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.

Risk to residents, infrastructure, and water bodies from flash floods and sediment transport.

Intense rainfall-runoff events and subsequent soil erosion can cause serious damage to the infrastructure in residential areas in Europe countries and all over the world. In the Czech Republic, the Ministry of the Interior has supported an analysis dealing with the risks to residents, infrastructure, and water bodies from flash floods and sediment transport. A total of more than 150,000 risk points were identified by GIS morphology and land-use analysis. The threat, the vulnerability, and the resulting risk category were determined for each of these points. The WaTEM/SEDEM model was used to assess the threat with 10-m data resolution. The summarized vulnerability of real objects on individual runoff trajectories was combined with the threat of sediment transport, resulting in the overall risk represented by a 5-degree scale, from lowest (1) to highest (5). The output of the project lies stored in the WEB application. Nineteen percent of the sites in the Czech Republic, i.e., more than 23,000 sites, have been assigned to categories 4 and 5, with a high level of risk. Thirty-four percent of cadastral units are classified as the high risky (4416 cadasters, with a total area 24,707 km2). Approximately 30% of the population of the Czech Republic lives in high-risk cadastral areas. Four scenarios of protection were modeled. To reduce the high-risk and very high-risk sites (categories 4 and 5), the most effective solution is the implementation of technical measures or conversion to grassland within the contributing watersheds. This could reduce the number of high-risk sites from 23,400 to 3700.Methods of sediment transport modeling and risk evaluation, based on presented USLE input data and documented WaTEM/SEDEM model, can be used worldwide. Especially in post-soviet union countries with shared arable land development and erosion consequences.