There are a few lakes in the country, but no permanent rivers. However, wadis (The channel of a watercourse that is dry except during periods of rainfall.) are very numerous. Saudi Arabia's geography is dominated by the Arabian Desert, associated semi-desert and shrubland and several mountain ranges and highlands. It is, in fact, a number of linked deserts and includes the 647,500 km2 (250,001 sq mi) Rub' al Khali ("Empty Quarter") in the southeastern part of the country, the world's largest contiguous sand desert. The fertile areas are to be found in the alluvial deposits in wadis, basins, and oases.

Saudi Arabia gets most of their drinking water supply from desalinated water from the sea.

The CIA World Factbook estimates land area at 2,149,690 km2 (830,000 sq mi) and lists Saudi Arabia as the world's 13th largest state.

Except for the southwestern province of Asir, Saudi Arabia has a desert climate with very high day-time temperatures and a sharp temperature drop at night. Average summer temperatures are around 113 °F (45 °C), but can be as high as 129 °F (54 °C). In the winter the temperature rarely drops below 32 °F (0 °C). In the spring and autumn the heat is temperate, temperatures average around 84 °F (29 °C). Annual rainfall is extremely low.

More Info: en.wikipedia.org