Saudi Arabia's geography: a tilted plateau with no rivers

Why Saudi Arabia's area has no single figure

The southern and south-eastern borders, with Yemen, Oman and the United Arab Emirates, run for long stretches through the Rub' al Khali, and Wikipedia's country article notes that because they are not precisely marked, the exact area of the country is undefined. The Saudi government's estimate is 2,217,949 square kilometres. The United Nations Statistics Division uses 2,149,690, and other estimates quoted by Wikipedia reach 2,240,000. Britannica puts it more simply: the kingdom occupies about four-fifths of the Arabian Peninsula.

Most of the land borders were settled by treaty over the twentieth century, one neighbour at a time. The Treaty of Ta'if fixed about 320 kilometres of the Yemen line along a mountain ridge to the area of Najran in 1934, and Britannica records that the dispute over the rest was nearly resolved in 2000. Jordan and Saudi Arabia exchanged territory in 1965, Jordan gaining 19 kilometres of coast on the Gulf of Aqaba. The Qatar border was determined in 1965, with Britannica dating the end of the dispute to 2001, and the Oman border defined by an agreement of 1990. In 1975 Abu Dhabi accepted sovereignty over six villages of the al-Buraymi oasis and a share of the Zararah oil field, and Saudi Arabia gained an outlet to the Gulf through Abu Dhabi; Britannica describes the border with the United Arab Emirates as still undefined. This catalogue records those positions and takes none of its own.

Two diamond-shaped neutral zones date from the 1922 Uqair agreements. The one with Kuwait, of 5,790 square kilometres, was divided by an agreement of 1965 (1969 in Britannica's account), with each country administering half while sharing the oil of the whole. The one with Iraq, about 7,000 square kilometres, was divided by agreement in 1981, though Britannica notes that the line was never finally marked on the ground.

How the Red Sea rift tilted the peninsula

The Arabian Peninsula is a massif of stable crystalline rock. When the Red Sea opened along the Great Rift, the whole mass tilted: the western and southern edges were lifted and the land slopes away from them towards the Gulf. That single movement explains most of the map. The watershed sits only about 40 kilometres from the Red Sea in the north and about 130 kilometres from it near Yemen, and the longer, branching wadis run east.

The western edge is an escarpment that runs the length of the Red Sea coast. Below it lies the Tihamah, a coastal plain whose width averages 65 kilometres, though Britannica notes it is virtually absent in the north; Mecca and Jeddah both sit in its northern part. North of a gap near Mecca the Hejaz mountains seldom exceed 2,100 metres. South of the gap, in Asir, several peaks pass 2,400 metres and the upper slopes are terraced for farming.

The highest point is Jabal Sawda near Abha. Its height is not agreed: Britannica gives estimates from 3,133 to 3,207 metres, while Wikipedia's country article gives 3,015 metres.

East of the mountains the plateau surface is scarred by lava fields, known as harrat. Wikipedia counts more than 2,000 dormant volcanoes and puts the lava fields of the Hejaz at about 180,000 square kilometres of alkali basalt. The Khaybar field lies north of Medina.

What lies across the Najd plateau

Najd, the interior plateau, falls from about 1,370 metres in the west to about 760 metres in the east, by Britannica's figures. It is mostly rock, with lava, small sandy patches and isolated mountain groups such as the Jabal Shammar, north-west of Riyadh, where the Al Rashid ruled from the oasis of Ha'il. Dry riverbeds cross it from west to east: Wadi Hanifa, Wadi ar-Rummah, Wadi as-Surr and Wadi ad-Dawasir.

Its eastern edge is a line of long, low ridges with steep western faces, which Britannica measures at about 1,200 kilometres from north to south. The Tuwayq escarpment is the most prominent, rising about 250 metres above the plateau and reaching more than 1,100 metres south-west of Riyadh. The main oases of the region, Buraydah, Unayzah, Riyadh and al-Kharj, lie in this area. Outside them Najd is thinly settled, with salt marshes, sabkhas, scattered across it.

In the far north the land drops into the Wadi as-Sirhan, a basin up to 300 metres below the surrounding plateau and the remnant of an ancient inland sea, used for millennia by caravans between the Mediterranean and central Arabia.

How three sand seas enclose the interior

Three bodies of sand wall Najd off on its north, east and south, as the escarpment does on the west.

The Rub' al Khali, the Empty Quarter, covers most of the south. Wikipedia gives it as about 1,000 kilometres long and 500 wide, shared with Oman, the United Arab Emirates and Yemen, its surface falling from about 800 metres in the south-west to near sea level in the north-east. It is hyper-arid, with annual rain generally below 50 millimetres, and Britannica notes that a decade may pass in parts of it with no rain at all. Raised beds of marl and gypsum mark lakes that formed there 6,000 to 5,000 and 3,000 to 2,000 years ago, with fossils of hippopotamus and water buffalo.

