Hydrological Outlook UK

Transcription

1 Hydrological Outlook UK Period: From Rainfall: For the UK as a whole, rainfall during November was slightly below the long-term average, but as so often there were marked variations. Areas with below average rainfall included central and southern England, and in particular coastal areas of south-east England, and also south Wales, the Pennines, and the catchments of the Forth and Tay in Scotland. The Met Office 3-month Outlook issued on 24 th November indicated that for December, belowaverage precipitation is slightly more likely than above-average precipitation. For December-January- February as a whole, above-average precipitation is more likely than below-average precipitation. The probability that UK-average precipitation for December-January-February will fall into the driest of our five categories is between 10% and 15% and the probability that it will fall into the wettest of our five categories is between 30% and 35% (the probability for each of these categories is 20%). Issued on using data to the end of November 2017 SUMMARY In south-east England groundwater levels are likely to be below normal, and potentially exceptionally low in places, during December and for the next three months. In south-central and south-eastern parts of the UK river flows are likely to be normal to below normal during December and for the next three months. Elsewhere in the UK river flows and groundwater levels are likely to be normal to above normal in the same period. Across most of the UK, river flows and groundwater levels are likely to be normal to above for December and the coming three months. HYDROLOGICAL OUTLOOK UK River flows: In large areas of south-east England river flows were below normal during November, with some exceptionally low flows. Elsewhere flows were generally normal to below normal, with only a few rivers in Northern Ireland and northern Scotland recording above average flows. A marked contrast is likely in river flows for December, and beyond, with northern and western parts of the UK experiencing normal to above normal flows, while elsewhere flows are most likely to be normal to below normal. Groundwater: Groundwater levels in November remained below normal to notably low across the majority of the southern Chalk aquifer. Elsewhere, groundwater levels were within the normal range, with the exception of south-western Scotland where levels remained above normal. Although water levels in many aquifers are currently within their seasonal range, and likely to remain normal over one month, there is a notable exception in the Chalk, where Kent, areas north of London and East Anglia will almost inevitably remain below normal, and potentially exceptionally low through December and over the next 3 months, even if rainfall is significantly above normal. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website: Shaded areas show principal aquifers In south-east England, groundwater levels in the Chalk aquifer are likely to be below normal in December and for the next three months. River flows are likely to be normal to below normal in December and for the next three months, in south-central and south-eastern parts of the UK..

2 Hydrological Outlook UK About the Hydrological Outlook: This document presents an outlook for the UK water situation for the next 1 3 months and beyond, using observational datasets, meteorological forecasts and a suite of hydrological modelling tools. The outlook is produced in a collaboration between the Centre for Ecology and Hydrology (CEH), British Geological Survey (BGS), the Met Office, the Environment Agency (EA), Natural Resources Wales (NRW), the Scottish Environment Protection Agency (SEPA), and the Northern Ireland Rivers Agency (RA). Data and Models: The Hydrological Outlook depends on the active cooperation of many data suppliers. This cooperation is gratefully acknowledged. Historic river flow and groundwater data are sourced from the UK National River Flow Archive and the National Groundwater Level Archive. Contemporary data are provided by the EA, SEPA, NRW and RA. These data are used to initialise hydrological models, and to provide outlook information based on statistical analysis of historical analogues. Climate forecasts are produced by the Met Office. Hydrological modelling is undertaken by CEH using the Grid-to-Grid, PDM and CLASSIC hydrological models and by the EA using CATCHMOD. Hydrogeological modelling uses the R-groundwater model run by BGS and CATCHMOD run by the EA. Supporting documentation is available from the Outlooks website: Presentation: The language used in the summary presented overleaf generally places flows and groundwater levels into just three classes, i.e. below normal, normal, and above normal. However, the underpinning methods use as many as seven classes as defined in the graphic to the right, i.e. the summary uses a simpler classification than some of the methods. On those occasions when it is appropriate to provide greater discrimination at the extremes the terminology and definitions of the seven class scheme will be adopted. Exceptionally high flow Notably high flow Above normal Normal range Below normal Notably low flow Exceptionally low flow Percentile range of historic values for relevant month Disclaimer and liability: The Hydrological Outlook partnership aims to ensure that all Content provided is accurate and consistent with its current scientific understanding. However, the science which underlies hydrological and hydrogeological forecasts and climate projections is constantly evolving. Therefore any element of the Content which involves a forecast or a prediction should not be relied upon as though it were a statement of fact. To the fullest extent permitted by applicable law, the Hydrological Outlook Partnership excludes all warranties or representations (express or implied) in respect of the Content. Your use of the Content is entirely at your own risk. We make no warranty, representation or guarantee that the Content is error free or fit for your intended use > < 5 Copyright: Some of the features displayed on the maps contained in this report are based on the following data with permission of the controller of HMSO. (i) Ordnance Survey data. Crown copyright and/or database right Licence no (ii) Land and Property Services data. Crown copyright and database right, S&LA 145. (iii) Met Office rainfall data. Crown copyright. All rights reserved. Unauthorised reproduction infringes crown copyright and may lead to prosecution or civil proceedings. Further information: For more detailed information about the Hydrological Outlook, and the derivation of the maps, plots and interpretation provided in this outlook, please visit the Hydrological Outlook UK website. The website features a host of other background information, including a wider range of sources of information which are used in the preparation of this Outlook. Contact: Hydrological Outlooks UK Centre for Ecology & Hydrology Wallingford Oxfordshire t: OX10 8BB e: enquiries@hydoutuk.net Reference for the Hydrological Outlook: Hydrological Outlook UK, 2017, December, Centre for Ecology and Hydrology, Oxfordshire UK, Online, Other Sources of Information: The Hydrological Outlook should be used alongside other sources of up-to-date information on the current water resources status and flood risk. Hydrological Summary for the UK: provides summary of current water resources status for the UK: Environment Agency Water Situation Reports: provides summary of water resources status on a monthly and weekly basis for England: Flood warnings are continually updated, and should be consulted for an up-to-date and localised assessment of flood risk: Environment Agency: Scottish Environment Protection Agency: UK Met Office forecasts for the UK: HYDROLOGICAL OUTLOOK UK

