207 
detailed resource maps. 
Physical Setting 
The area can be divided into two geological regions. 
1. The Western Tihama is a raised marine surface of Plio-Pleistocene 
age typically 3-15 kms wide, with reefal limestones and marine 
sands and clays. 
2. The Eastern Tihama is a heavily faulted and deeply dissected zone 
of pre-Cambrian rocks (igneous and sedementary) in some areas 
overlain by Tertiary strata and also by basalt lavas - the Harrats. 
The area's tectonic history (see Skipweth, 1973) and a previously wetter 
climate have contributed to its geomorphological character. 
The coastal plain is severely affected by climatic and edaphic 
limitations. The average annual temperature is 28 C, and rainfall is 
too low (100-150 mm/year) for rainfed agriculture; occasional floods, 
however, are a serious hazard. 
The soils derived from loose sands and gravels of mainly fluvial origin 
are generally poor with low organic content. Coastal soils are 
frequently salt affected (surface crusts or pans) whereas the deeper 
wadi soils are often rendered infertile through secondary salinisation 
or alkalisation. 
The permanent vegetation is sparse and consists of low trees and shrubs 
with ephemeral ground cover following rains. 
Land use is a mixture of extensive nomadic and sedentarised stock raising 
with fallow-irrigated agriculture in wadis. 
METHODOLOGY 
Appropriate Landsat scenes of the area were obtained from EROS Data 
Centre. These were unenhanced standard film products for two seasons to 
allow temporal comparisons. The registrations listed in Table 1 were 
used. 
Band 
Scale 
Date 
7 
1:250,000 
19.2.73 
7 
1:250,000 
7.6.73 
4,5,7 (FCC) 
1:250,000 
19.2.73 
4,5,7 (FCC) 
1:250,000 
7.6.73 
R.B.V.Subscene B/D 
1:250,000 
I8.7.8O 
Table 1. Standard EROS Landsat scenes. 
Also available were four examples of different enhancement techniques 
applied to Landsat data of the area. These are detailed in Table 2. 
* 
Made available for reference by USGS Remote Sensing Centre, Jeddah.