Simulation of river flow in the Thames over 120 years: evidence of change in rainfall-runoff response?

Study region: The Thames catchment in southern England, UK. Study focus: Modelling with 124 years of rainfall, potential evaporation (PE), temperature and naturalised flow data. Daily rainfall-runoff flow simulation using current and three historic land cover scenarios to determine the stationarity of catchment response examined through three time-frames of analysis – annual, seasonal and flow extremes. The criterion of response stationarity is often assumed in climate change impact studies. New hydrological insights: The generally close correspondence between observed and simulated flows using the same model parameter values for the whole period is indicative of the temporal stability of hydrological processes and catchment response, and the quality of the hydrometric data. Changes that have occurred are a decrease in flood peak response times, typically two to three days pre and post the early 1940s, from change in agricultural practices and channel conveyance, and an increase of about 15% in summer flow from increase in urban land cover between the first decade of the 20th and 21st centuries. The water balance was found to be sensitive to the PE data used, with care needed to avoid discontinuity between two parts of the data record using different methods for calculation. Long-term mean annual rainfall shows little change but contrasting patterns of variation in seasonal rainfall demonstrate a variable climate for which simulated flow is similar to observed flow

An investigation of the effect of transient climate change on snowmelt, flood frequency and timing in northern Britain

Climate change is almost certain to affect snow and ice processes. Even at lower latitudes, changes in snow cover at high altitudes can significantly affect catchment hydrology. This paper uses data from a transient Regional Climate Model projection (HadRM3Q0) for 1950-2099 (A1B emissions) to drive hydrological models for three nested catchments on the river Dee in north-east Scotland, to assess potential changes in flood frequency and timing using annual maxima and moving-window analyses. Some results are also shown for an upland catchment in northern England. Modelling is performed both with and without a snow module, to demonstrate the effects of snowfall/melt and how these change through time and vary between catchments. Modelled changes in flood magnitude and timing are non-linear, with most changes for daily mean flows not significant. For longer duration (30-day) flows with snow there are significant decreases in peak magnitude, particularly for the smaller higher altitude Dee catchments, with peaks occurring months earlier in future (changes without snow are generally not significant). There is a general convergence in results with and without snow later in the period, as snow processes become less important, but convergence occurs at different times for different catchments and occurs differently for daily and 30-day peak flows due to the differential effects of snow at different durations. This highlights the importance of including snow processes for such catchments, particularly for longer duration flows, but also highlights the complexity of interactions: Physical catchment properties, the balance between precipitation occurrence and temperature, and how this balance alters as the climate changes will each be critical in determining the impact on the magnitude and timing of peak flows, making it hard to generalise results

Regionalised impacts of climate change on flood flows: regionalising the flood response types in Britain. Milestone report 4. Revised November 2009

The primary objective of this project is to assess the suitability of current FCDPAG3 guidance given the advances in climate change science since its publication. PAG3 requires an allowance of 20% to be added to peak flows for any period between 2025 and 2115 for any location across Britain. This guidance was considered a precautionary value and its derivation reflected the evidence available at that time. FD2020 has been designed to increase this evidence base, and 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. A scenario-neutral approach based on a broad sensitivity analysis to determine catchment response to changes in climate as chosen for FD2020. The method separates the climate change that a catchment may be exposed to (the hazard) from the catchment response (change in peak flows) to changes in the climate (the vulnerability). By combining current understanding of climate change likelihood (the ‘hazard’) with the vulnerability of a given catchment, it is possible to evaluate the risk of flood flow changes. The vulnerability of a catchment is to be characterised in two steps: first, the response of a set of catchments to a range of climatic changes are modelled, then analysed for similarity, and second the main responses are characterised according to catchment properties. This is possible by defining a sensitivity framework of changes to the mean and seasonality of precipitation and temperature and modelling the response of each catchment within this fixed framework. This milestone report describes the second step of the vulnerability assessment. This is achieved by identifying the relationships identified between a catchment’s characteristics (geographic, geologic or climatic) and the vulnerability of its flood peak to changes in the climate. The work follows the identification of nine flood response types for catchments in Britain, after a comprehensive ‘scenario-neutral’ sensitivity study based on 4,200 patterns of changes in rainfall, temperature and potential evaporation. These nine flood response types were found to fully describe the range of changes in flood peak obtained in 154 catchments, and represent five main families of behaviour from the most ‘damping’ (low vulnerability), through ‘neutral’, to the most ‘enhancing’ (high vulnerability) catchments. One of the response types, with a very damped response to changes in climate, was removed from the analysis, as the group was too small for a reliable model to be built; leaving eight flood response types to characterise. Using a hierarchical partitioning technique and digital catchment descriptors from the Flood Estimation Handbook and the Hydrometric Register databases, decision trees were identified to discriminate the flood response type from nine descriptors including mean annual rainfall, area, northing and easting, elevation, and measures of permeability and catchment losses. At the 2-year return period level, all eight flood response types could be discriminated. For changes in the 20- and 50-year return period floods, the flood response types had to be merged into four main categories before they could be discriminated by the catchment characteristics. This merging was also necessary to ensure that uncertainty due to the impact of seasonality in rainfall change was fully incorporated into the flood response types. For the most enhancing catchments (i.e. where the changes in flood peak are proportionally much greater than the maximum increases in rainfall), the difference between the mean annual rainfall and the losses in the catchment was found to be an important discriminatory factor. For changes in higher return period floods, mean annual rainfall was found to be less critical. Wetter catchments were found to be in general less enhancing than drier catchments. The decision trees were successful for between 67.5% and 84% of the study catchments, depending on the flood indicator. Amongst the misclassified catchments, a larger proportion was misclassified as more enhancing, resulting in a potential over-estimation of changes in flood peaks, or an over-precautionary assessment. When evaluating the ability to discriminate between the more general families of ‘resilient/damping catchments’ (i.e. associated with a damped flood response type), ‘neutral catchments’ and ‘vulnerable/enhancing catchments’ (i.e. associated with an enhanced response type), 80% of the catchments were found to be correctly classified across all four flood indicators. Large catchments seem to be slightly more difficult to classify, suggesting they might not be well represented by single value descriptors which smooth out spatial variations important in the response of the river to climatic changes. Following the decision trees (sets of partitioning rules and paths for each of the flood response types), it is possible to quickly identify, for any catchment (gauged or ungauged but with available descriptors), the expected flood response type in response to climate change. This regionalised vulnerability assessment can be used in combination with an evaluation of potential climatic changes (or the hazard) to provide a measure of the risk of changes in flood peaks. In particular, this framework will enable a quick update of the potential risk of changes in peak floods when new climate change projections become available, such as for example the UKCP09 scenarios, without the need to undertake an extensive hydrological modelling and impact study

