Assessment of nutrient entry pathways and dominating hydrological processes in lowland catchments

International audienceThe achievement of a good water quality in all water bodies until 2015 is legally regulated since December 2000 for all European Union member states by the European Water Framework Directive (EU, 2000). The aim of this project is to detect nutrient entry pathways and to assess the dominating hydrological processes in complex mesoscale catchments. The investigated Treene catchment is located in Northern Germany as a part of a lowland area. Sandy, loamy and peat soils are characteristic for this area. Land use is dominated by agriculture and pasture. Drainage changed the natural water balance. In a nested approach we examined two catchment areas: a) Treene catchment 517 km2, b) Kielstau catchment 50 km2. The nested approach assists to improve the process understanding by using data of different scales. Therefore these catchments serve not only as an example but the results are transferable to other lowland catchment areas. In a first step the river basin scale model SWAT (Soil and Water Assessment Tool, Arnold et al., 1998) was used successfully to model the water balance. Furthermore the water quality was analysed to distinguish the impact of point and diffuse sources. The results show that the tributaries in the Kielstau catchment contribute high amounts of nutrients, mainly nitrate and ammonium. For the parameters nitrate, ammonium and phosphorus it was observed as a tendency that the annual loads were increasing along the river profile of the Kielstau

Land use change impacts on floods at the catchment scale: Challenges and opportunities for future research

Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology and geomorphology

A method to assess instream water quality – the role of nitrogen entries in a North German rural lowland catchment

The objective of this study was the detection and assessment of nutrient entry pathways in a complex mesoscale lowland catchment. The central aim was the development of a transferable method for an efficient estimation of instream water quality in rural environments. The investigated Kielstau catchment with a size of approximately 50 km<sup>2</sup> is located in the North German lowlands. Land use is dominated by agriculture as well as pasture and is characterised by extensive artificial drainage systems. Additionally wastewater treatment plants influence the river water quality. <br><br> Six measuring campaigns were carried out over the period of one year. Each time, water quality was assessed at 16 locations along a 12 km longitudinal transect of the stream to consider the influence of each tributary or inflow. The results showed that the applied measuring method was useful to assess nutrient entry pathways into surface waters from diffuse and point sources. Some tributaries increased the main stream NO<sub>3</sub>-N concentrations especially in autumn. Using a German classification system (LAWA, 1998), the NO<sub>3</sub>-N results can mostly be assigned to water quality class III (heavily contaminated). Water balance and nitrate loads were modelled with the river basin scale model SWAT (Soil and Water Assessment Tool, Arnold et al., 1998). The measured and modelled discharges showed a good correlation. While nitrate load range and dynamics are well represented during the summer periods, they show a poor correlation during the winter period

<i>Preface</i> <br>Transdisciplinary concepts and modelling strategies for the assessment of complex environmental systems <br> <i>Proceedings of the 12th Workshop on Large-scale Hydrological Modelling</i>

No abstract available

Modelling point and diffuse source pollution of nitrate in a rural lowland catchment using the SWAT model

The assessments of potential environmental impacts of point and diffuse source pollution at regional scales are necessary to achieve the sustainable development of natural resources such as land and water. Nutrient related diffuse source pollutant inputs can enhance crop growth and improve soil eutrophication. However, excessive nutrient input can result in the impairment of water quality. The objectives of this study were to evaluate the long-term impact of point and diffuse source pollution on nitrate load in a lowland catchment using the ecohydrological model SWAT (Soil and Water Assessment Tool) and to determine the contribution of point and diffuse sources to nitrate load in the entire catchment. The study area Kielstau catchment has a size of approximately 50km2 and is located in the North German lowlands. The water quality is not only influenced by the predominating agricultural land use in the catchment as cropland and pasture, but also by six municipal wastewater treatment plants. Diffuse entries as well as punctual entries from the wastewater treatment plants are implemented in the model set-up. The model was first calibrated and then validated in a daily time step. The values of the Nash-Sutcliffe efficiency for the simulations of flow and nitrate load range from 0.68 to 0.75 for the calibration period and from 0.76 to 0.78 for the validation period. These statistical results revealed that the SWAT model performed satisfactorily in simulating daily flow and nitrate load in lowland catchment of Northern Germany. The results showed that diffuse sources are the main contributor to nitrate load in the entire catchment accounting for about 95% of the total nitrate load, while only 5% results from point sources. The model results also indicated that agriculture is the dominant contributor of diffuse sources and the percentage of agricultural land area is considerably positively correlated to nitrate load at the different subbasins. The area covered by forest is found to be negatively correlated with nitrate load.Lowland hydrology Point and diffuse sources Nitrate load SWAT

Process verification of a hydrological model using a temporal parameter sensitivity analysis

To ensure reliable results of hydrological models, it is essential that the models reproduce the hydrological process dynamics adequately. Information about simulated process dynamics is provided by looking at the temporal sensitivities of the corresponding model parameters. For this, the temporal dynamics of parameter sensitivity are analysed to identify the simulated hydrological processes. Based on these analyses it can be verified if the simulated hydrological processes match the observed processes of the real world. <br><br> We present a framework that makes use of processes observed in a study catchment to verify simulated hydrological processes. Temporal dynamics of parameter sensitivity of a hydrological model are interpreted to simulated hydrological processes and compared with observed hydrological processes of the study catchment. The results of the analysis show the appropriate simulation of all relevant hydrological processes in relation to processes observed in the catchment. Thus, we conclude that temporal dynamics of parameter sensitivity are helpful for verifying simulated processes of hydrological models