Physiographical and Hydrological Analysis... 151 JKAU: Earth Sci., vol. 13, pp. 151-177 (1422 A.H. / 2001 A.D.) 151 Physiographical and Hydrological Analysis of Yalamlam Basin, Makkah Al-Mukarramah Area ALI M. SUBYANI and TARIQ H. BAYUMI Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia Received: 9/6/2002 Revised: 25/11/2002 Accepted: 31/12/2002 ABSTRACT. Wadi Yalamlam is one of the Arabian Shield Wadis, locat- ed 70 km south of Makkah Al-Mukarramah city. The main rock units consist of Precambrian crystalline, meta−volcanic and metasedi- mentary rocks, with local Tertiary and Quaternary basalt flows ˘The geographic setting of Wadi Yalamlam basin can be divided into three main zones: the Red Sea coastal plain (Tihamah), the hills, and the Scarp-Hijaz Mountains. The catchment area starts from the Hijaz Es- carpment (Ashafa area) in the east to the Red Sea at the west ˘Morpho- metric analysis of the basin shows that it drains a large catchment area of about 1600 km 2 , very elongated, high relief, and of a moderate drainage density˘ The mean annual and seasonal rainfalls of hydrological stations around the study area have been analyzed ˘Seasonal rainfall occurs in the mountains area, but varies from year to year. It often occurs as thunderstorms of high intensity during a local storm followed by dry periods. The average annual rainfall exceeds 200 mm in the moun- tains and decreases to 80 mm towards the Red Sea coast. In addition, Wadi Yalamlam has surface running water in the upstream most of the year. Runoff hydrographs of 0.5 hour duration were constructed, based on Snyder method, for three sub-basins of wadi Yalamlam. Introduction Wadi Yalamlam, called Wadi Sa’diyah in some reports, is one of the important Wadis in Makkah Al-Mukarramah area and lies about 125 km south east of Jed- dah, and 70 km south of Makkah Al-Mukarramah city, Fig. 1. It flows into the