Algeria's geography: a country divided by its rainfall

Why rainfall decides where Algerians live

Two regions make up the country, and the tectonic collision of the African and Eurasian plates along the Mediterranean made both. The north, known as the Tell, is a set of young mountain chains and the basins between them. The south is the Sahara, an old, flat platform of basement rock that holds the oil and gas. Population density is highest in the plains and coastal mountains of the northern Tell, where rain is heaviest and most reliable, and falls away southward until large parts of the desert have no settled population at all.

What sets the boundaries between those zones is the amount of rain and when it falls. Algiers receives about 760 mm a year, four-fifths of it between October and March, with July and August usually dry. From a point about 80 km west of the capital to the Tunisian border, annual rainfall exceeds 600 mm, and in Great Kabylia, Little Kabylia and the Edough massif it reaches about 1,000 mm. West of that point, much of the Chelif plain and the coast around Oran receives less than 580 mm.

That east-west divide is the main line in Algerian farming. In the east, Britannica notes, dry farming pays and forests survive. In the west, cereals need irrigation, herding takes over and the forest disappears. The sirocco, a hot dust-laden wind off the Sahara that can blow at gale force, adds a second hazard: its timing in a given year can decide a harvest.

How the Tell folds into parallel strips

From the coast inland, Britannica counts five zones that run roughly parallel to the sea: coastal massifs, coastal plains, the Tell Atlas, the High Plateaus and the Saharan Atlas. The coast is hilly and in places mountainous, with a few natural harbours, and plains such as those of Oran and Annaba push inland between the ridges.

In the west the ranges are broken often enough to let traffic through. The Maghnia plain separates the Tlemcen mountains from the Traras, and the plains of Sidi Bel Abbès and Mascara lie between hill ranges. The Dahra massif runs from the mouth of the Chelif to Mount Chenoua, with the Chelif valley between it and the Ouarsenis.

The centre is harder going. There the Blida Atlas joins the Titteri, and Great Kabylia joins the Bibans and Hodna mountains, and only the valley of the Wadi Soummam gives a route through to the port of Béjaïa. East of Béjaïa one ridge follows another between the high cereal plains of Sétif and Constantine and the sea. The Mitidja plain behind Algiers was a malarial swamp until the French cleared it, and its alluvial soils then carried vineyards and citrus groves.

Only the northern ranges, which lie along the plate boundary, have much seismic activity. Earthquakes destroyed the town of Chlef, formerly El-Asnam, in 1954 and again in 1980. Another in 1989 caused severe damage between the Chenoua massif and Algiers, and one in 2003 struck just east of the capital.

What lies between the two Atlas ranges

The High Plateaus stretch more than 600 km east from the Moroccan border. They are steppe, dry enough that the plains are sometimes counted as part of the Sahara, and they carry sabkhahs, lake beds crusted with salt. Rain there is 100 to 400 mm a year. Summer afternoons typically pass 38 °C and nights fall to about 10 °C, while winter runs from about 16 °C by day to about minus 2 °C at night. Esparto grass grows wild across the western plains.

The Saharan Atlas to the south is a set of ranges running south-west to north-east. Heights fall from Mount Aïssa in the Ksour mountains, at 2,236 m, to the lower Amour and Ouled Naïl ranges, and rise again in the Aurès, where Mount Chelia reaches 2,328 m. The Aurès and the Nemencha fill the north-east up to the Tunisian border, and their steep cliffs and long ridges have made an almost impenetrable refuge since Roman times. Watercourses coming off the southern slopes disappear into the desert and feed the wells of oases along its edge, among them Biskra, Laghouat and Béchar.

South of the Aurès the ground falls below sea level. Chott Melrhir, a salt marsh, is the lowest point: the country's record gives it as 35 m below sea level and Wikipedia's geography article as 40 m.

From the mid-1970s the state planted a barrage vert, a green barrier of mostly Aleppo pine, roughly along the ridge line of the Saharan Atlas from the Moroccan to the Tunisian frontier, to hold back the desert. Wikipedia describes a zone 1,500 km long and up to 20 km wide; Britannica gives about 1,600 km and up to 19 km and calls the result only somewhat successful. By the early 1980s the desert had reached Bou Saâda, well inside the High Plateaus, and the project was ended in the late 1980s for lack of money.

How the Sahara divides into sand and rock

The Algerian Sahara runs about 1,500 km south from the Saharan Atlas to the borders of Niger and Mali. Much less of it is sand than the name suggests. Dune fields called ergs cover about a quarter of it; most of the rest is bare rock platform, called hamada. The Grand Erg Oriental and the Grand Erg Occidental sit in two basins either side of the M'zab, a limestone rise where the Mozabite community settled. Britannica gives the average height of the ergs as 400 to 600 m, declining northward from the foot of the Ahaggar.

