Hydraulic Modelling of the Irrigation System of Medieval Basrah

Abstract

Ancient Basrah has been the center of anthropological research studies due to its significance in the development of the culture, politics and economics of the region at the time. Agriculture was one of the main income revenues for ancient Basrah even though it received only an average of 60mm of rainfall per year, indicating a large reliance on irrigation. Therefore, this study focuses on how the landscape would have offered options for irrigated farming. The unique element of this irrigation system is the existence of many large ridges, however their significance is not yet understood.
Within this study a HEC-RAS hydraulic model was set up to explore the hypothesis of the ridges themselves having served the function of elevated agricultural fields. Therefore, multiple scenarios were constructed and sensitivity analysis of the boundary conditions performed, to explore how easily the landscape within the ridge area could be flooded and transformed into a marshland. The results showed that although there were always some dry spots between the fields, most of it was flooded all day and all year round, with a mean water depth ranging from 0.3m to 0.6m. The maximum water depth stayed below 5m in all model variations (except for one sensitivity analysis run), therefore the ridges (which have been hypothesized to originally have been 5 m tall) would have stayed dry.
The main limitations were set by the lack of data available. Except for the elevation data, very little data was available in general and additionally no data from before the 1940’s was available. Therefore, this study should be viewed as an exploration of how the system may have functioned given certain hypothesized boundary conditions. Further research for confirmation of the utilized boundary conditions is necessary to confirm the study results, for example decreasing the river cross-sections by 25% lead to an increase in mean depth of up to 26%.