Habitat hydraulic models - a tool for Danish stream quality assessment? a case study in a small Danish stream, stream Ledreborg.

In relation to the European Water Framework Directive (WFD), Danish water management has to change to a holistic management approach considering both groundwaters and surface waters at the same time.

Furthermore the WFD introduces the concept "Good ecological status" where the quality of the biological community to a high degree has to resemble the biological community which would be expected under minimal anthropogenic impact. In order to establish hydromorphological and chemical standards for "Good ecological status" the relation between the quality of the biological community and hydromorphological and chemical characteristics has to be enlightened (EUROPA, 2005).

This study links catchment hydrology, stream discharge and physical habitat in a small Danish stream, the stream Ledreborg, and discusses the utility of habitat hydraulic models in relation to the present criteria and methods used in Danish stream management and stream quality assessment.

The stream Ledreborg catchment is modelled using a precipitation-run-off-model (NAM) and as an addition to the normal calibration procedure (Kronvang et al., 2000) the model is calibrated using DAISY adjusted evaporation data. The impact from groundwater abstraction upon stream discharge is assessed and in relation to this the relative importance of variations in precipitation, evaporation/temperature and groundwater abstraction are discussed.

Physical habitat preferences for trout in the stream Ledreborg are assessed through a series of field observations and "site-specific" habitat suitability indices (HSI) are constructed. "Site-specific" HSI's are compared to other HSI's for Danish streams (Søholm and Jensen, 2003) and general HSI's used in other habitat hydraulic modelling projects (Lund, 1996; Fjordback et al. 2002; Thorn and Conallin, 2004). 

Hydromorphological conditions in the stream are measured through field study, using a habitat mapping approach and modelled using a habitat hydraulic model (RHYHABSIM).

Using RHYHABSIM and both "site-specific" and general HSI's, Weighted Usable Area (WUA) for the trout population at different discharges is assessed and differences between simulated WUA using "site-specific" and general habitat preferences are discussed.

In RHYHABSIM it is possible to use two different approaches to investigate the hydromorphological conditions in a river, the habitat mapping approach used in this project and the representative reach approach used in Thorn and Conallin (2004) for the same stream (stream Ledreborg). Using the same general HSI's but two different hydromorphological approaches for the same stream, WUA for the trout population at different discharges is assessed and differences between simulations using habitat mapping approach and representative reach approach are discussed.

As a last part of the project simulation of WUA is done for macro invertebrates using a general HSI (Waters, 1976) for macro invertebrates. Data from three sites in stream Ledreborg used for biological assessment of the stream quality (DFI and DVFI) from 1995-2004 and the simulated WUA are correlated between years for the whole stream and between stretches in the stream to estimate the relation between the present measures of biological quality and the habitat hydraulic simulation.

The applicability/utility of habitat hydraulic models in relation to the present criteria and methods used in Danish stream management and stream quality assessment are discussed.