For additional info:
http://person.au.dk/en/hath@dmu.dk

Selected publications:
Andersen, H. E., Heckrath, G., and Thodsen, H. (2008) Mapping areas at risk of diffuse phosphorus losses to water: a pilot study of Lake Haderslev Dam, Denmark. Water Science and Technology, 58 (11): 2173-2178

Haderslev Dam is a 272 ha lake in southern Denmark with a high recreational value. For decades the lake has been severely eutrophicated due to excessive phosphorus loading. Major point sources were cut off in the early 1990s and an upstream wetland was recreated. However, the ecological quality remains unsatisfactory. In this study we estimate the importance of agriculture on diffuse phosphorus (P) input to the lake by modelling combined with independent estimates for contributions from scattered dwellings not connected to a sewer and from background losses. We apply a newly developed Danish P index to the lake catchment for mapping of risk areas for diffuse phosphorus losses. For risk areas we suggest mitigation measures and estimate the effect of the mitigation measures on the total P loading of the lake as well as the associated costs.

Thodsen H. , Hasholt B. and Kjærsgaard J. H. (2007) The influence of climate change on suspended sediment transport in Danish rivers. Hydrological Processes, 22: 764-774

The HIRHAM regional climate model suggests an increase in temperature in Denmark of about 3 °C and an increase in mean annual precipitation of 6—7%, with a larger increase during winter and a decrease during summer between a control period 1961—1990 and scenario period 2071—2100. This change of climate will affect the suspended sediment transport in rivers, directly through erosion processes and increased river discharges and indirectly through changes in land use and land cover. Climate-change-induced changes in suspended sediment transport are modelled for five scenarios on the basis of modelled changes in land use/land cover for two Danish river catchments: the alluvial River Ansager and the non-alluvial River Odense. Mean annual suspended sediment transport is modelled to increase by 17% in the alluvial river and by 27% in the non-alluvial for steady-state scenarios. Increases by about 9% in the alluvial river and 24% in the non-alluvial river were determined for scenarios incorporating a prolonged growing season for catchment vegetation. Shortening of the growing season is found to have little influence on mean annual sediment transport. Mean monthly changes in suspended sediment transport between -26% and +68% are found for comparable suspended sediment transport scenarios between the control and the scenario periods. The suspended sediment transport increases during winter months as a result of the increase in river discharge caused by the increase in precipitation, and decreases during summer and early autumn months.

Thodsen H. (2007) The influence of climate change on stream flow in Danish rivers. Journal of Hydrology 333: 226— 238

The influence of climate change on river discharges in five major Danish rivers divided into 29 sub-catchments is investigated for the future period of 2071—2100. Climate changes are modelled by the HIRHAM regional climate model on the basis of the IPCC A2 scenario. A hydrological model (NAM) is used to convert precipitation to river discharges. Difficulties are found in the direct use of climate model generated precipitation and potential evapotranspiration (reference evaporation) because of too many rainy days, deviations from mean annual values of precipitation and potential evapotranspiration from observed values, and poor agreement on seasonality. Therefore climate model generated data is corrected to match observed mean annual values and the mean monthly distribution. Mean annual precipitation is found to increase 7%, potential evapotranspiration to increase 3% and river discharges to increase 12% on average, between a control period (1961—1990) and the future period. Because of increased precipitation from October to March and reduced precipitation from July to September the monthly river discharges are found to increase from December to August and decrease in September and October. Extreme values of precipitation and river discharge are examined and the level of the highest precipitation and the highest river discharge events are estimated to increase. The precipitation amount exceeded 0.1% of all days increases by an average of 7%, the river discharge exceeded 0.1% of all days increases approximately 15%. The 100-year flood is modelled to increase 11% on average.