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Before the invention of modern, large-scale engineering projects, terrace systems were rarely built in single phases of construction, but instead developed gradually, and could even be said to have evolved. Understanding this process of landscape change is therefore important in order to fully appreciate how terrace systems were built and functioned, and is also pivotal to understanding how the communities that farmed these systems responded to changes; whether these are changes to the landscape brought about by the farming practices themselves, or changes to social, economic or climatic conditions. Combining archaeological stratigraphy, soil micromorphology and geochemistry, this paper presents a case-study from the historic and extensive terraced landscape at Konso, southwest Ethiopia, and demonstrates - in one important river valley at least - that the original topsoil and much of the subsoil was lost prior to the construction of hillside terraces. Moreover, the study shows that alluvial sediment traps that were built adjacent to rivers relied on widespread hillside soil erosion for their construction, and strongly suggests that these irrigated riverside fields were formerly a higher economic priority than the hillside terraces themselves; a possibility that was not recognised by numerous observational studies of farming in this landscape. Research that takes into account how terrace systems change through time can thus provide important details of whether the function of the system has changed, and can help assess how the legacies of former practices impact current or future cultivation.

Agricultural landscapes are human-manipulated landscapes, most obviously in areas modified by terracing and/or irrigation. Examples from temperate, arid, and desert environments worldwide have attracted the attention of many disciplines, from archaeologists, palaeoecologists, and geomorphologists researching landscape histories to economists, agronomists, ecologists, and development planners studying sustainable resource management. This article combines these interdisciplinary interests by exploring the role archaeology can play in assessing sustainability. Our case study is Engaruka, Tanzania, archaeologically famous as the largest abandoned irrigated and terraced landscape in East Africa. The site has been cited as an example of economic and/or ecological collapse, and it has long been assumed to have been irrigated out of necessity because agriculture was presumed to be nearly impossible without irrigation in what is now a semiarid environment. Geoarchaeological research refutes this assumption, however, demonstrating that parts of the site flooded with sufficient regularity to allow the construction of more than 1000 ha of alluvial sediment traps, in places greater than 2 m deep. Soil micromorphology and geochemistry also record changes in irrigation, with some fields inundated to create paddylike soils. Geoarchaeological techniques can be applied to both extant and abandoned agricultural systems, thereby contributing to an understanding of their history, function, and sustainability.

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