Human induced soil erosion and the implications on crop yield in a small mountainous Mediterranean catchment (SW-Turkey)

Many hillslopes in the limestone dominated Taurus Mountain Range (SW Turkey) are characterized by severely depleted soils, while a significant amount of sediment is being stored in the valley bottoms. The same holds true for the 11.4 km² endorheic Gravgaz basin in the vicinity of the Hellenistic-Roman city of Sagalassos in SW Turkey. Previous palaeo-environmental research in this basin already yielded both detailed sedimentological and palynological information on sedimentation in the valley bottom and vegetation changes that took place during the last several millennia. An adapted WATEM/SEDEM geomorphic model version was used to simulate the impact of the observed changes in vegetation cover, climate and hillslope soil properties on hillslope soil erosion and valley bottom sediment deposition over the last 4000 years. The calibrated WATEM/SEDEM model is able to reconstruct the temporal changes of sedimentation in the valley bottom reasonably well. To simulate the impact of historic soil erosion on crop productivity a simple crop yield model was coupled to the reconstructed soil thickness maps. The main outcomes are that soil erosion was mainly driven by deforestation and hence anthropogenic activity, but the resulting soil erosion did not cause a complete collapse of crop yields. On the contrary, we were able to quantify that the sediment accumulation in the lower lying valley bottoms compensated at least a part of the loss in crop yield from the hillslopes: potential crop yield value changed from 2.80 t ha^{− 1} a^{− 1} before widespread deforestation to 2.58 t ha^{− 1} a^{− 1} during Roman-Imperial times and 2.19 t ha^{− 1} a^{− 1} at present. These model approaches are tools that allow us to quantify human impact in the past, going beyond traditional qualitative descriptions, which will ultimately lead to a better understanding of human-environment interactions in the past.