Future Flows Hydrology: an Ensemble of Daily River Flows and Monthly Groundwater Levels for Use for Climate Change Impact Assessment Across Great Britain

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Future Flows Hydrology: an ensemble of daily river flows and monthly groundwater levels for use for climate change impact assessment across Great Britain

Christel Prudhomme*1, Tracey Haxton2, Sue Crooks1, Christopher Jackson3, Andrew Barkwith3, Jennifer Williamson1,4, Jon Kelvin2, Jonathan Mackay3, Lei Wang3, Andy Young2, Glenn Watts5

Supplementary Material

Table 1 Future Flows Hydrology river flow sites organised by regions

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2 2

Region

tationName

Station(Hydrometric RegisterNumber) Name River S Catchment(km area Station(Hydrometric RegisterNumber) Name River StationName Catchment(km area

2001 Helmsdale Kilphedir 551.4 9003 Isla Grange 176.1 4003 Alness Alness 201 10002 Ugie Inverugie 325 4005 Meig Glenmeannie 120.5 11001 Don Parkhill 1273 6008 Enrick Mill of Tore 105.9 12002 Dee Park 1844 7002 Findhorn Forres 781.9 12003 Dee Polhollick 690

7004 Nairn Firhall 313 12005 Muick Invermuick 110 7005 Divie Dunphail 165 12008 Feugh Heugh Head 229

7006 Lossie Torwinny 20 90003 Nevis Claggan 69.2 SEPA North SEPA 7009 Mosset Burn Wardend Bridge 28.3 92002 Shiel Shielfoot 256 8004 Avon Delnashaugh 542.8 93001 Carron New Kelso 137.8 8006 Spey Boat o Brig 2861.2 94001 Ewe Poolewe 441.1 8009 Dulnain Balnaan Bridge 272.2 95001 Inver Little Assynt 137.5 9001 Deveron Avochie 441.6 97002 Thurso Halkirk 412.8 9002 Deveron Muiresk 954.9 13001 Bervie Inverbervie 123 18005 Allan Water Bridge of Allan 210 13005 Lunan Water Kirkton Mill 124 19006 Water of Leith Murrayfield 107 13007 North Esk Logie Mill 732 19011 North Esk Dalkeith Palace 137

13008 South Esk Brechin 488 20001 Tyne East Linton 307

13009 West Water Dalhouse Bridge 127.2 21003 Tweed Peebles 694 14001 Eden Kemback 307.4 21006 Tweed Boleside 1500

SEPA SEPA East 15006 Tay Ballathie 4587.1 21009 Tweed Norham 4390 15014 Ardle Kindrogan 103 21012 Teviot Hawick 323 15023 Braan Hermitage 210 21013 Gala Water Galashiels 207 15024 Dochart Killin 239 21015 Leader Water Earlston 239 15025 Ericht Craighall 432 21017 Ettrick Water Brockhoperig 37.5 EA North East Region SEPA West 23004 22009 22004 22001 83011 83010 83007 83005 82001 81007 81005 81002 80005 79006 79003 79002 78999 78006 78005 78003 77304 77004 77003 77002 18001 17016 17015 17005 17003 16007 16003 24002 23011 23006 Station (Hydrometric Register Number)

Gaunless Burn Kielder Tyne South Tyne South Coquet Aln Coquet Ayr Irvine Lugton Water Irvine Girvan Fleet of Water Burn Piltanton Cree Lane Dargall Nith Nith Nith Evan Water Annan Kinnel Water Annan Water Liddell Kirtle Water Water Liddel Esk Allan Water Burn Lochty Queich North Avon Bonny Water Water Ruthven Water Ruchill River Name

Bishop Auckland Bishop Kielder Featherstone Bridge Haydon Rothbury Hawkhill Morwick Wellwood Newmilns Eglinton Castle Shewalton Robstone Rusko Barsolus Stewart Newton Dee Loch Drumlanrig Hall Bridge Carse Friars Beattock Woodfoot Bridgemuir Brydekirk Newcastleton Mossknowe Rowanburnfoot Canonbie Kinbuck Whinnyhall Lathro Polmonthill Bonnybridge Aberuthven Cultybraggan Station Name

