Construction and deflation of irrigation soils from the Pharaonic to the Roman period at Amheida (Trimithis), Da

Most of the environmental studies performed in the Western Desert of Egypt have dealt with climate change since the Late Pleistocene, notably with the humid Holocene (10000 to 4500 cal. BC) and following desertification. In the last dry period of the Holocene, human societies had to cope with changing environments and they selected refuges where water was permanently available. Some of the archaeological investigations in the Western Desert have studied the available water resources and the role of irrigation in the building of soils. In some cases, they have explained the abandonment of sites by the progressive depletion of water resources. This paper focuses on the site of Amheida (the Roman Trimithis) located in the Dakhla depression in the Western Desert of Egypt. It is an elongated area located south of a limestone plateau, where spring mounds provided abundant water during the period of occupation, which lasted from the Old Kingdom (2700 cal. BC) to the Late Roman period (end of the 41th century AD). Trimithis was abandoned after a long period of prosperity. The reasons for its abandonment, as well as that of other settlements in the oasis in the same period, have not been clarified yet. Geoarchaeological surveys carried out in 2011 and 2013 point to several phases including 1) the use of a spring mound during part of the Pharaonic period (2700 to 1069 cal. BC) and the formation of irrigation soils in the vicinity of spring mounds from the Late Pharaonic times (712 to 332 BC) to the beginning of the Roman period (30 BC); 2) the degradation of these soils by wind deflation in the irrigated perimeter during the early Roman period; 3) a recovery until the 3rd century AD, made possible by the digging of wells, before; 4) a severe phase of deflation and sand drifting which destroyed the irrigation soils and forced farmers to till the poorly irrigated bedrock.The Trimithis case exemplifies the way former societies could cope with harsh constraints (aridity, drifting sand and wind erosion) as long as water was plentiful. Water scarcity due to the progressive depletion of groundwater resources was probably compensated by an increase in human labour until thresholds were crossed, recovery being impossible due to water scarcity. A model summarizes the main findings of this study.