Climatic conditions

The whole territory of the Confederación Hidrográfica del Segura presents great climatic contrasts, frequent droughts, torrential rains, recurrent floods, high temperatures and heavy frosts. Important climatic differences are observed; from one face of a mountainous slope to another, from the high lands to the littoral fringe, or from one geographical area to another; in occasions they are local variations directly related to topographic factors that give origin to local eco-climates; in others they are factors that affect wide areas.

In general, one can affirm that the factors that condition the climate of the basin (latitude, climatic atmospheric components, topography, orientation and exposure, together with distance to the coast) combine and multiply giving place to a rich array and diversity of conditions both generally and in more specific areas.

The spatial distribution of the mean annual precipitation suggests only a slight relationship between relief and precipitation. It is in the mountains located in the northwest of the basin, which are subjected to the action of the humid winds of the Atlantic storms of polar fronts, where the maximum precipitation values are registered. In these areas the mean annual precipitation is over 1000 mm. Although altitude favours precipitation, the orientation southwest-northeast of the high mountain ranges of the upper Segura river basin, (Sierra of Segura, Alcaraz, Taibilla, etc.) hinder the advance of the Atlantic influences dragged by westerly currents, resulting in a decrease of the mean annual precipitation in a diagonal running in a northwest-southeast direction from the mountains to the coast, where minimum values below 300 mm are reached.

The mean annual precipitation in the depression of Aguilas is attenuated as a result of the effects of surrounding hills of Carrasquilla, Almenara and Lomo de Bas. The mean annual precipitation in this zone hovers around the 200 mm mark. In these extreme cases, indirect forms of precipitation must be considered (condensation of dew, high environmental humidity of the coastal areas), factors that can cause an appreciable reduction of the evapotranspiration, and facilitate water for plants, data not reflected by the observed precipitation.

As exceptions to this general scheme of precipitation distribution are the zones of Sierra Espuña with values over 500 mm, and a wide area located in the vicinities of the polygon Hellín-Jumilla-Fortuna-Cieza, where the mean annual precipitation figure is around the 300 mm mark. As for the intensity of precipitation generating floods, frequently values of 100 mm in one day can be observed, although on occasions values of 300 mm or more have been registered. The torrential character of the rain together with the ramified structure of the fluvial network and the density of urban areas close to riverbanks results in frequent flash floods with serious consequences.

Mean annual precipitation (mm/year). Source: Hydrological Plan of the Segura Basin

Contrary to the mean annual precipitation, the daily maxima are often registered in the middle to lower reaches of the basin as well as in the coastal fringe, instead of the headwaters. This can be explained by the Mediterranean origin of convective phenomena that produces the strongest torrential storms.

Temperatures are also related to the factors previously mentioned such as the latitude, altitude and orientation. These originate the thermal diversity that exists in the Segura basin. As extreme values, one can mention the 10 degree Celsius value of the mean annual isotherm that passes through the Sierra de Segura, and the 18 degree Celsius mean annual isotherm in the vicinities of Albatera, Dolores, and some coastal areas. The lowest temperatures in the Segura basin are registered in the northwest mountainous areas. From these mountains, and descending towards the coast, the mean annual temperature increases, reaching 18 degrees Celsius.

Exceptions to this general outline are on the one hand Sierra Espuña, where the annual mean temperature falls to 14 degrees Celsius and on the other, the littoral fringe around the Mar Menor at 17 degrees Celsius. .

The annual regime of temperatures frequently presents a winter minimum in the months of December and January. The annual maxima correspond to the months of July and August, although in general terms, the former is usually the hotter. In the following figure a general summary of the mean annual temperatures of the basin can be observed.

Mean annual temperature in degrees Celsius. Source: Hydrological Plan of the Segura Basin

As a result of the geographical situation of the Segura basin in the Southeast of the Iberian Peninsula, the absolute maximum temperatures correspond to the North African winds. Temperatures reach values close to 40 degrees Celsius and under certain circumstances up to 45 degrees Celsius.

The currents of cold air from the polar region result in strong temperature drops giving rise to heavy frosts and catastrophic effects on crops. The lowest values for evapotranspiration correspond to the mountainous headwaters of the Segura. This zone close to the headwaters is typified of a mean annual ETP below values of 600 mm according to Thornthwaite.

The rest of the mountain ranges in the northwest of the Segura basin are covered by isolines of mean annual ETP from 650 mm to 750 mm. As with the precipitation and the mean annual temperatures, one can trace a line from the northwest to the southeast of the basin, from the headwaters towards the coastal plains, in which, the ETP (Thornthwaite) increases to reach a maximum of 950 mm, around the city of Murcia, the river Guadalentín and the confluence with the river Segura, descending to values of 850 mm in the zone of the Mar Menor. The exception to this tendency is the area of Sierra Espuña where mean annual ETP is lower than 700 mm.

Potential ETP mm/year. Source: Hydrological Plan of the Segura Basin.

Of the groups established by Papadakis, the Segura basin can be identified as Mediterranean with the subtypes temperate Mediterranean, continental Mediterranean, subtropical Mediterranean, and subtropical semi-arid Mediterranean.

Within the first type practically half of the Segura basin is included from the headwaters of the river Guadalentín via the mountains of the northwest, continuing to Corral Rubio, and Yecla up to the Sierra del Carche. The continental Mediterranean climate, occupies two well differentiated and adjacent areas. The first is located in the river Turrilla, south of the Sierra de Ponce or Cambrón and headwaters of the river Pliego. The second stretches from the Cierva reservoir, via the lower reaches of the Murtas stream, half of Judío wadi, Fortuna and Ojós reservoir.

The subtropical Mediterranean climatic type, is the second most important in terms of extension, covering an area from the limits of the previous established, to the coast, except a fringe running from the surroundings of Águilas, to Cabo Tiñoso, via Mazarrón. This region is classified climatically as having a subtropical semi-arid Mediterranean climate.