The south-eastern quarter is taken up by the Ahaggar and Tassili n'Ajjer highlands, parts of which rise above 2,000 m. The Tassili n'Ajjer is a sandstone plateau of about 72,000 km² at the borders with Libya, Niger and Mali. Its eroded sandstone forms what UNESCO calls forests of rock, and it holds some 15,000 engravings, recorded since their discovery in 1933, that follow changes in climate and animal life on the edge of the Sahara from about 6000 BC. UNESCO inscribed it as a mixed natural and cultural site in 1982.

Rain in the deep south is irregular to the point of absence: three years can pass without any in the Tademaït and as many as five on the Ahaggar plateau. Nights are cool even when midday is hot, because the dry air loses heat fast.

How high Mount Tahat stands

The summit of the Ahaggar, Mount Tahat, is the highest point in Algeria, and three of the sources used here give three heights. The country's record and Wikipedia's geography article give 2,908 m, Britannica gives 2,918 m and Wikipedia's main Algeria article gives 3,003 m. The first figure is the one the record prints beside this article.

Where the land borders run

Wikipedia's geography article gives 6,734 km of land border and 2,148 km of coast. The longest land border, 1,559 km, is with Morocco; then come Mali at 1,376 km, Libya at 982 km, Tunisia at 965 km, Niger at 956 km, Mauritania at 463 km and Western Sahara at 42 km. Most of that length crosses the desert, and in the south-west the line with Morocco and Western Sahara follows a meridian, so the country has no single westernmost point.

Where the water comes from

Most rivers of the Tell Atlas are short and swing sharply between flood and trickle. The Chelif rises on the High Plateaus, crosses the Tell Atlas and reaches the sea east of Mostaganem. It is drawn off so heavily for irrigation and drinking water that its lower course stops flowing in summer.

South of the Tell Atlas the rivers are wadis that run only after rain, and much of the runoff ends in chotts, salt marshes in closed basins. In the Sahara, valleys cut during the wet periods of the Pleistocene still guide water off the Ahaggar, and some southward wadis recharge the water table beneath the desert. Oases stand where that water rises under pressure through artesian wells or springs.

Algeria had about 20 large dams at independence, and a building programme had nearly doubled that number by the late 1980s. Britannica describes the water resources as meagre and under growing pressure from the cities and industry.

What grows and lives on the land

Every plant community in Algeria has to survive a dry season, and the vegetation follows the rainfall gradient from north to south. Forest is scarce, and two sources measure it differently: Britannica gives about 2% of the land area, and Wikipedia gives around 1%, or 1,949,000 hectares in 2020, up from 1,667,000 hectares in 1990. Woodland covered about 40,000 km² in 1830 and no more than 24,000 km² in 1967, most of it brush and scrub by then.

What forest survives is in the less accessible mountains: holm oak, cork oak, Aleppo pine, juniper and small stands of Atlas cedar, which has been cut for fuel and timber since antiquity. Much of the old Tell woodland is now maquis of laurel, rosemary and thyme, or on limestone a lower garrigue of gorse, lavender and sage. Farther south the steppe is esparto, the wadis carry Barbary fig and date palm, and the desert holds tufts of drinn and cram-cram, tamarisk, acacia and jujube.

Barbary macaques live in the northern mountains, with one refuge in the Djebel Babor reserve, beside wild boar and the Barbary stag, a subspecies of red deer. Gazelles, jackals, hyenas and the fennec, the national animal, live in the desert. Barbary lions and Atlas bears are extinct. The national parks include Ahaggar, Tassili n'Ajjer, Djurdjura, El Kala, Gouraya, Taza, Belezma, Chréa, Théniet El Had and Tlemcen.

How war and policy moved the population

Rural Algeria was traditionally a country of scattered hamlets and isolated houses, with nomads in the Sahara and its fringes and compact villages at the oases and in uplands such as the Aurès and Great Kabylia. French settlers added several hundred villages of colonisation in the second half of the nineteenth century, often laid out on a grid.

The war of independence rearranged all of it. Britannica records nearly 8,000 villages and hamlets destroyed and some three million people displaced between 1954 and 1962, many into resettlement centres that became ordinary villages after the war. The agrarian reform of the 1970s built some 400 socialist villages before it was dropped in the 1980s. In the desert, drought and government policy sharply reduced nomadism, and many Tuareg now live settled around Djanet and Tamanrasset.

The upland pattern still maps onto language and community. Britannica places the Kabyle Imazighen in the mountains east of Algiers, the Shawia mainly in the Aurès, the Mozabites on the northern edge of the desert and the Tuareg in the Ahaggar, with some one-fifth of Algerians identifying as Amazigh.

The movement since then has been into the towns. Britannica puts almost three-quarters of the population in urban areas, concentrated along the coast, with Algiers, Oran, Constantine and Annaba the main cities. Algeria: economy follows the gas that pays for those cities out of the desert beneath them.