イエーメン・アラブ共和国の環境地質 ＩＩ ティハマ平原

Abstract

The Tihamah Plain is a coastal plain bordering on the Red Sea, located in the western part of Yemen Arab Republic. The length of the plain from north to south and the width from east to west are about 400 km and about 30 to 40 km respectively and the altitudes range 0 to 500 m.Most of this plain has still uncultivated like a semi-desert area because of hydrometeorologic conditions such as high temperature, small rainfall, dryness, etc., geologic conditions such as silty, sandy, sand and gravel strata, and bad conditions for water use. However, this plain has high abilities for further development.The purposes of this paper are to describe the physical environments in this Tihamah Plain and the actual circumstances of land and water uses which are strictly provided under such environments and recommendation for further development concerned with water in an arid region.This paper is summarized as follows : 1. Geology of the Tihamah Plain is mainly Alluvium composed of silt, sand, and gravel. Alluvium strata are declining westward in about 1/300 gradient. The depths of Alluvium are considered to be about 50 m in the central part of the plain and more than 100 m in the coastal part. Main aquifer in this plain exists among these Alluvium strata.2. According to tritium analyses, residence time of groundwater in this plain is found to be extremely long except limited regions such as irrigation areas and those along wadi courses. Groundwater except such areas is only recharged by a great flood which happens once for several years.3. Remote sensing is emphasized to be a convenient method to get many available informations about groundwater occurrences through observations such as soil moisture, vegetation, land use, etc..4. The fact that infiltration from wadi courses and irrigation canals occurs in large quantity suggests the possibility of artificial recharge for groundwater in this plan.The authors propose the following systematic water use plan : that wadi water must be artificially recharged into groundwater before evaporation loss when it runs down from mountainous regions to the plain and it should be withdrawn in the lower part of the plain.The following effects may be expected by this plan.i) Avoidance of invalid runoff during flood.ii) Decreasing of evaporation loss.iii) Possibility of utilizing high quality groundwater by purification mechanism of water holding layers.