The first documented crossings by outsiders were made by the British explorers Bertram Thomas and St John Philby in the early 1930s, and Wilfred Thesiger crossed it several times between 1946 and 1950. A road between Saudi Arabia and Oman through the sand was completed in September 2021, 580 kilometres of it on the Saudi side. At its western edge, 'Uruq Bani Ma'arid became the kingdom's first natural World Heritage site in 2023; it holds the country's only wild Arabian oryx, reintroduced between 1995 and 2013.

Where the Gulf coast and its oases begin

East of Ad Dahna, the As Summen plateau, about 120 kilometres wide, is limestone riddled with caves, the dahls, some of them mazes of passages several kilometres long; they were first systematically studied in 1981. Beyond it the land drops to a flat coastal lowland about 60 kilometres wide, gravel in the north and the sand of al-Jafurah in the south.

The coast itself is irregular and shallow, with sand, marsh and salt flat merging into the sea and reefs well offshore, and only long moles at Ras Tanura opened it to seagoing tankers. Near the border with the United Arab Emirates, the Batn al-Tarfa sabkha reaches 24 metres below sea level, the lowest ground in the country by Wikipedia's account.

Al-Hasa, inland from this coast, is a pair of neighbouring oases around the town of Hofuf, fed by artesian springs that draw on groundwater from the eastern watershed of the Tuwayq. UNESCO inscribed the Al-Ahsa Oasis on the World Heritage List in 2018 as an evolving cultural landscape. The same coast carried the damage of the 1991 Gulf War, when a spill of up to 8,000,000 barrels of oil fouled much of the Saudi shoreline around Jubail.

What lives in the deserts and along the reefs

The Gulf of Aqaba and the Red Sea give the kingdom a western coast of almost 1,800 kilometres, and Wikipedia counts about 1,300 islands in the Red Sea and the Gulf. Fringing reefs of stony coral run along some 2,000 kilometres of the Red Sea shore, and the sea holds more than 1,200 species of fish, about 10% of them found nowhere else. UNESCO lists the coral reefs of the Gulf of Aqaba and the Red Sea on Saudi Arabia's tentative list.

On land the wildlife is thinner than it was. The Arabian leopard, the Arabian wolf, the striped hyena, the sand cat and the jerboa remain. Gazelles, oryx, leopards and cheetahs were once relatively numerous, and the two main sources date their collapse differently: Wikipedia to hunting in the nineteenth century, Britannica to about 1950, when hunting from motor vehicles reduced them almost to extinction. Plants are mostly small herbs and shrubs that need little water. The date palm grows across the country, and Britannica places about a third of the palms in the Eastern Province.

How a country without rivers finds water

There are virtually no permanent surface streams, and the country's water has come from three places: rain caught in wadis and oases, groundwater and the sea. Much of the groundwater is fossil water, laid down in prehistoric times and not renewed. Until the 1980s there were lakes at Layla in al-Aflaj and deep waterholes at al-Kharj; heavy pumping for farms and towns has since emptied them.

Farming took most of it. Agriculture consumed about 90% of the kingdom's water demand in 1987, and in the early 1990s large projects drew more than 80% of their water from aquifers. Wheat grown on that water was phased out by 2016. Wikipedia's undated figures put desalinated seawater at about 50% of drinking water, non-renewable groundwater at 40% and surface water from the mountainous south-west at 10%. Less than 7% of the land is suitable for cultivation.

How hot and dry the climate runs

Britannica divides the kingdom into three climatic zones: desert almost everywhere, a strip of steppe along the western highlands, and a small area of milder, humid conditions in the highlands next to Yemen. Summers, June to August, bring shade temperatures above 38 °C in almost the whole country and up to 55 °C in the desert. Winters are cool, with average temperatures for December to February of 23 °C at Jeddah, 14 °C at Riyadh and 17 °C at Dammam. Snow falls in some winters in the mountains of Tabuk province, and the lowest temperature on record, minus 12 °C, was measured at Turaif.

Rain is scarce and irregular. Britannica gives mean annual totals of about 65 millimetres at Jeddah and about 75 at Riyadh and Dammam, while the highlands of Asir can receive more than 480 millimetres, most of it from the summer monsoon between May and October.

The World Bank's Climate Change Knowledge Portal gives a national mean of 26.04 °C and 63.06 millimetres of rain a year for 1995 to 2014. Temperatures have risen by 0.54 °C a decade since 1970, and under its high-emission scenario the median projection for 2040 to 2059 is a further 1.98 °C.

Where Saudi Arabia's people live

The census of 2022, the kingdom's fifth and the first carried out electronically, counted 32,175,224 people, with results announced on 31 May 2023. Three regions, Riyadh, Makkah and the Eastern Province, held 68% of them. The census names Riyadh, Jeddah, Makkah, Madinah and Dammam as the main cities: one in the interior, three in the Hejaz and one on the Gulf.

The pattern follows the water and the tilt of the land. The two holy cities and Jeddah lie in the western lowlands and valleys; Riyadh stands on the eastern bank of Wadi Hanifa; Dammam and the oil towns sit on the Gulf. The kingdom is divided into 13 regions, each under an emir with the rank of minister appointed by royal decree under the Law of Provinces. Most of the Rub' al Khali is waterless and uninhabited except by a few Bedouin tribes, among them the Al Murrah.