Use of numerical models for determination of best sampling locations for monitoring of large lakes.

The implementation of the European Union Water Framework Directive (WFD) that requires all surface waters to achieve at least good status by year 2015 presents a challenge to local authorities responsible for the monitoring programmes. Obtaining a single representative value for each water quality parameter may be extremely difficult to achieve in the case of large lakes, which may be characterised by significant spatial and temporal variation of water quality parameters. The aim of this paper is to present a methodology, which can support the decision making as to the best locations for water quality sampling when developing monitoring systems of large lakes. The methodology is based on three-dimensional hydrodynamic and a conservative tracer transport numerical modelling and utilises the concept of flushing and residence time. The 3D numerical model of Lough Corrib located in the west of Ireland was created and used to calculate its residence time and flushing pathways. Based on the results it was possible to identify the areas in the lake that are characterised by the local residence time values close to the lake's average as well as by the values significantly lower and higher. Three water classes have been introduced and the authors recommend that sampling takes place in all three classes at various frequencies. Spatial distribution of the flushing pathways allows identifying the areas in the lake that best represent the lake's average conditions both in the long term and seasonally, where monitoring can be carried out at the required frequency; worst affected areas can also be identified. The methodology can be used to improve robustness of the monitoring programme and may also contribute towards the reduction of both sampling and analytical costs, both at the design and operational stages.