Soil erosion as a source of sediment and phosphorus in rivers and reservoirs - Watershed analyses using WaTEM/SEDEM

Spatially distributed modelling of sediment and phosphorus fluxes on a scale of thousands of square kilometers always involves a compromise between the quality of the data input and the complexity of the model that can be applied. WaTEM/SEDEM offers an approach that allows us to target on spatially focused outputs that can easily be implemented in the decision-making process for effective watershed control. The results for a study area covering the watersheds of 58 large reservoirs threatened by eutrophication within the Czech Republic are presented here as an example of the available analyses. The total area of the watersheds is 27,472 km². After building a complex river topology scheme and estimating the trap efficiencies in all reservoirs within the river networks, we are able to estimate the total transport efficiency of each river unit for any outlet point (terminal reservoir). The sources of the greatest amounts of sediment (phosphorus) can be identified on the scale of single parcels. According the model, the total soil loss in the study area is 7487 Gg year^-1 (2.73 Mg ha^-1 year^-1). The total sediment entry into the river systems in the target area is 1705 Gg year^-1 (15.2% of the total soil loss). The total deposition in the 9890 water reservoirs of various sizes in the target area is 1139 Gg year^-1. This means that the deposition in the landscape is 5.1× higher than the deposition in the reservoirs within the study area. The mean annual sediment transport by all watershed outlets is 566 Gg year^-1. The cost of dredging the sediment would be about 12.8 million EUR year^-1. There is great spatial variability in the deposition and transport processes, but it is imperative to provide strengthened soil protection directly on-site, especially in watersheds where the sediment delivery ratio is much higher than the average value. Phosphorus transported by water erosion is an important element in the balances of phosphorus sources in basins. Sewage waters usually play the predominant role in triggering the eutrophication effect, but there are also reservoirs where erosion-based phosphorus plays a major role.