Changing sediment dynamics due to natural reforestation in the Dragonja catchment, S.W. Slovenia

Under the influence of socio¿economic changes in many regions in Europe, a trend of decreasing agricultural activity has been observed since the Second World War. The resulting reforestation profoundly changes water and sediment supply to river channels, deposition rates on the floodplains and erosion rates on the hillslopes. We studied these changes in the 91 km2 Dragonja catchment in southwestern Slovenia. With the spatially distributed erosion and sediment delivery model WATEM/SEDEM, the hillslope sediment delivery to the river channel was calculated on the basis of parameters (soil and precipitation parameters, a DEM and land use) measured in the field and laboratory in 2002 and land use maps based on aerial photographs from 1954, 1975, 1985 and 1994. For two independent calibrations WATEM/SEDEM modelled a sharp decline of 69% in total hillslope sediment delivery from 1954 to 2002. As the sub-catchments Rokava and Upper-Dragonja did not reforest in the same way, the sediment yield response is different as well. Separate calculations show the same reduction (45%) in sediment yield from 1954 to 1975. After 1975 the sediment yield was stable in the Rokava sub-catchment. In the Upper-Dragonja the trend continued, to a total reduction of 76% of sediment outflow since 1954. The sources of fine sediment were determined by analysing the hysteresis of the discharge waves, and the suspended sediment texture. The sediment that leaves the catchment originates from three sources: hillslopes, erosional bedrock banks and sedimentary riverbanks. The analysis of the suspended sediment texture suggests that during a discharge wave the suspended sediment originates predominantly from the hillslopes. During low stage the sparse sediment in the water column largely originates from large bedrock banks. The sedimentary riverbanks are not an important source of suspended sedimen