Regionalisation of climate impacts on flood flows to support the development of climate change guidance for Flood Management

Current Defra / Environment Agency guidance (FCDPAG3 supplementary note: http://www.defra.gov.uk/environ/fcd/pubs/pagn/climatechangeupdate.pdf) requires all flood management plans to allow for climate change by incorporating, within a sensitivity analysis, an increase in river flows of up 20% over the next 50 years, and beyond. This guidance is the same for all of England and Wales, making no allowance for regional variation in climate change or catchment type. This reflects the lack of scientific evidence to resolve the spatial distribution of potential impacts on flood flows with enough confidence to set such policy regionally. The 20% allowance was first raised in 1999 for MAFF and subsequently reviewed following the release of the UKCIP02 scenarios. Although the 20% figure is a memorable precautionary target, there is the risk that it leads to a significant under- or over-estimation of future flood risk in individual catchments. Defra and the Environment Agency procured project FD2020 (Regionalisation of climate change impacts on flood flows) to provide a more rigorous science base for refreshing the FCDPAG3: supplementary note guidance. The FD2020 approach is exploring the relationships between catchment characteristics and climate change impacts on peak flows in a “scenario neutral” way. This is done by defining a regular set of changes in climate that encompass all the current knowledge from the new scenarios available from the IPCC Fourth Assessment Report. For each of the 155 catchments included in the research, this broad approach will provide multiple scenarios to produce a “vulnerability surface” for change in the metrics of peak flows (e.g. the 20-year flood flow). Some of the UKCP09 products have also been used to understand what these projections may mean for changes to peak flow. The catchment-based analysis will be used to generalise to other gauged sites across Britain, using relationships with catchment characteristics, providing the scientific evidence for the development of regional guidance on climate change allowances. Specifically the project is: Investigating the impact of climate change on peak river flows in over 150 catchments across Britain to assess the suitability of the FCDPAG3 20% climate change allowance. Investigating catchment response to climate change to identify potential similarities such that the FCDPAG3 nationwide allowance could be regionalised. Investigating the uncertainty in changes to future peak river flows from climate change. Developing an approach that has longevity beyond the project timeframe and the lifetime of the latest generation of climate model results

Using sub-daily precipitation for grid-based hydrological modelling across Great Britain: assessing model performance and comparing flood impacts under climate change

•Study region: Great Britain. •Study focus: National-scale grid-based hydrological models are usually run at fine spatial and temporal resolutions, but driving data are often not available at the required resolutions. Here, a recent observation-based hourly 1 km gridded precipitation dataset is applied with a 1 km hydrological model to simulate daily mean river flows. Performance is compared to use of equally-disaggregated and profile-disaggregated daily data, for a large number of catchments. Hourly and daily precipitation from a high-resolution convection-permitting climate model (CPM) are then used to drive the hydrological model for baseline (1980–2000) and future (2060–2080) periods, to investigate differences in potential peak flow changes. •New hydrological insights: On average, use of observation-based hourly data provides a clear improvement over equally-disaggregated daily data for high flows and peak flow bias, a small improvement for average flows and mean flow bias, but little difference for low flows. Performance in faster-responding catchments typically improves more; performance in some catchments degrades. Use of profile-disaggregated daily data provides the small mean flow bias improvement and some peak flow bias improvement, but other factors degrade. On average, future changes in peak flows from hourly CPM precipitation are only slightly larger than from equally-disaggregated daily data. Future work will look at simulation of hourly mean flows