195.3 321.9 751.1 569.8 380.7 245.5 2 23.1 50.5 99.5 58.8 72.8 54.6 34.2 Catchment area (km ) 207 319 495 161 346 205 368 471 155 799 217 229 925 2.1 72 14 50 93 60 77 80

84012 84005 84004 84003 21032 21031 21027 21023 21022 21021 27042 27041 27035 27034 27021 27009 27007 89009 89008 89005 89004 89003 85003 85002 84037 84029 84026 84022 84020 84018 84016 84015 84013 Station (Hydrometric Register Number)

White Cart Water White Cart Clyde Clyde Clyde Glen Till Water Blackadder Water Leet Water Whiteadder Tweed Dove Derwent Aire Ure Don Ouse Ure Eas Ghaill a' Eas Daimh Lochy Strae Orchy Falloch Water Endrick Water Douglas Water Cander Water Allander Duneaton Water Glazert Clyde Water Luggie Kelvin Clyde

River Name

Hawkhead Blairston Clyde Sills of Hazelbank Kirknewton Etal Bridge Mouth Coldstream Castle Hutton Sprouston Kirkby Mills Buttercrambe BridgeKildwick Bridge Kilgram Doncaster Skelton Lock Westwick Succoth Eas Daimh Inverlochy Strae Glen Orchy Glen Falloch Glen Gaidrew Happendon Candermill Milngavie Maidencots Campsie of Milton Mill Tulliford Condorrat Dryfield Daldowie Station Name

1704.2 1092.9 1256.2 1903.1 234.9 741.8 198.9 282.3 510.2 914.6 251.2 219.9 110.3 932.6 235.4 2 3330 1586 3315

59.2 47.7 36.2 80.3 24.5 32.8 51.9 33.9 Catchment area (km ) 648 159 113 503 9.7 4.5 97

EA Thames EA Midlands Region EA Anglian 39001 38014 38003 30018 33049 33044 33029 33027 33026 33019 33018 33014 33012 31020 31010 30018 28055 28046 28033 28031 28030 28022 28018 28008 25020 25019 25007 25005 24009 24005 39016 39008 39006 Station (Hydrometric Register Number)

Kennet Thames Windrush Thames Brook Salmon Mimram Witham Water Stanford Thet Stringside Rhee Ouse Bedford Thet Tove Lark Kym Brook Morcott Chater Beck Honington Ecclesbourne Dove Dove Manifold Brook Black Trent Dove Dove Skerne Leven Clow Beck Leven Wear Browney River Name

Theale Eynsham Newbridge Kingston Edmonton Park Panshanger Colsterworth Tofts Buckenham Bridgham Whitebridge Wimpole Offord Bridge Melford Bridge Cappenham Temple Farm Meagre Luffenham South Bridge Fosters Honington Duffield Walton Izaak Hollinsclough Ilam Onebarrow Muskham North Dove on Marston Weir Rocester le Preston Easby Croft Bridge Leven leStreet Chester Burn Hall Station Name

Skerne

1008.3 1033.4 1616.2 148.5 883.2 196.3 178.5 362.6 133.9 277.8 119.1 138.1 137.5 8231 9948 2570 2 14.8 78.2 20.5 51.3 43.5 98.8 19.6 68.9 50.4 Catchment area (km ) 399 147 316 272 8.4 83 51.3 8

54022 54018 54008 54001 28066 27084 27071 27055 27049 27043 39103 39096 39092 39090 39081 39076 39057 37019 37011 37001 36007 36005 35008 34018 34014 34011 34006 34002 33063 54057 54038 54036 Station (Hydrometric Register Number)

Severn Rea Brook Teme Severn Cole Beck Eastburn Swale Rye Rye Wharfe Kennet Brook Wealdstone Brook Dollis Cole Ock Windrush Crane Beam Chelmer Roding Brook Belchamp Brett Gipping Stiffkey Wensum Wensum Waveney Tas Ouse Little Severn Tanat Isbourne River Name