3 Met Office 3-month Outlook Period: February 2018 Issue date: The forecast presented here is for December and the average of the December-January-February period for the United Kingdom as a whole. The forecast for December will be superseded by the long-range information on the public weather forecast web page ( starting from 1. This forecast is based on information from observations, several numerical prediction systems and expert judgement. SUMMARY PRECIPITATION: For December, below-average precipitation is slightly more likely than above-average precipitation. For December-January-February as a whole, above-average precipitation is more likely than below-average precipitation. The likelihood of impacts from heavy rainfall and high winds is greater than usual. The probability that UK-average precipitation for December-January-February will fall into the driest of our five categories is between 10% and 15% and the probability that it will fall into the wettest of our five categories is between 30% and 35% (the probability for each of these categories is 20%). Fig P1 PRECIPITATION CONTEXT: As discussed in the temperature section, for December the Met Office long-range prediction system and other systems from other prediction centres show an increased likelihood of weather patterns with northerly or easterly winds. These types of weather pattern favour drier-than-average conditions for the UK. The outlook for December-January-February overall shows an increased likelihood of weather systems from the North Atlantic and above-average precipitation (see figure P2). There still remains a chance of drier-than-average conditions, though this is less likely than wetter-than-average conditions. The outlook also indicates that the likelihood of impacts from heavy rainfall and high winds is greater than usual. Fig P2 Fig P3 This Outlook provides an indication of possible temperature and rainfall conditions over the next 3 months. It is part of a suite of forecasts designed for contingency planners. The Outlook should not be used in isolation but should be used with shorter-range and more detailed (30-day, 15-day and 1-to-5-day) forecasts and warnings available to the contingency planning community from the Met Office.

