Taming the torrents: The hydrological impacts of ancient terracing practices in Jordan

Abstract

Extensive terracing was an important component of the water and agricultural management regime of Nabatean Petra. The exact function(s) of these terraces, qualitatively and quantitatively, is herein investigated. Field studies and computer modeling for two sites within the drainage basin of Petra show that these terraces were highly effective at reducing surface runoff. Different design features seem to have been adapted in order to achieve different objectives at varying sites. For example, the Beidha site in the lower reaches of the catchment area falls within the higher relief Ordovician Disi Sandstone formation area. Here, the terrace system was designed to slow water flow and trap sediments. The terracing greatly reduced surface water flow through the entrapment of sandy loam sediments behind it. On the other hand, the Baqa’a site falls in the upper reaches of the drainage basin within the Upper Cretaceous Amman Silicified Limestone formation. The effect of the terrace system here on runoff is significant, but less than that seen at Beidha. This system was more suited for soil conservation and agriculture, as the soil here in its natural state already had a high infiltration rate. Neglect and abandonment of these terraces at both sites have led to increased runoff, as evidenced by the development of gullies and by periodic flooding downstream in the core of the ancient city. Based on the six modeled cases, terraces can minimize the surface flow by an average of 28%, with values as high as 90% when 2010 precipitation data were used. Conducting an intensive survey within the catchment area surrounding the ancient city of Petra, mapping the ancient terraces, and reviving their function may produce significant results in flood mitigation and control there.