Plynlimon flume Hookagate Tenbury Bewdley Coleshill Crosshills Crakehill Foot Broadway Ness Addingham Newbury Wembley Bridge Lane Hendon Inglesham Abingdon Worsham Park Cranford Farm Bretons Churchend Redbridge Bridge Bardfield Hadleigh Stowmarket Saints All Warham Total Swanton Morley Fakenham Mill Needham Shotesham Knettishall Haw Llanyblodwel Green the on Hinton Station Name

Bridge

1134.4 131.7 238.7 548.1 303.3 128.9 397.8 161.9 146.5 2 4325 1363 9895 43.4 21.8 25.1 61.7 49.7 72.6 58.6 87.8 90.7 Catchment area (km ) 178 130 427 140 234 296 156 370 101 229 8.7

EA Wales EA South West Region EA Southern 56013 56007 56005 56003 56002 55029 55007 55004 55003 55002 47001 46006 46005 46003 45011 45009 45005 45004 45001 44002 43021 43007 43006 43005 43003 40023 40017 40011 40003 39049 39034 58005 57004 56019 Station (Hydrometric Register Number)

Ogmore Cynon Ebbw Yscir Senni Lwyd Honddu Ebbw Monnow Wye Irfon Lugg Wye Tamar Erme EastDart Dart Barle Exe Otter Axe Exe Piddle Avon Stour Nadder Avon Avon EastStour Dudwell Stour Great Medway Stream Silk Evenlode River Name

Brynmenyn Abercynon Aberbeeg Pontaryscir Pont Hen Ponthir Brecon Forge The Rhiwderyn Grosmont Erwood Abernant Lugwardine Belmont Gunnislake Ermington Bellever Bridge Austins Brushford Pixton Dotton Whitford Thorverton Baggs Mill Mill Knapp Throop Wilton Amesbury EastMills Willesborough South Burwash Horton Teston Lane Colindeep Mill Cassington Station Name

Hafod

1477.8 1256.1 1282.1 1895.9 220.6 323.7 216.5 885.8 916.9 247.6 159.7 202.5 288.5 600.9 183.1

1706 1073 2 58.8 27.5 74.3 71.7 62.8 19.9 98.1 62.1 72.8 43.5 21.5 Catchment area (km ) 345 430 106 354 128 29

50002 49001 48003 47014 47008 42012 41026 41022 41011 39131 39105 66011 65006 65001 64002 64001 63004 63001 62002 62001 61001 60006 60004 60002 53018 53017 53006 53005 52010 52004 51001 50007 50006 Station (Hydrometric Register Number)

Torridge Camel Fal Walkham Thrushel Anton Brook Cockhaise Lod Rother Brent Thame Conwy Seiont Glaslyn Dysynni Dyfi Ystwyth Ystwyth Teifi Teifi Cleddau Western Gwili Dewi Fawr Cothi Avon Boyd Frome(Bristol) Brook Midford Brue Isle Stream Doniford Taw Mole

River Name

Torrington Denby Tregony Horrabridge Tinhay Fullerton Holywell Halfway Mill Iping Greenford Lane Costons Wheatley Cwmlanerch Peblig Mill Beddgelert Pont Dyfi Bridge Cwm Ystwyth Pont Llolwyn Llanfair Teifi Glan Mill Prendergast Glangwili Ford Glasfryn Mynachdy Felin Bathford Bitton Frenchay Midford Lovington Mill Ashford BridgeSwill Bridge Taw Woodleigh Station Name - y

Garth

Bridge

208.8 112.7 146.2 533.8 344.5 471.3 169.6 893.6 197.6 129.5 297.8 148.9 147.4 135.2 327.5 2 1552 44.6 36.1 74.4 68.6 75.1 32.1 36.7 47.9 90.1 75.8 71.4 Catchment area (km ) 663 185 154 510 87 52

EA North West Region

73005 73003 72015 72014 72009 72004 71009 71006 71001 69042 68005 68003 68001 60009 59001 58012 58008 58007 Station (Hydrometric Register Number)

Kent Kent Lune Conder Wenning Lune Ribble Ribble Ribble Ding Brook Weaver Dane Weaver Sawdde Tawe Afan Dulais Llynfi River Name