4 Fig T2 Fig T1 Fig T3 Met Office 3-month Outlook Period: February 2018 Issue date: The forecast presented here is for December and the average of the December-January-February period for the United Kingdom as a whole. The forecast for December will be superseded by the long-range information on the public weather forecast web page ( forecast/#?tab=regionalforecast), starting from 1. This forecast is based on information from observations, several numerical prediction systems and expert judgement. SUMMARY TEMPERATURE: For December, below-average temperatures are more likely than above-average temperatures. The likelihood of impacts from cold weather is greater than normal. For December-January-February as a whole, above-average temperatures are more likely than below-average temperatures. Impacts from cold weather remain possible, but they are less likely than normal. Overall, the probability that the UK-average temperature for December-January-February will fall into the coldest of our five categories is between 5% and 10% and the probability that it will fall into the warmest of our five categories is around 30% (the probability for each of these categories is 20%). Fig T1 TEMPERATURE CONTEXT: La Niña, the cold counterpart to El Niño, is developing in the Central and Eastern tropical Pacific Ocean. Long-range prediction systems indicate that it is likely to persist throughout the period of the outlook. La Niña moderately increases the likelihood of weather patterns that block the progress of mild air from the North Atlantic in early winter, leading to colder-than-average conditions; in late winter this tendency reverses, giving a greater likelihood of patterns with winds from the west and milder-than-average conditions. Predicted patterns of rainfall in other parts of the tropics increase the likelihood of the positive phase of the North Atlantic Oscillation (NAO) through this winter. The positive phase of the NAO brings milder-than-average conditions to the UK. The Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial winds in the stratosphere, is in an easterly phase. An easterly phase of the QBO tends to increase the likelihood of a negative phase of the NAO, which in turn increases the chances of below-average temperatures. For December, the Met Office long-range prediction system and systems from other prediction centres show an increased chance of weather patterns with northerly or easterly winds. This implies an increased likelihood of below-average temperatures. Nevertheless, there remains a smaller but still realistic chance of other weather patterns that would lead to above-average temperatures (see lefthand graph of figure T2). For December-January-February overall, long-range forecast systems mostly show increased chances of the positive phase of the NAO and milder-than-usual conditions. This is reflected in our outlook which shows an increased likelihood of above-average temperatures over the 3-month period (see right-hand graph of figure T2). As a result, significant cold weather impacts, such as from frost, fog and snow, are less likely than normal, although they are not ruled out. Fig T2 Fig T3 This Outlook provides an indication of possible temperature and rainfall conditions over the next 3 months. It is part of a suite of forecasts designed for contingency planners. The Outlook should not be used in isolation but should be used with shorter-range and more detailed (30-day, 15-day and 1-to-5-day) forecasts and warnings available to the contingency planning community from the Met Office.

5 Outlook based on hydrological persistence and analogy Period: February 2018 Overview Issued on using data to the end of November SUMMARY The outlook for December and for December to February is for normal to below normal river flows across most of the country, but please note that there are very few forecasts available for the northwest. RIVER FLOW ANALOGY 1-month flow outlook Outlooks from hydrological analogues are based on a comparison of river flow during recent months with flows during the same months in previous years at a set of approximately 90 sites from across the UK. These sites are depicted on the two maps. Years with observed flows that most closely resemble current conditions are identified as the best analogues and the outlook is based on extrapolating from current conditions based on these analogues. It is, however, often the case that a simpler forecast based on the persistence of river flow provides a better forecast than provided by analogy. This is particularly true for slowly responding catchments associated with aquifer outcrops. Both methods are considered at each site and the forecast from the method with the higher confidence is presented. A simple classification of flows is used (high, medium and low) as indicated by the colours of the dots, with the confidence 3-month flow outlook of the forecast being represented by the size of the dot. A tag on the dot indicates which method has been used in each instance. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

6 These figures provide insight into the hydrological analogue methodology for a set of sites from across the UK. In each of the time series graphs the bold black line represents the observed flow during the past nine months. The grey band indicates the normal flow range (the normal band includes 44% Outlook based on hydrological persistence and analogy Period: of observed flows in each month). The selected analogues are shown as thin lines and the trajectories that flows took in the following month are also shown. The forecast is shown as the dashed red line, and in each plot it states whether this has come from the analogues or has been generated on the basis of persistence. Site-based: 1 month outlook Issued on using data to the end of November RIVER FLOW ANALOGY No forecast available The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

7 These figures provide insight into the hydrological analogue methodology for a set of sites from across the UK. In each of the time series graphs the bold black line represents the observed flow during the past nine months. The grey band indicates the normal flow range (the normal band includes 44% Outlook based on hydrological persistence and analogy Period: February 2018 of observed flows in each month). The selected analogues are shown as thin lines and the trajectories that flows took in the following three months are also shown. The forecast is shown as the dashed red line, and in each plot it states whether this has come from the analogues or has been generated on the basis of persistence. Site-based: 3 month outlook using data to the end of November RIVER FLOW ANALOGY No forecast available The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

8 Outlook based on modelled flow from historical climate Period: November 2018 Overview Issued on using data to the end of November In central, southern, and south-eastern England, river flows are likely to be below normal to low in the majority of catchments over the next three months. Below normal to low flows are expected to persist in south-eastern England over the next six to twelve months. RIVER FLOW FROM HISTORICAL CLIMATE This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evapotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average river flow over the forecast period (1 to 12 months ahead), at each location. The simulations are generated by conceptual rainfall-runoff models from CEH (PDM and CLASSIC) and the EA (CATCHMOD) calibrated on observed or naturalised flows. The bar plot maps show the outlook distribution for 3, 6 and 12- month period for 28 catchments across England and Wales. Each bar plot represents the probabilistic distribution of the simulated river flow compared to the historical river flow, for the same n- month period. The probabilities fall within five categories, classified as: low, below normal, normal, above normal and high. This outlook is based entirely on historical sequences and therefore does not contain any knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current largescale atmospheric conditions and would therefore be unlikely to occur in the next few months. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

