Suspended sediment yield in Italian rivers

Abstract

The knowledge about river sediment yield in Italy is rather scarce and mainly based on small reservoir sedimentation data. This study used the suspended sediment yield field data measured by the national and regional hydrological services. The data of nine rivers and 11 selected flow gauges were used to investigate the control of basic parameters such as catchment area, annual precipitation and a characteristic discharge on sediment yield. The average sediment yield of Italy is close to the world mean but the relationship between catchment area and sediment yield is not significant, though a general decreasing trend with increasing catchment area can be distinguished. The variation of sediment yield with annual precipitation is poorly significant, although a general increasing trend with increasing catchment area can be observed. Sediment yield varies widely. The largest range of variability occurs for annual precipitation between 700 and 800 mm. The data indicate an average suspended sediment yield of 220 t km−2yr−1 and an upper limit of 3000 t km−2yr−1 which is observed in all the study rivers, no matter the catchment size which ranges from 580 to 70091 km2. Such a high value seems to be the maximum sediment yield possible in Italy given the local climatic, tectonic and land use characteristics. The discharge exceeded 10 days a year (2.7% of time) was found to exert a substantial control on sediment yield. Groups of rivers with similar catchment area show significant correlations and almost parallel trendlines. The occurrence of data gaps and multiple years interruptions allowed the investigation on the decadal variation of sediment yield of only very few rivers. The reduction of sediment yield is remarkable. The decrease in annual precipitation and the unprecedented expansion of forests are considered as the main factors causing such marked decrease of sediment yield in Italy.