Sedgwick Burneside Bridge Lunes Galgate Wennington Caton Rock New Jumbles Henthorn Samlesbury Reservoir Naden Audlem Rudheath Ashbrook Felin Ynystanglws Weir Marcroft Cilfrew Coytrahen Station Name - y

cwm

407.1 227.7 141.5 1145 1053 2 77.5 87.8 50.2 73.6 28.5 Catchment area (km ) 456 207 622 209 142 983 2.2 43

74001 73014 73013 73011 73009 73006 67013 67010 67005 76008 76007 76005 75017 74007 74006 74005 Station (Hydrometric Register Number)

Duddon Brathay Rothay Mint Sprint Beck Cunsey Hirnant Gelyn Ceiriog Irthing Eden Eden Ellen Esk Calder Ehen River Name

Duddon Hall Duddon Knotts Jeffy House Bridge Miller Bridge Mint Sprint Mill Bridge Eel House Plas Rhiwedog Cynefail Weir Brynkinalt Greenholme Sheepmount Sowerby Temple Bullgill How Cropple Hall Calder Braystones Station Name

2286.5 113.7 334.6 616.4 125.5 2 85.7 57.4 65.8 34.6 18.7 33.9 13.1 70.2 44.8 Catchment area (km ) 96 64

Table 2 Future Flows Hydrology groundwater sites

ObservationBorehole (Hydrometric Register Number) ObservationBorehole Name Aquifer SY68/34 Ashton Farm Chalk TA10/63 Aylesby Chalk SU81/1 Chilgrove House Chalk SU34/8D Clanville Lodge Gate Chalk SE94/5 Dalton Holme Chalk ST88/62A Didmarton 1 Inferior Oolite SD27/6B Furness Abbey Permo-Triassic Sandstone TL89/37 Grimes Graves Chalk SJ62/112 Heathlanes Permo-Triassic Sandstone SK17/13 Hucklow South Carboniferous Limestone TR14/9 Little Bucket Farm Chalk SJ15/13 Llanfair Dyffryn Clwyd Permo-Triassic Sandstone TQ41/82 Lower Barn Cottage Lower Greensand TF03/37 New Red Lion Lincolnshire Limestone NX97/2 Newbridge Permo-Triassic Sandstone SU17/57 Rockley Chalk NY63/2 Skirwith Permo-Triassic Sandstone SU78/45A Stonor Park Chalk NZ21/29 Swan House Magnesian Limestone TL33/4 Therfield Rectory Chalk TF81/2A Washpit Farm Chalk TQ25/13 Well House Inn Chalk TV59/7C West Dean No. 3 Chalk SU01/5B West Woodyates Manor Chalk

Example of river flow catchment fact sheet - the Llynfi at Coytrahen (58007)

Future Flows and Groundwater Levels – SC090016 – Briefing Note for River fact sheets

The fact sheets are designed to provide a brief overview on the ability of the river flow or groundwater models to reproduce (simulate) some of the most important components of the water cycle when using observed and modelled climate. This overview is given by sets of statistics (measuring the differences between two time series) and graphs (providing a visual comparison). Detailed information on the meaning of the statistics and graphs is provided in the Modelling protocol report (Crooks et al., 2012, SC090016/PN4) accessible from the FF webpages (www.ceh.ac.uk). This

note briefing note summarises the meaning and relative importance of the statistics and graphs; it does not provide any interpretation for specific catchments/model. One fact sheet is delivered for each site and river flow or groundwater level model combination. If two hydrological models are used to simulate flow at the same site, two catchment fact sheets are provided for this site. Note that different models use different methods of calibration ranging from catchment specific to regionalised parameters. The advantage of a regionalised parameter model is to extend the climate range under which the model parameters are evaluated; this is particularly important in a warming climate for catchments where evaporation processes may change from a surplus of summer precipitation over evaporation to a deficit. The advantage of catchment calibrated models is that they are designed to reproduce well the local hydrological processes. The calibration method may affect the statistical measures of model performance. A catchment fact sheet is divided in three parts. Top front page: general information section with the main physical characteristics of the catchment, its location and the availability of observed flow data. Front: how well the observed flow

Overview of the fact sheets and briefing briefing and sheets fact the of Overview time series are reproduced by the models when using observed climate; or a measure of the confidence in the hydrological model. Back: how well flow time series are reproduced by the models when using modelled climate; or a measure of the confidence in the climate/hydrological model combination. Both front and back must be looked at to fully understand the factors affecting the Future Flows Hydrology (FFH) time series. This is very important when the FFH time series are used to assess climate change impact on a catchment ecosystem. The FFH flow time series are in m3s-1.

