Anker Lajer Højberg

Uncertainty in the environmental modelling process - A framework and guidance

A terminology and typology of uncertainty is presented together with a framework for the modelling process, its interaction with the broader water management process and the role of uncertainty at different stages in the modelling processes. Brief reviews have been made of 14 different (partly complementary) methods commonly used in uncertainty assessment and characterisation: data uncertainty engine (DUE), error propagation equations, expert elicitation, extended peer review, inverse modelling (parameter estimation), inverse modelling (predictive uncertainty), Monte Carlo analysis, multiple model simulation, NUSAP, quality assurance, scenario analysis, sensitivity analysis, stakeholder involvement and uncertainty matrix. The applicability of these methods has been mapped according to purpose of application, stage of the modelling process and source and type of uncertainty addressed. It is concluded that uncertainty assessment is not just something to be added after the completion of the modelling work. Instead uncertainty should be seen as a red thread throughout the modelling study starting from the very beginning, where the identification and characterisation of all uncertainty sources should be performed jointly by the modeller, the water manager and the stakeholders.

Fate of herbicides in a shallow aerobic aquifer: A continuous field injection experiment (Vejen, Denmark)

A continuous, natural gradient, field injection experiment, involving six herbicides and a tracer, was performed in a shallow aerobic aquifer near Vejen, Denmark. Bentazone, (±)-2-(4-chloro-2-methylphenoxy) propanoic acid (MCPP), dichlorprop, isoproturon, and the dichlobenil metabolite 2,6-dichlor-benzamide (BAM) were injected along with 2-methyl-4,6-dinitrophenol (not discussed in this paper) and the tracer bromide. The injection lasted for 216 days and created a continuous plume in the aquifer. The plume was monitored in three dimensions in 96 multilevel samplers of 6–9 points each for 230 days, with selected individual points for a longer period. The bromide plume followed a complex path through the monitoring network downgradient of the injection wells. The plume movement was controlled by spatially varied hydraulic conductivities of the sand deposit and influenced by asynchronous seasonal variation in groundwater potentials. An average flow velocity of 0.5 m/d was observed, as depicted by bromide. Bentazone, BAM, MCPP, and dichlorprop retardation was negligible, and only slight retardation of isoproturon was observed in the continuous injection experiment and a preceding pulse experiment. No degradation of bentazone was observed in the aerobic aquifer during the monitoring period. BAM and isoproturon were not degraded within 5 m downgradient of the injection. The two phenoxy acids MCPP and dichlorprop were both degraded in the aerobic aquifer. Near the source a lag phase was observed followed by fast degradation of the phenoxy acids, indicating growth kinetics. The phenoxy acids were completely degraded within l m downgradient of the injection wells, resulting in the plumes being divided into small plumes at the injection wells and pulses farther downgradient. During the lag phase, phenoxy acids had spread beyond the 25 m long monitoring network. However, the mass of the phenoxy acids passing the 10–25 m fences never matched the corresponding bentazone or bromide masses, and the pulse was observed to shrink in size. This indicates that this pulse of phenoxy acids was being partially degraded at a low rate as it traveled through the aquifer.

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.

Transport and time lag of chlorofluorocarbon gases in the unsaturated zone, Rabis Creek, Denmark

