Regionalised impacts of climate change on flood flows: hydrological models, catchments and calibration. Milestone report 1

The primary objective of this project is to assess the suitability of the PAG3 guidance of a 20% change to peak flows for any period between 2025 and 2115. The guidance was set as a precautionary upper limit to changes in river flows over the next 50 years, and was applicable to all regions of Britain. This has since been extended to the period up to 2115 reflecting the lack of scientific evidence to suggest any alternative figure. This project has been designed to provide this evidence. Furthermore, it is anticipated that the research will lead to the development of regional, rather than national, guidelines for changes to peak flows due to climate change. The hydrological modelling tasks within this project provide the fundamental building blocks for the subsequent analysis of the potential implications of climate change on flood flows, and the regionalisation of those impacts. This means that it is essential that the hydrological models are set up and calibrated as robustly as possible. In particular, the inclusion of snowmelt within the hydrological models was considered crucial, given the project’s aim to regionalise the impacts of climate change on flooding, as the winter flow regime of upland catchments can be considerably affected by snowfall and snowmelt, even in the UK, and changes in temperature will almost certainly alter the balance between snowfall and rainfall processes in such catchments in the future. This milestone report describes the hydrological models (their structure and data requirements), details the 154 catchments to be modelled across Britain: 120 with the PDM (a lumped conceptual hydrological model), and 35 (generally larger) catchments with CLASSIC (a semi-distributed hydrological model), and presents their calibration results. One catchment is modelled by both hydrological models. The final calibrations include the use of a snowmelt module, which has been applied (with a fixed set of module parameters) for all catchments, to avoid an arbitrary decision on which catchments are affected. The hydrological models with the snowmelt module require input time-series of precipitation, potential evaporation and temperature to simulate mean daily flow. Overall, model performance improves when the snowmelt module is applied. The calibrated models were used to simulate baseline time series of mean daily flows from which a set of independent flood peaks was extracted for each catchment. For the majority of catchments there is good comparison between flood frequency curves fitted to the observed and modelled mean daily flood peak data sets. Reasons are identified where there are considerable differences between the observed and modelled curves. The final calibrated parameter sets are used in the next part of the project: the application of a large, regular, set of perturbations to observed precipitation time-series, alongside a smaller set of (linked) perturbations to potential evaporation and temperature time-series, to investigate the relative sensitivity of different catchments to the potential range of climate change. The development of this scenario method, and its application is described in milestone report 2 (Prudhomme & Reynard, 2009).