A toolkit for assessing unsaturated soil time lag with respect to Water Framework Directive deadlines
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Time lag is the delay between the implementation of water quality mitigation measures and observed effects on water quality. This delay arises as a result of the long hydrological/hydrogeological pathways between nutrient sources and water receptors (Fenton et al., 2011; Sousa et al., 2013). Although time lag is an intrinsic component of nutrient transport through the vertical pathway (through the unsaturated zone), which has been identified as a controlling factor on the response to programmes of mitigation measures (POM). However, it is not currently accounted for within the European Union Water Framework Directive (EU-WFD) (EC, 2000), nor is there any established and accepted method of time lag assessment or prediction within the Republic of Ireland. This represents not only a knowledge gap from a theoretical perspective, but also a major limitation for assessing the efficacy of current measures on water quality (Schulte et al., 2006), ascertaining the implementation of legislative requirements (Wall et al., 2012), and developing appropriate policies for future iterations of this directive (Schröder et al., 2004; Bournaoui and Grizzetti, 2014). Although reporting periods within the EU-WFD are fixed at three- to six-year intervals (2012-2015, 2021 and 2027) (EC, 2000), time lag may frequently exceed these periods, leading to a perceived failure of either the POM or their implementation (Fenton et al., 2011).