Central Region/Najd Area

The central region or the ‘Najd’ is characterized by hillocks, valleys (wadis), meadows (‘raudhas’), salt pans(‘sabkhas’), gravel (‘reg’), rocky (‘hammada’) and sandy deserts. The entire area is a large plateau which lies within a low precipitation area, often receive less than 100mm rain. An array of hillocks and notably the arch-shaped Tuwayq Mountain range (c. 1000m) that stretches about 800 km along the central region is the main landmark of this Province. Several wadis also present in the region. Among these Wadi Hanifa is the prominent one running for a length of about 100 km from north to south direction, passing along the western sides of Riyadh City. The area is devoid of any perennial streams, although there are a few underground aquifiers exist in the southern part of Riyadh.

Huge areas of sand dunes (mostly the extensions of Dahna sands) are present in the northern, eastern and southern borders. Isolated longitudinal dunes of 100km or more in length occur along the middle and western edges. The entire area fall within the Saharo-Arabian (Saharo-Sindian in a broad sense) phytochorion and therefore the vegetation is mainly dominated by annuals and drought resistant xeromorphs. Vegetation of rocky deserts are generally sparse due to the impermeability of water and its impenetrable surface to plant roots.  Plant growth and seed deposits are largely confined to the periphery of the gravel areas, where fine soil accumulate over a considerable period and generate a habitat conducive for certain shallow rooted plants. Plateau and other deserts are also less favourable for plant growth due to exposure to wind and other evaporating factors. Vegetation in such areas are restricted to notches and crevices.  Vegetation is virtually nil on hillocks except for a few grass species and some perennial plants. However the west facing slopes hold significant number of annuals and perennials. Wadis on the other hand, which receive additional supply of runoff water along with fine soil from the elevated places, are the real seat of the main vegetation. Acacias are the dominant group in such wadis. In depressions where the soil is composed of loam and silt, the vegetation is dominated by annuals. There are several raudhas in the region, most of which are located near sand dunes and are influenced by the vegetations of both sand-dunes and rocky habitats. Tree species are rare in the central region. Out of 70 tree species present in Saudi Arabia, the central region contains only Acacia gerrardii and A. raddiana, in wadis and a few stands of Ziziphus spina-christi, Maerua crassifolia, Moringa peregrina, Capparis decidua and Ficus palmata in open plains or in gullies or ravines of hilly areas.