the headwaters are not very important. But after said confluence, and especially after receiving the Genil, which provides it with the melt waters of the Sierra Nevada snows, which means that in its upper part this tributary is of a transitional nival regime that later passes to nivo -rain and whose influence is accused in the curve of the Guadalquivir from that point. The initial maximum of February of the Guadalquivir dragged to the month of March, which indicates the influence of the snow. At its mouth, the Guadalquivir has a regime very similar to the lower course of the Tagus, with a flow of 164 m³/s in Cantillana, shortly before Seville.

Irregularity and large avenues

The difference between the maximum and minimum coefficients of each river specifies the value of the irregularity, which, as expected, is very high in peninsular rivers, especially in Mediterranean ones.

On the other hand, in the rivers of the Cantabrian slope, where the climatic fluctuations are much less pronounced, the irregularity oscillates only between values 2 and 3. In the Meseta, with very dry summers and consequently with very dry periods accentuated, the irregularity already rises from 9 to 12 and is somewhat higher still in the south, especially in torrential courses,

The great avenues of our rivers are above all a characteristic phenomenon of the peninsular periphery, both due to the importance of the slope of the courses that descend to the Plateau and peripheral mountain ranges, as well as due to the climate. With the irruption of stormy and abundant clouds, so typical of Mediterranean meteorology, the imposing discharge of its aqueous content occurs before the first reliefs that oppose its passage inland, being capable of causing true flows due to the amount of catastrophes. The great floods caused by the Mediterranean rivers take place especially in autumn. On the other hand, in the Meseta, the rivers of the Tagus and Duero basins, and in the Atlantic slope, exceptional floods usually occur from December to March.

In the Ebro basin and in some rivers on the Iberian mountain slopes, the avenues are usually spring-like, coinciding with exceptionally high temperatures that cause an accelerated melting of the winter snows.

Water resources

Transfer Tajo-Segura in the vicinity of Balazote, Albacete.

The low rainfall characteristic of the Mediterranean climate imposes a delicate policy on the use of its water resources, both surface and underground, on Spain. Since ancient times, water was carefully used and necessary both for irrigation and for supplying populations; think of the expensive Roman aqueducts of Segovia, Mérida, Tarragona and Barcelona, for example; and as regards irrigation, they were already extended at least by the Arabs in the orchards of Valencia and Murcia, the fields of Lérida and in a good part of the Andalusian countryside. Irrigation water legislation has given rise to a rich and varied traditional legislation as very few countries can offer. Then the use of water as a driving force as industrialization occurs, either directly or through electricity, has determined the construction of large hydroelectric works in which Spain is an advanced country. Finally, the consumption of water for industry and for the supply of large urban concentrations is an obsessive concern of the present times to the point of having to resort in less favored places, such as in some of the Canary Islands, to the distillation of sea water.

In general it can be said that the entire Spanish Mediterranean, in one aspect or another, is deficient in water resources compared to the Atlantic climate zone. Thus, the deficit in the Spanish Mediterranean basin is around 5,000 hm³ per year, while on the Atlantic slope there is a surplus of close to 25,000 hm³ because, as has been seen, the large rivers discharge into the ocean. For this reason, it has been necessary to compensate this deficit with the costly and long transfer of the Tagus, to alleviate the painful situation of the Levante. For this reason also the large reservoirs either for irrigation or above all for obtaining hydroelectricity. All the main rivers have been used for the construction of dams that have transformed the peninsular fluvial landscape, so rich in gorges, into long lakes embedded in the course of tortuous valleys, as occurs with the Ebro, in its Mequinenza reservoir, the Esla, in Ricobayo and especially with the large Guadiana reservoirs such as Cíjara and Tajo, Entrepeñas and Buendía, several kilometers long.

All these swamps have a capacity of 1,000 to 1,500 million m³. These rivers, with relatively flat areas or with little slope, have required the construction of dams of considerable height, in order to compensate for the little slope with a large flow. Instead, the Pyrenean lakes, suspended at heights of more than 2000 m s. no. m., they compensate for their reduced capacity with a large unevenness, and are used in chains, through forced pipes that carry the water to the power stations located downstream, at the bottom of the valley. Thus we see how the morphological conditions of the peninsular rivers determine the type of hydroelectric power plants.

But, in turn, these artificial swamps have repercussions with a series of physical events. For example, the catastrophic floods so characteristic of Spanish rivers are becoming increasingly rare, as they have reservoirs in the upper/middle course to regulate the flow, damming it in periods of abundant rainfall and yielding it in periods of marked hardship. In many rivers, thanks to these reservoirs, a more regular and uniform type of regime has been achieved. On the other hand, the reduction of debris, which now remains accumulated to a large extent in the swamp basin, by decreasing the transported load, has increased the erosive force of the rivers, and in the delta mouths and alluvial plains the decrease in transport is manifested due to a retreat of the coastline, due to the attack of waves and marine currents. Thus, for example, it can be seen in the Llobregat and Ebro deltas, whose points advanced years ago at a rate of one and ten meters respectively, while now they are receding at a rather marked rate.