Soil erosion control on sloping olive fields in northwest Syria

Land degradation caused by soil erosion has been an environmental problem for many regions around the world for the past millennia. Dryland ecosystems are specifically prone to land degradation. The dryland areas are found in all continents, including the Mediterranean region, and are inhabited by almost 40% of the world’s population. One of the countries were the process of land degradation in olive orchards is highly relevant is Syria, since it is already affected for large areas by soil degradation. One of the most important crops for income in Syria are olives. In the last decade, the number of olive trees in Syria has increased by 50%, now reaching 554.000 hectares. The problem with the recent expansion of olive trees in Syria, is that it expands into marginal and degraded lands. This study therefore focuses on the quantification of the amount of hill slope erosion for an olive orchard with soil conservation structures in northwest Syria. The selected village already had been experimenting SWC techniques before and the selected fields contained semi-circular terraces around trees. Three olive orchards in northwest Syria, with a mean area of 0.42 ha, were used for Assessment of Current Erosion Damage (ACED) measurements. In one selected field, three 2m Gerlach troughs and 8 small collectors were used for the measurement of surface runoff and sediment concentration. The annual runoff and erosion values were modeled using the a spatially adjusted revised Morgan-Morgan-Finney (RMMF) model in PCRaster. Based on the results of the different measurement techniques compared with the literature results, it has been concluded that the small collectors gave a better indication of the surface runoff and soil erosion values than the much lower Gerlach troughs measurements, due to the influence of a scale effect and the spatial heterogenity of the plots. The results from small collector measurements led to an annual expected runoff of approximately 879 m3/ha. The corresponding runoff coefficients were approximately 15.3 %. The soil loss rates led to an annual expected soil loss of 5.9 ton/ha, based on small collector measurements versus an expected 9 ton/ha for the results based on the ACED measurements. Regarding the soil conservation structures, the ACED measurements indicate a strong decrease of soil erosion due to rill forming in fields with semi-circular terraces. However, the specific extent of the erosion control by the SWC remains under discussion. Correlations among rainfall, runoff and sediment data further showed that the rainfall intensity is the driving factor influencing the runoff and sediment load, indicating a common process of surface runoff and soil loss generation. Unexpected runoff results were sometimes explained by the Antecedent Soil Moisture (AMC). The model outcomes showed that the RMMF model is very well suited for annual surface runoff and soil erosion prediction on the field scale, predicting annual values of 826 m3/ha for surface runoff and 8 ton/ha for soil loss, which are close to the measured values. Inner-field erosion pattern prediction was more difficult, leading to a slight to fair map overlap between the ACED erosion maps and the model outcomes based on the weighted kappa coefficient. This difference is expected to be the result of the dependence of the model on factors such as the DEM resolution, together with high spatial variability of field characteristics