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Assessment and prediction of sediment dynamics within olive orchards

Final Activity Report Summary - OLSEDYN (Assessment and prediction of sediment dynamics within olive orchards)

Water erosion has been identified as one of the major environmental threats to the sustainability of Mediterranean agricultural areas (Kirkby, 2004). In particular, olive growing areas are often pinpointed as one of the most degrading land uses. Because of water competition with the trees, the soil surface of orchards is often kept bare, which is known to promote soil erosion. At present, however, considerable uncertainty exists to the exact contribution of olive orchards to the soil erosion problem. This project aimed at quantifying, understanding and predicting within-field sediment dynamics in olive plantations. Hereby, the attention is not merely focused on what happens in small areas, but rather at integrating all involved processes at the catchment scale. This implies that different erosion and deposition processes have been studied for it has been often observed that an important fraction of the eroded soil is re-deposited before it reaches the streams.

The main research result is that soil erosion processes in olive orchards are now much better understood. All measurements show that soil erosion in olive orchards is undoubtedly a serious environmental threat. Long-term historical average erosion rates that were derived from comparing ancient surfaces with the actual ones, indicate that each year 0.8 cm of fertile soil is lost. Given the shallow depth (<1 m) of many Mediterranean soils, in places, the fertile soil cover has already been lost completely. Measures of actual processes indicate the same dramatic soil losses.

During some extreme events, between 1.5 and 4.8 cm of soil loss was measured. These dynamics show how difficult prediction is. Nevertheless, using some novel modelling techniques, such as cellular automata-based soil erosion models, relatively good predictions were obtained over the longer term. Also, some general trends in sediment transport equations have been tested and it was found that a single equation describes well the behaviour of small rills to large gullies under field conditions. This will contribute to improved soil erosion models in the future.