et al., 1995, 1996; Szabo et al., 1996), flow and groundwater quality (Johnston et al., 1998; Böhlke and Denver, Transport of chlorofluorocarbon (CFC) gases through the unsatu1995), and groundwater–surface water interactions (Katz rated zone to the water table is affected by gas diffusion, air–water et al., 1995). A critical component in such analyses is the exchange (solubility), sorption to the soil matrix, advective–dispersive transport in the water phase, and, in some cases, anaerobic degradaconsideration of the transport and fate of the CFC tracers tion. In deep unsaturated zones, this may lead to a time lag between in the overlying unsaturated zone. For example, what entry of gases at the land surface and recharge to groundwater. Data are the CFC concentrations in recharge water and how from a Danish field site were used to investigate how time lag is aflong were the CFC gases in the unsaturated zone before fected by variations in water content and to explore the use of simple reaching the groundwater system? The residence time analytical solutions to calculate time lag. Numerical simulations demof a CFC tracer in the unsaturated zone is also called onstrate that either degradation or sorption of CFC-11 takes place, the time lag (Cook and Solomon, 1995). An accurate whereas CFC-12 and CFC-113 are nonreactive. Water flow did not estimate of the age of a groundwater sample also relies appreciably affect transport. An analytical solution for the period with on an accurate estimate of the time lag. a linear increase in atmospheric CFC concentrations (approximately For very shallow unsaturated zones of only a few early 1970s to early 1990s) was used to calculate CFC profiles and time lags. We compared the analytical results with numerical simulations. meters thickness, diffusion and barometric pumping sufThe time lags in the 15-m-deep unsaturated zone increase from 4.2 to ficiently mix the gases in the unsaturated zone so soilbetween 5.2 and 6.1 yr and from 3.4 to 3.9 yr for CFC-11 and CFC-12, gas CFC concentrations are similar to those in the trorespectively, when simulations change from use of an exponential to posphere. The input to the groundwater system is thus a linear increase in atmospheric concentrations. The CFC concentravery close to the atmospheric changes in CFC concentions at the water table before the early 1990s can be estimated by trations. This simple approach often has been used for displacing the atmospheric input function by these fixed time lags. A dating groundwater, where the concentrations measured sensitivity study demonstrates conditions under which a time lag in in groundwater are used directly together with the atmothe unsaturated zone becomes important. The most critical parameter spheric concentrations to estimate the time of recharge, is the tortuosity coefficient. The analytical approach is valid for the thus neglecting any time lag of the CFCs in the unsatulow range of tortuosity coefficients ( 0.1–0.4) and unsaturated zones greater than approximately 20 m in thickness. In these cases rated zone. the CFC distribution may still be from either the exponential or linear In deeper unsaturated zones the time lag can be imphase. In other cases, the use of numerical models, as described in portant, and various processes affecting CFC transport our work and elsewhere, is an option. become important. Gas diffusion and air–water exchange (solubility) are especially important in controlling the migration and attenuation rate. Both processes are a C hlorofluorocarbons are volatile organic comfunction of the water content and, thus, seasonal and pounds used, for example, as aerosol propellants year-to-year changes in infiltration and depth to the and refrigerants since the 1930s (Plummer and Busenwater table. Cook and Solomon (1995) investigated nuberg, 1999) and now found in the subsurface because of merically the conditions under which the time lag of the release to the atmosphere. There are numerous exCFCs in the unsaturated zone is of importance by examamples of the use of CFCs as tracers in groundwater ining the relative effects of various soil parameters on studies, including studies of dating recharge water (Ekthe distribution of gases in the unsaturated zone. Buwurzel et al., 1994; Plummer et al., 2000), dating young senberg and Plummer (2000) used the same model as groundwater ( 50 yr) (Busenberg and Plummer, 1992; Cook and Solomon (1995) to study lag times of SF6 in Oster et al., 1996; Plummer et al., 2001), groundwater unsaturated zones and found that the lag times of SF6 flow and transport processes (Reilly et al., 1994; Cook are smaller than for CFCs because of its low solubility. Oster et al. (1996) measured 57 profiles of both CFC-11 and CFC-12 in a 4.5-m-thick unsaturated zone in a forest P. Engesgaard and K.H. Jensen, Geological Institute, Univ. of Copensoil in Germany. A clear damping of the annual changes hagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark; A.L. in CFC atmospheric concentrations were found with a Højberg, K. Hinsby, and T. Laier, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark; relaxation time (time lag) of 30 d for 4 m. Weeks et al. F. Larsen, Environment and Resources, Technical Univ. of Denmark, (1982) used analytical and numerical transport models Building 204, 2800 Lyngby, Denmark; E. Busenberg and L.N. Plumto simulate the observed distribution of CFC-11 and mer, USGS, 12201 Sunrise Valley Drive, Reston, VA 20192, USA. CFC-12 in a 50-m-deep unsaturated zone. The analytiReceived 4 July 2003. Original Research Paper. *Corresponding author (pe@geol.ku.dk). cal model was a pure diffusion model, whereas the nuPublished in Vadose Zone Journal 3:1249–1261 (2004).

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...

Delay in catchment nitrogen load to streams following restrictions on fertilizer application

A MIKE SHE hydrological-solute transport model including nitrate reduction is employed to evaluate the delayed response in nitrogen loads in catchment streams following the implementation of nitrogen mitigation measures since the 1980s. The nitrate transport lag times between the root zone and the streams for the period 1950-2011 were simulated for two catchments in Denmark and compared with observational data. Results include nitrogen concentration and mass discharge to streams. By automated baseflow separation, stream discharge was separated into baseflow and drain flow components, and the nitrogen concentration and mass discharge in baseflow and drain flow were determined. This provided insight on the development of stream nitrogen loads, with a short average lag time in drain flow and a long average lag time in baseflow. The long term effect of nitrogen mitigation measures was determined, with results showing that there is a 15 years long delay in the appearance of peak nitrogen loads in streams. This means that real time stream monitoring data cannot be used alone to assess the effect of nitrogen mitigation measures.