9 Avon Wye Severn Outlook based on modelled flow from historical climate Overview Lune RIVER FLOW FROM HISTORICAL CLIMATE Ure This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evapotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average river flow over the forecast period (1 to 12 months ahead), at each location. The simulations are generated by conceptual rainfall-runoff models from CEH (PDM and CLASSIC) and the EA (CATCHMOD) calibrated on observed or naturalised flows. The stack diagrams show the variation over time of the outlook distribution for a number of individual catchments. Each graph represents variation over time of the number of simulated river flows, in each month ensemble, that fall within each of seven categories: exceptionally low, notably low, below normal, normal, above normal, notably high and exceptionally high. The categories represent cumulative flow conditions, e.g. For 3-month, the simulated total 3-month flow compared to the historical 3-month flow distribution. The monthly variations can be compared to the long-term average distribution of river flows (shown as columns on the right of each timeline graph). Trent This outlook is based entirely on historical sequences and therefore does not contain any knowledge of the state of the atmosphere and ocean. It is hence possible that some of the historical sequences used might be inconsistent with current largescale atmospheric conditions and would therefore be unlikely to occur in the next few months. The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

10 1-month flow outlook Outlook based on modelled flow from rainfall forecasts Period: February 2018 SUMMARY: During December river flows are likely to be in the Normal range in southeast Scotland and southeast England. Elsewhere river flows are likely to be in the Normal range or above except in Anglian where they are more likely to be in the Normal range or below. Over the next 3 months river flows are likely to be in the Normal range in southeast England and in the Normal range and above elsewhere, except in Anglian where they are likely to be in the Normal range or below. Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast Overview Issued on using data to the end of November These forecasts are produced by using five members of the Met Office rainfall forecast ensemble as input to a water balance hydrological model to provide the five estimates of river flows shown on the left for one month and three months ahead. Regional forecast monthly-mean river flows are derived from the average of 1km river flow estimates within each region and ranked in terms of 54 years of historical flow estimates ( ). The five maps illustrate the wide range of possible flows and while there is a 50% chance of flows between the 1 st and 3 rd quartiles, actual flows may be more extreme than the flows derived using the highest or lowest rainfall forecasts. Key Exceptionally high flow Notably high flow Above normal Normal range Below normal Notably low flow Exceptionally low flow Percentile range of historic values for relevant month > < 5 RIVER FLOW FROM RAINFALL FORECASTS Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast 3-month flow outlook SCOTLAND HR Highlands Region NER North East Region TR Tay Region FR Forth Region CR Clyde Region TWR Tweed Region SR Solway Region ENGLAND N Northumbria NW North West Y Yorkshire ST Severn Trent A Anglian T Thames S Southern W Wessex SW South West WALES WEL Welsh NORTHERN IRELAND This method cannot currently be used in Northern Ireland The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

11 Outlook based on Modelled Flow from Rainfall Forecasts The regional maps illustrating the regional river flows for five members of the Met Office ensemble of rainfall forecasts give some indication of the range of possible river flows in the coming months. As noted previously, the actual flows could be more extreme than the flows generated by either the lowest or highest members of the rainfall ensemble. The bar charts (below) give further insight into the range of river flow forecasts by considering all members of the forecast rainfall ensemble. The regional bar charts show the percentage of ensemble forecasts falling in each of the flow categories as generated by the monthly-resolution water-balance model. As before results are averaged by region then ranked in terms of 54 years of historical regional flow estimates ( ). SUMMARY: During December river flows are likely to be in the Normal range in southeast Scotland and southeast England. Elsewhere river flows are likely to be in the Normal range or above except in Anglian where they are more likely to be in the Normal range or below. Over the next 3 months river flows are likely to be in the Normal range in southeast England and in the Normal range and above elsewhere, except in Anglian where they are likely to be in the Normal range or below. expected example Period: February 2018 Issue date: month 3-month 1- and 3-month variability SCOTLAND HR Highlands Region NER North East Region TR Tay Region FR Forth Region CR Clyde Region TWR Tweed Region SR Solway Region ENGLAND N Northumbria NW North West Y Yorkshire ST Severn Trent A Anglian T Thames S Southern W Wessex SW South West WALES WEL Welsh Exceptionally high flow Notably high flow Above normal Normal range Below normal Notably low flow Exceptionally low flow NORTHERN IRELAND This method cannot currently be used in Northern Ireland Percentile range of historic values for relevant month > < 5 RIVER FLOW FROM RAINFALL FORECASTS The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