Table Graphs Summary of differences in modelling the flow with observed The graphs illustrate how well the model simulates the climate. Differences (except Nash Sutcliffe) give the % flow time series by plotting together observed and departure between statistics calculated from simulated and simulated flow. observed flow time series. Two types of graphs are shown: Names represent the considered statistics; Qx = % difference Hydrographs of mean daily flow for two 2-year periods in flow percentile value (i.e. in flow exceeded x% of the time); (for most catchments representative of contrasting climatic Nash Sutcliffe measures if the modelled time series describes conditions): (i) The 1975-1977 period illustrative of a dry the observed time series better than the long term average. A episode and subsequent re-wetting; (ii) The 2000-2002 value of 1 shows a perfect match. period illustrative of a wet episode and subsequent Three parts of the hydrological regime are of interest: (i) average conditions. Water balance, seasonality, and day to day variability (upper They give a visual assessment of the reproduction of part of table); (ii) Low flows (flow percentiles Q75 and Q90); different hydrological processes under contrasting (lower left); (iii) High flows (flow percentiles Q25 and Q5) and conditions (e.g. drying during the recession phase; flood peaks (RP2 to RP20, not all models) (lower right). Sets temporal variability typical of the flashiness of the of statistics are given for two time periods. Statistics are only catchment). Daily precipitation is also shown in these calculated when there is observed flow data which may be graphs.

Front page: Simulation from observed climate observed from Simulation page: Front limited within the 1962-1991 period. Mean monthly flows and flow duration curves (shown Differences include measurement errors and other factors on the back page). These graphs provide a visual affecting the observed flow but generally the smaller the assessment of how well the long-term variability and difference the better the model simulation. seasonality is reproduced by the simulation. Model performance Assessment of model performance is given for the statistics for the 1971-2005 period using three Bands as defined in Table 1 of the Modelling Protocol. Interpretation of the Performance Bands; (i) Define the purpose for which the FFH time series are being used; (ii) Select the statistics most relevant to the purpose; (iii) Assess the performance bands for these statistics. For example: for low flows look at the performance for Q90 possibly in conjunction with that in Jul, Aug and Sep;

Because of the year-to-year variability of the climate of the UK (also called climate variability) it is possible that several climate time series differ while representing different plausible realisations of the climate. In addition, because knowledge of the physics of the atmosphere is limited and it is not yet feasible to accurately model small-scale climate features, it is now recommended that several climate models projections are considered together when assessing future projections in hydrology. For both reasons, an ensemble of climate models has been used to drive the FF hydrological models and generate an ensemble of FFH time series for each of the sites. The FFH ensemble is derived from the ensemble of Future Flows Climate (FFC) which contains information on both climate variability and climate modelling uncertainty; no single projection should be considered in isolation of the others as this might mask some important information given by the other ensemble members. Note that as FFC is derived from a climate model, the day-to-day sequencing of the climate and resulting flow is not the same as that of observed flow when directly comparing time series. Long-term statistics, such as the flow duration curves, should match more closely those derived from simulations using the observed climate.

