A First Step Towards the Use of Radionuclides in Assessing the Effectiveness of Terraced Agricultural Land to Control Erosion and Enhance its Productivity in the Yemen Highlands

The Global Assessment of Human Induced Soil Degradation (GLASOD) survey carried out during the 1980's by the United Nations Environment Programme (UNEP) and the International Soil Reference and Information Centre (ISRIC) established that over 45% of the Yemeni territory is characterised by severe to very severe human induced land degradation (FAO 2005). The highlands in Yemen are particularly threatened by land degradation especially through soil erosion. Soil terraces in the highlands are of ancient origin, and these arable lands have been cultivated for the last 5000 years. So far, the oldest AMS-14C dated organic material (i.e. humic acids) from a soil of such old terraces (i.e. Anthrosol) in the Wadi Al-Jidar area has an age of 4500-4700 cal a BP (ERL 11865). Due to high precipitation (∼ 700 mm a-1) coupled with high rainfall erosivity, as well as the breakdown of many of the soil conservation measures, fertile soil is being lost through erosion (Pietsch, 2001). As effective erosion control starts with the knowledge of soil erosion rates and mechanisms, the authors proposed to use nuclear techniques (i.e. the 137Cs technique) to in-vestigate soil removal by water erosion processes under the specific agro-ecological conditions of Yemen. Measurements of NOR (Naturally Occurring Radioisotopes) and FRNs (Fallout RadioNuclides) in agricultural Yemeni soils are, however, scarce, therefore the initial fallout of these soil tracers was determined. The two major objectives of this study were to evaluate the external gamma dose rate through radioisotopic background information based on NOR (i.e. 40K, 226Ra, 232Th) soil content in the upper terrace soil, which was not used for agriculture, and to assess the initial FRN (i.e. 137Cs) at the same location for the preparation of a future study on the magnitude of soil erosion affecting the terraced Yemen Highlands