12 Outlook based on Modelled Flow from Rainfall Forecasts Period: February 2018 Issue date: The maps illustrating the regional river flows for five members of the Met Office ensemble of rainfall forecasts give some indication of the range of possible river flows in the coming months. As noted previously, the actual flows could be more extreme than the flows generated by either the lowest or highest members of the rainfall ensemble. The tables below give further insight into the range of river flow forecasts by considering all members of the forecast rainfall ensemble. The numbers in the tables are the percentage of ensemble forecasts falling in each of the flow categories as generated by the monthly-resolution water-balance model. As before results are averaged by region then ranked in terms of 54 years of historical regional flow estimates ( ). SUMMARY: During December river flows are likely to be in the Normal range in southeast Scotland and southeast England. Elsewhere river flows are likely to be in the Normal range or above except in Anglian where they are more likely to be in the Normal range or below. Over the next 3 months river flows are likely to be in the Normal range in southeast England and in the Normal range and above elsewhere, except in Anglian where they are likely to be in the Normal range or below. 1- and 3-month variability SCOTLAND HR Highlands Region NER North East Region TR Tay Region FR Forth Region CR Clyde Region TWR Tweed Region SR Solway Region ENGLAND N Northumbria NW North West Y Yorkshire ST Severn Trent A Anglian T Thames S Southern W Wessex SW South West WALES WEL Welsh NORTHERN IRELAND This method cannot currently be used in Northern Ireland RIVER FLOW FROM RAINFALL FORECASTS 1-month ahead A NW N ST SW S T Welsh W Y CR FR HR NER SR TR TWR Exceptionally high flow Notably high flow Above normal Normal range Below normal Notably low flow Exceptionally low flow months ahead A NW N ST SW S T Welsh W Y CR FR HR NER SR TR TWR Exceptionally high flow Notably high flow Above normal Normal range Below normal Notably low flow Exceptionally low flow The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

13 Outlook based on modelled groundwater level & climate forecast Period: February 2018 Although water levels in many aquifers are currently within their seasonal range, and likely to remain normal over one month, there is a notable exception in the Chalk, where Kent, areas north of London and East Anglia will almost inevitably remain below normal, and potentially exceptionally low through December and over the next 3 months, even if rainfall is significantly above normal. Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast Overview Issued on using data to the end of November These forecasts are produced by running five members of the Met Office ensemble climate forecast through groundwater models of observation borehole hydrographs at 42 sites across the country. The sites are distributed across the principal aquifers. Based on the distribution of observed historical groundwater levels in a given month, seven categories have been derived for each site: very low, low, below normal, normal, above normal, high, and very high. The forecast groundwater level is assigned to one of these seven categories depending on where it falls within the distribution of the historically observed values. Key Percentile range of historic observed values for relevant month Exceptionally high levels > 95 Notably high levels Above normal Normal Below normal Notably low levels 5-13 Exceptionally low levels < 5 GROUNDWATER FROM CLIMATE FORECASTS 1-month outlook Lowest rainfall forecast 1 st quartile Median 3 rd quartile Highest rainfall forecast 3-month outlook The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website:

14 Rockley (SU17/57) Heathlanes (SJ62/112) Skirwith (NY63/2) Outlook based on modelled groundwater from historical climate Period: November 2018 Issued on using data to the end of November The likely persistence of notably or exceptionally low levels in the Chalk of parts of South Eastern England is well illustrated by the hydrograph of Little Bucket Farm, and to an extent by Washpit Farm. In other parts of the Chalk the 3 to 12 month outlook is close to climatology. Dalton Holme (SE94/5) Washpit Farm (TF81/2A) GROUNDWATER FROM HISTORICAL CLIMATE This outlook is based on monthly ensembles of historical sequences of observed climate (rainfall and potential evpotranspiration) that form input to hydrological models. The outputs are probabilistic simulations of the average groundwater level over the forecast horizon (3 to 12 months ahead), at each location. that fall within each the seven categories: exceptionally low, notably low, below normal, normal, above normal, notably high and exceptionally high. The monthly variations can be compared to the long-term average distribution of levels, which are shown as columns on the left and right of each graph. Little Bucket Farm (TR14/9) the atmosphere and ocean..it is hence possible that some of the historical sequences used might be inconsistent with current large-scale atmospheric conditions and would therefore be unlikely to occur in the next few months. The graphs show variation over time of the number of simulated groundwater levels in each monthly ensemble, This outlook is based entirely on historical sequences and therefore does not contain any knowledge of the state of The Hydrological Outlook UK provides an outlook for the water situation for the UK over the next three months and beyond. For guidance on how to interpret the outlook, a wider range of information, and a full description of underpinning methods, please visit the website: