Monitoring and assessment of environmental impacts of droughts (SC120024): work package 4. Final report

The Environment Agency is responsible for managing water resources in England and Wales. During droughts, the Agency monitors, reports and mitigates their effect on the environment. Monitoring and reporting is undertaken to provide the evidence-base for decisions on appropriate mitigation measures. The NDSN (National Drought Surveillance Network) was established by the Environment Agency in 2010 to provide this evidence-base. A recent (2012) literature review highlighted the limitations of our current understanding of the ecological effects of drought as there are few long-term drought studies. Similarly, a review of the NDSN, informed by the literature review, recommended the NDSN be enhanced by including macrophyte, fish and diatom data (in addition to macro-invertebrate data) and by collecting water quality, temperature and hydraulic information at all NDSN sites. It was also recommended that monitoring frequency be increased. In an earlier phase of this work, an existing methodology, DRUWID (DRIEDUP With Incremental Drought), was used as a framework for developing regional ecohydrological models with a particular emphasis on assessing and predicting the effects of long-term drought. It was demonstrated that DRUWID could be successfully applied to assess the ecological effects of drought, but that further testing and validation was required. A conceptual framework was developed for rivers based on the understanding that drought progressively reduces the volume of water and the wetted habitat of a water body, variably affecting lateral, vertical and longitudinal connectivity. Conceptual abiotic and biotic response curves to drying and re-wetting were developed which were aligned to the Environment Agency’s four state drought model (normal situation – developing drought – drought – severe drought – recovering from drought – normal situation). This study represents an extension of the aforementioned drought work. An assumption was made that the objective of the NDSN is to develop drought curves for different river types that in future would allow for the development of critical thresholds that would either flag Ecological Quality Ratio failures and / or deliver an early warning signal to trigger specific management measures. Accordingly, this study has been tailored as an adaptive management approach, with the work representing a ‘proof of concept’ of an adaptive approach. Thus, DRUWID has been applied to assess the ecological effects of drought. DRUWID outputs have been input to a GIS which plots drought effects at a national-scale. This information has been used to iteratively update the conceptual drought curves for rivers. With further work and refinement, these curves could be used to help set early warning signals or critical thresholds for management intervention. In adopting this approach, significant learning has been achieved, which has highlighted knowledge, information and management gaps. This learning has been taken forward by making a number of general strategy recommendations for enhancing the NDSN including the need for an appropriate river typology, more frequent and consistent collection of species-level and River Habitat Survey data and the importance of collecting site-level hydraulic, temperature and light information. Ecohydrological models could be enhanced by the incorporation of hydraulic information rather than just flow. Ultimately, however, a monitoring strategy should seek to capture effects and manage drought at the landscape-scale. A preliminary set of curves has also been developed to represent the response of an ‘average’ lake to drought based on expert opinion. Recommendations have been made for a lake monitoring programme, including the importance of bathymetric survey data and hydraulic modelling.