Use of very high resolution climate model data for hydrological modelling: baseline performance and future flood changes

Increasingly, data from Regional Climate Models (RCMs) are used to drive hydrological models, to investigate the potential water-related impacts of climate change, particularly for flood and droughts. Generally, some form of further downscaling of RCM data has been required, but recently the first decadal-length runs of very high resolution RCMs (with convection-permitting scales) have been performed. Here, a set of such runs for southern Britain has been used to drive a gridded hydrological model. Results using a 1.5km RCM nested in a 12km RCM driven by European-reanalysis boundary conditions show that the 1.5km RCM generally performs worse than the 12km RCM for simulating river flows in 32 example catchments. The clear spatial patterns of bias are consistent with bias patterns shown in the RCM precipitation data. Results using 1.5km and 12km RCM runs for the current climate and a potential future climate (driven by GCM boundary conditions) show clear differences in projected changes in flood peaks. The 1.5km RCM tends towards larger increases than the 12km RCM, particularly in spring and winter. If robust, this could have important consequences for adaptation planning under climate change, but further research is required, particularly given the greater biases in the baseline flow simulations driven by 1.5km RCM data, and the use of only a single short future climate projection

Grid-based simulation of soil moisture in the UK: future changes in extremes and wetting and drying dates

Soil moisture, typically defined as the amount of water in the unsaturated soil layer, is a central component of the hydrological cycle. The potential impacts of climate change on soil moisture have been less specifically studied than those on river flows, despite soil moisture deficits/excesses being a factor in a range of natural hazards, as well as having obvious importance for agriculture. Here, 1 km grids of monthly mean soil moisture content are simulated using a national-scale grid-based hydrological model, more typically applied to look at changes in river flows across Britain. A comparison of the soil moisture estimates from an observation-based simulation, with soil moisture deficit data from an operational system developed by the UK Met Office (Meteorological Office Rainfall and Evaporation Calculation System; MORECS), shows relatively good correspondence in soil drying and wetting dates, and in the month when soils are driest. The UK Climate Projections 2018 Regional projections are then used to drive the hydrological model, to investigate changes in occurrence of indicative soil moisture extremes and changes in typical wetting and drying dates of soils across the country. Analyses comparing baseline (December 1981–November 2011) and future (December 2050–November 2080) time-slices suggest large increases in the spatial occurrence of low soil moisture levels, along with later soil wetting dates, although changes to soil drying dates are less clear. Such information on potential future changes in soil moisture is important to enable the development of appropriate adaptation strategies for a range of sectors vulnerable to soil moisture levels

An assessment of the possible impacts of climate change on snow and peak river flows across Britain

A temperature-based snow module has been coupled with a grid-based distributed hydrological model, to improve simulations of river flows in upland areas of Britain subject to snowfall and snowmelt. The coupled model has been driven with data from an 11-member perturbed-parameter climate model ensemble, for two time-slices (1960-1990 and 2069-2099), to investigate the potential impacts of climate change. The analysis indicates large reductions in the ensemble mean of the number of lying snow days across the country. This in turn affects the seasonality of peak river flows in some parts of the country; for northerly regions, annual maxima tend to occur earlier in the water year in future. For more southerly regions the changes are less straightforward, and likely driven by changes in rainfall patterns rather than snow. The modelled percentage changes in peak flows illustrate high spatial variability in hydrological response to projected climate change, and large differences between ensemble members. When changes in projected future peak flows are compared to an estimate of current natural variability, more changes fall outside the range of natural variability in southern Britain than in the north

Framework for setting up a Hydro-JULES perturbed parameter ensemble (PPE)

Land surface and hydrological processes and feedbacks that act at sub-grid scales need to be parameterised within models. These parameterisation schemes are an important source of uncertainty in model simulations. One of the aims of Hydro- JULES (HJ) is to quantify uncertainties in river flows induced by land surface and hydrological model parameterisations

Climate change impacts on peak river flows: combining national-scale hydrological modelling and probabilistic projections

Potential future increases in flooding due to climate change need to be taken into consideration when designing flood defences or planning new infrastructure or housing developments. Existing guidance on climate change allowances in Great Britain was based on research that developed a sensitivity-based approach to estimating the impacts of climate change on flood peaks, which was applied with catchment-based hydrological models. Here, the sensitivity-based approach is applied with a national-scale grid-based hydrological model, producing modelled flood response surfaces for every river cell on a 1km grid. This provides a nationally consistent assessment of the sensitivity of flood peaks across Britain to climatic changes. The flood response surfaces are then combined with the most recent climate change projections, UK Climate Projections 2018 (UKCP18), to provide location-specific information on the potential range of impacts on floods across the country, for three flood return periods, three future time-slices and four emissions scenarios. An accompanying web-tool provides a convenient way to explore the large amount of data produced. Consideration is now being given to how to use the latest work to update guidance on climate change and flood peaks, including a workshop held to gather stakeholder views