Table Graphs Summary of the percentage differences in modelling the Two sets of graphs are shown. flow with observed and modelled climate (FFC time series; Mean monthly flows and flow duration curves note that FFC is a version of HadRM3-PPE where (observed and modelled climate) systematic biases in precipitation and temperature have been corrected, a snowmelt module has been applied and The upper pair of graphs gives a visual assessment of which has been downscaled at a hydrologically-relevant how well the long-term variability of observed flows is scale). Naming convention and units are as on the Front reproduced using the observed climate (1971-2005, or page. period of observed flow record if this is shorter). The lower pair shows how similar the flow simulated from the 11 Comparisons are made for a 30-year period representative modelled climate time series is to the flow simulated from of 1962-1991, called control. This gives an assessment of observed climate for the control period (1962-1991). the difference introduced by the use of modelled rather than observed climate when simulating flow. This is important because FFH time series, as they project into the future, can

Change in mean monthly flow and flow duration curve only be derived from modelled climate. These differences help identify two possible features: The lower pair of graphs shows the percentage change in mean monthly flow and flow exceeded x% of the time Systematic differences in the climate-hydrological chain for between two 30 year periods - the 1970s (1961-1990) a specific part of the regime; e.g. if all summer flows show and 2050s (2040 – 2069) for the 11 modelled climate a large difference, this might suggest that modelled summer series (FFC). The line of zero change is also shown. The climate (rainfall and/or potential evaporation (PE)) is range of change is indicative of uncertainty in the climate different from observed; modelling Systematic differences in the climate-hydrological chain for specific ensemble member; e.g. if all statistics associated with afixa show a large difference, this might suggest that afixa climate (rainfall and/or PE) has different characteristics from the observed climate;

Back page: Simulation from modelled climate climate modelled from Simulation Backpage: In both cases, the statistics should only suggest caution when interpreting the results of the whole FF ensemble, in particular if runs/periods with large differences in the control period are associated with a future signal different from the rest of the FF ensemble. Large differences in some statistics of the control runs should not be used to automatically reject one of the ensemble members.

Example of groundwater level catchment fact sheet – Washpit Farm (TF81/2A)

Future Flows and Groundwater Levels – SC090016 – Briefing Note for Borehole fact sheets

note briefing note summarises the meaning and relative importance of the statistics and graphs. It does not provide any interpretation for specific catchments/model. One fact sheet is delivered for each site and river flow / groundwater level model combination. If two hydrological models are used to simulate flow at the same site, two catchment fact sheets are provided for this site. Note that different models use different methods of calibration ranging from catchment specific to regionalised parameters. The advantage of a regionalised parameter model is to extend the climate range under which the model parameters are evaluated; this is particularly important in a warming climate for catchments where evaporation processes may change from a surplus of summer precipitation over evaporation to a deficit. The advantage of catchment calibrated models is that they are designed to reproduce well the local hydrological processes. The calibration method may affect the statistical measures of model performance. A fact sheet is divided into three parts. Top front page: general information section with the main physical characteristics of the borehole, its location and the availability of observed level data. Front: how well level time series are reproduced by the

Overview of the fact sheets and briefing briefing and sheets fact the of Overview models when using observed climate; or a measure of the confidence in the hydrological model. Back: how well level time series are reproduced by the models when using modelled climate; or a measure of the confidence in the climate/hydrological model combination. Both front and back must be looked at to fully understand the factors affecting the Future Flows Hydrology (FFH) time series. This is very important when the FFH time series are used to assess climate change impact on a catchment ecosystem. The FFH level time series are in m.

Table Graphs Summary of differences in modelling groundwater levels with The graphs illustrate how well the model simulates the observed climate. Differences (except Nash Sutcliffe) are level time series by plotting together observed and given in metres between statistics calculated from simulated simulated levels. and observed level time series. Two types of graphs are shown: Names represent the considered statistics; Lx = difference in Hydrographs of groundwater levels for the whole level percentile value (i.e. in level exceeded x% of the time); observed period and for two periods representative of Nash Sutcliffe measures if the modelled time series describes contrasting climatic conditions: (i) The 1975-1977 period the observed time series better than the long term average. A illustrative of a dry episode and subsequent re-wetting; (ii) value of 1 shows a perfect match. The 2000-2001 period illustrative of a wet episode. Three parts of the hydrological regime are of interest: (i) They give a visual assessment of the reproduction of Water balance and seasonality, (upper part of table); (ii) Low different hydrological processes under contrasting and high levels (level percentiles L90 to L10), (lower left); (iii) conditions (e.g. drying during the recession phase); Difference in range in level (%), (lower right). Mean monthly levels and level duration curves. These Differences include measurement errors but generally the graphs provide a visual assessment of how well the long- smaller the difference the better the model simulation. term variability and seasonality is reproduced by the

Front page: Simulation from observed climate observed from Simulation page: Front simulation.

