Lars Troldborg

Methodology for construction, calibration and validation of a national hydrological model for Denmark

An integrated groundwater/surface water hydrological model with a 1 km2 grid has been constructed for Denmark covering 43,000 km2. The model is composed of a relatively simple root zone component for estimating the net precipitation, a comprehensive three-dimensional groundwater component for estimating recharge to and hydraulic heads in different geological layers, and a river component for streamflow routing and calculating stream–aquifer interaction. The model was constructed on the basis of the MIKE SHE code and by utilising comprehensive national databases on geology, soil, topography, river systems, climate and hydrology. The present paper describes the modelling process for the 7330 km2 island of Sjaelland with emphasis on the problems experienced in combining the classical paradigms of groundwater modelling, such as inverse modelling of steady-state conditions, and catchment modelling, focussing on dynamic conditions and discharge simulation. Three model versions with different assumptions on input data and parameter values were required until the performance of the final, according to pre-defined accuracy criteria, model was evaluated as being satisfactory. The paper highlights the methodological issues related to establishment of performance criteria, parameterisation and assessment of parameter values from field data, calibration and validation test schemes. Most of the parameter values were assessed directly from field data, while about 10 ‘free’ parameters were subject to calibration using a combination of inverse steady-state groundwater modelling and manual trial-and-error dynamic groundwater/surface water modelling. Emphasising the importance of tests against independent data, the validation schemes included combinations of split-sample tests (another period) and proxy-basin tests (another area).

Stakeholder driven update and improvement of a national water resources model

It is generally acknowledged that water management must be based on an integrated approach, considering the entire freshwater cycle. This has in particularly been endorsed in Europe by the European Water Framework Directive (WFD) imposing integrated management considering all waters. Although not prescribed by the WFD, integrated hydrological modelling may be necessary to support the management according to the directive as also suggested by several research projects initiated by the EU commission. To ensure a coherent and consistent management across various institutions and authorities, having different responsibilities and operating at various scales, a common tool integrating all relevant knowledge and data is imperative. By the end of 2003, a numerical national water resources model was constructed for Denmark, which has been applied in several national assessments. At the regional level there has, however, been some reluctance to use the model, primarily because the model did not contain the most recent data and understanding obtained from detailed local studies. The model has therefore been subject to a comprehensive update focussing on utilising the system understanding from the local studies. This process was largely stakeholder driven by involvement of predominantly the technical staff at the regional water authorities. Local knowledge is continuously improved urging the model update to be an on-going process. Based on experience from the update of the Danish national water resources model, three levels of model updating have been identified: 1) Basic data update - keeping the model up-to-date with respect to input data, 2) improving the model description by including new or more detailed data, and 3) reconstructing the model concept. The three levels vary with respect to technical tasks, challenges and stakeholder involvement. Two utility programs developed to optimise the updating process and support the uptake of data and knowledge from local users are furthermore presented. Finally, some of the challenges in operating a national model with multiple users belonging to different institutions with varying demands are discussed.

Comparison and evaluation of model structures for the simulation of pollution fluxes in a tile-drained river basin.

The European Union Water Framework Directive requires an integrated pollution prevention plan at the river basin level. Hydrological river basin modeling tools are therefore promising tools to support the quantification of pollution originating from different sources. A limited number of studies have reported on the use of these models to predict pollution fluxes in tile-drained basins. This study focused on evaluating different modeling tools and modeling concepts to quantify the flow and nitrate fluxes in the Odense River basin using DAISY-MIKE SHE (DMS) and the Soil and Water Assessment Tool (SWAT). The results show that SWAT accurately predicted flow for daily and monthly time steps, whereas simulation of nitrate fluxes were more accurate at a monthly time step. In comparison to the DMS model, which takes into account the uncertainty of soil hydraulic and slurry parameters, SWAT results for flow and nitrate fit well within the range of DMS simulated values in high-flow periods but were slightly lower in low-flow periods. Despite the similarities of simulated flow and nitrate fluxes at the basin outlet, the two models predicted very different separations into flow components (overland flow, tile drainage, and groundwater flow) as well as nitrate fluxes from flow components. It was concluded that the assessment on which the model provides a better representation of the reality in terms of flow paths should not only be based on standard statistical metrics for the entire river basin but also needs to consider additional data, field experiments, and opinions of field experts.

Modelling diffuse nitrogen loadings of ungauged and unmonitored lakes in Denmark: Application of an integrated modelling framework

Abstract According to the EU Water Framework Directive all freshwater bodies must obtain good ecological status by 2015. In Denmark this means that all lakes with a surface area above 5 ha (∼600 lakes) must be evaluated individually and mitigation measures must be enforced if the ecological status is below “good”. In consequence, the nutrient pressures from point and diffuse sources must be assessed based on a quantification of the nutrient loading of each lake. In this study we focus on the loading of nitrogen. Few of the 600 lakes are monitored for runoff and nitrogen loading. Therefore, a national 3D hydrological model covering all major parts of the country was used to estimate runoff from the lake catchments. The diffuse nitrogen input to each lake was simulated with an empirical nitrogen model. Where lakes are located upstream/downstream of each other, a calculation chain involving the nitrogen retention in lakes was created. A case study was selected using a large river basin including 23 lakes in Denmark. The cost‐effectiveness of reducing nitrogen loadings from different sub‐catchments in the case study river basin to the sea was calculated. The cost‐effectiveness associated with reducing the N‐loading from the most expensive sub‐catchment is shown to be 20 times more expensive than from the cheapest sub‐catchment.

Chapter 9.3:Evaluation of the Quantitative Status of Groundwater–Surface Water Interaction at a National Scale

With the European Union (EU) Water Framework Directive (WFD) the achievement of a good ecological status of surface waters and a good quantitative and qualitative status of groundwater has become obligatory. The ecological status of surface water is here defined by biological, chemical, morphologica...