Continuous flow simulation modelling incorporating climate change

Transcription

1 Continuous flow simulation modelling incorporating climate change Nick Reynard Head, Risk Analysis and Modelling Group, Wallingford

2 Content Flood frequency estimation Hydrological modelling Climate change Concluding remarks Thames at Wallingford, January 2003

3 Flood frequency estimation Statistical method Continuous flow simulation method

4 Flow and rainfall data Catchment properties Runoff model Time series of river discharge; flood frequency curves Relating of catchment properties and model parameters Principle of continuous simulation approach SPATIALLY GENERALISED FLOOD FREQUENCY ESTIMATION for use at ungauged sites Long term rainfall Optional climate change information FLOOD FREQUENCY ESTIMATION to high recurrence interval floods for current and under climate change

5 Lumped catchment models PDM TATE Runoff production in a catchment determined by a distribution of soil moisture stores. Routing through storage reservoirs. Modelling basis for FD2106

6 Distributed, grid-based model The Grid-to-Grid model estimates river flow on a 1km grid covering the UK Applications include: * Assessing climate-change impacts on river flows * Estimating river flows for input to the Shelf-seas/coastal model The Grid-to-Grid model To the Sea River flow gauging station Example model output: space-time varying river flows over Wales on a 1km grid

7 Semi-distributed catchment modelling CLASSIC: Climate and LAnd use ScenarioS in Catchments Nested calibration. Runoff production at each grid-cell. Routing direct from cell to outlet. Ungauged estimates within catchment.

8 Observed Modelled Flow hydrograph mean daily flow (m 3 s 1 ) Flow duration Modelling the Severn to Bewdley percentage of time flow exceeded Flood frequency curve return period (years)

9 Flow and rainfall data Catchment properties Runoff model Time series of river discharge; flood frequency curves Relating of catchment properties and model parameters Principle of continuous simulation approach SPATIALLY GENERALISED FLOOD FREQUENCY ESTIMATION for use at ungauged sites Long term rainfall Optional climate change information Provide driving climate data to represent possible future conditions FLOOD FREQUENCY ESTIMATION to high recurrence interval floods for current and under climate change

10 Climate change - methodology Simulate flows under current conditions Generate climate change scenarios Simulate flows time series using future climate Look at CHANGE in flows, not actual future series mean daily flow (m 3 s 1 ) baseline UKCIP s Statistical downscaling 2080s return period (years)

11 Climate change scenarios Output from Global and Regional Climate Models Downscaling in space and time dynamic or statistical Extreme events? Short duration, high intensity rainfall Long duration, sustained heavy rainfall Coupled modelling (includes feedbacks)

12 UK results change in 20-year flow by the 2080s +22% -13% -4%

13 Percentage change in peak flow at 20-year return period 9 > +20% 9 +5% to +20% 0% to +5% % to 0% 8-20% to -5% 8 < -20% RCM-driven change Percentage change in flows for the 20-year return period by the 2080s

14 Annual Winter Spring Summer Autumn Main factors Location Catchment response Greater seasonality of rainfall Sequencing of wet and dry seasons/years

15 FD2020 Regionalisation of climate impacts on flood flows Hydrological modelling of 150 catchments Including snowmelt Climate change sensitivity, with specific scenarios Catchment-related change in peak flows Refined guidelines for climate change Reporting March 2009

16 Annual Winter Spring Summer Autumn Main factors Location Catchment response Greater seasonality of rainfall Sequencing of wet and dry seasons/years Uncertainty.

17 Uncertainty Future socio-economic change Future emissions of greenhouse gases Climate response to increased concentrations of gases Global climate modelling Natural variability Development of scenarios Hydrological model uncertainty Decision uncertainty (climate and non-climate related)

18 From: Kay, A.L., Davies, H.N. and Bell, V.A. Comparison of uncertainty sources for climate change impacts: flood frequency in the UK. Submitted to Climatic Change, October River Beult in South East England (Kent) Climate Impacts Uncertainty % change in flood frequency Natural variability: -34 to +17 Emissions: -14 to - 9 Global Climate Model structure: -13 to +41 GCM initial conditions: -25 to - 5 Downscaling: -22 to - 8 RCM structure: -5 to +8 Hydro model structure: -45 to - 22 Hydro model parameters: +1 to + 7 % change in flood frequency Return period (years)

19 Annual Winter Spring Summer Autumn Location Main factors Catchment response Greater seasonality of rainfall Sequencing of wet and dry seasons/years Uncertainty Longer recurrence intervals Multiple / ensemble rainfall series..

20 Rainfall modelling under climate change Provide multiple / long time series to explore current and future natural variability Allow exploration of changes in extreme / long recurrence interval rainfall Facilitate a probabilistic approach to change in rainfall and hence flood flows

21 What of the future???...fd2113 Current flood frequency to high recurrence intervals simulated rainfall flood frequencies simulated using stochastic rainfall series

22 Concluding remarks Flood frequency estimation Continuous flow simulation Climate change impact modelling Need to explore: Uncertainty Changes in natural variability Changes in extremes / long recurrence interval events Rainfall modelling under climate change