Model performance Assessment of model performance is given for each of the statistics for the period of groundwater level observations using three Bands as defined in the Modelling Protocol. Interpretation of the performance Bands; (i) Define the purpose for which the Future Flows level time series are being used; (ii) Select the statistics most relevant to the purpose; (iii) Assess the performance bands for these statistics. Where several statistics have performance Band 2 or 3 then particular care should be taken in use of the FFH data. 1

Because of the year-to-year variability of the climate of the UK (also called climate variability) it is possible that several climate time series differ while representing different plausible realisations of the climate. In addition, because knowledge of the physics of the atmosphere is limited and it is not yet feasible to accurately model small-scale climate features, it is now recommended that several climate models projections are considered together when assessing future projections in hydrology. For both reasons, an ensemble of climate models has been used to drive the FF hydrological models and generate an ensemble of FFH time series for each of the sites. The FFH ensemble is derived from the ensemble of Future Flows Climate (FFC) which contains information on both climate variability and climate modelling uncertainty; no single projection should be considered in isolation of the others as this might mask some important information given by the other ensemble members. Note that as FFC is derived from a climate model, the day-to-day sequencing of the climate and resulting levels is not the same as that of observed levels when directly comparing time series. Long-term statistics, such as the level duration curves, should match more closely those derived from simulations using the observed climate.

Table Graphs Summary of the differences in metres in modelling Three pairs of graphs are shown. groundwater levels with observed and modelled climate Groundwater level surplus and deficit statistics (FFC time series; note that FFC is a version of HadRM3- (observed climate) PPE where systematic biases in precipitation and temperature have been corrected, a snowmelt module The top pair shows two additional statistics for applied and which has been downscaled at a hydrologically- groundwater levels, referred to as surplus and deficit. relevant scale). Naming convention and units are as on the These are measures of the severity of an extreme event Front page. above and below a threshold level. The surplus and deficit are expressed in units of m.days and calculated as: Comparisons are made for a 30-year period representative of 1962-1991, called control. This gives an assessment of t2 t2 the difference introduced by the use of modelled rather than S h t dt , D h t dt observed climate when simulating levels. This is important t1 t1 because FF time series, as they project into the future, can where, is the threshold groundwater level, and t and t only be derived from modelled climate. These differences 1 2 are the start and end time of the period of extreme help identify two possible features: groundwater levels, h(t), above or below the threshold. Systematic differences in the climate-hydrological chain for This results in a number of S and D values for a given a specific part of the regime; e.g. if all summer levels time-series, and upper and lower threshold values. The show a large difference, this might suggest that modelled distributions of these values, expressed as an empirical summer climate (rainfall and/or potential evaporation (PE)) cumulative distribution function, calculated from the is different from observed; observed and simulated groundwater level time-series are compared. The threshold values are defined as the 10th Systematic differences in the climate-hydrological chain for and 90th percentile value of the observed groundwater specific ensemble member; e.g. if all statistics associated level time-series (shown on the front page). with afixa show a large difference, this might suggest that afixa climate (rainfall and/or PE) has different characteristics Mean monthly flows and flow duration curves from the observed climate; (modelled climate)

Back page: Simulation from modelled climate climate modelled from Simulation Backpage: In both cases, the statistics should only suggest caution The middle pair shows simulated levels using the when interpreting the results of the whole FF ensemble, in observed and modelled climate time series (1962-1991). particular if runs/periods with large differences in the control Change in mean monthly flow and flow duration curve period are associated with a future signal different from the rest of the FF ensemble. Large differences in some The bottom pair of graphs shows the change in metres in statistics of the control runs should not be used to mean monthly levels and levels exceeded x% of the time automatically reject one of the ensemble members. between two 30 year periods - the 1970s (1961-1990) and 2050s (2041 – 2070) for the 11 modelled climate series. The range of change is indicative of uncertainty in the climate modelling.