Storm Surge 2013 : One Year On - Part Three : Community Resilience

In Part 2 last week, the blog looked at some of the lessons learnt a year on after the 5 December 2013 storm surge in the Humber. This week the post come from guest blogger Jazmin Scarlett. Jazmin is a PhD student with a specialism in natural hazard response and mitigation, and offers her insights from her experience in how communities respond after natural disasters. In this post we expand our look, out of the Humber Estaury and southwards along the east coast of the UK, to Boston.

By Jazmin Scarlett

On the night of 5 December 2013 the Humber Estuary experienced its worst floods ‘since 1953.’ The main natural hazard that the country must co-exist with is flooding, and this post is going to discuss how despite its rather common occurrence across the country, we, as the British people, are not considering every factor when trying to mitigate and adapt to it.

In December 2014, £2.3billion was granted to the defence against flooding to protect 300,000 homes. We have known how bad flooding can get in this country and yet, in my opinion, the response has been rather slow. Even then, throwing money at the situation needs to be managed carefully and should address all the problems, not just the ones that everyone can physically see.

The Boston Stump (by Martin Clark)

The BBC Look North special report a year after the 2013 storm surge it showed how the people of Boston have been coping with some of the often unreported effects of flooding. Some people’s businesses were still suffering, some people’s homes were still being repaired, and some, unfortunately, were experiencing mental health problems. It is not unusual for mental health problems to arise after events like 5 December 2013, in fact it should be expected and mitigated for. It depends on a certain number of factors. One depends on factors, such as the actual experience the individual had: for example, farmers who lost their livelihoods and saw the deaths of their livestock in the 2003 East Gippsland bushfires in Victoria, Australia, experienced shock and post-traumatic stress (Whittaker et al., 2012). Those in Boston who lost properties and are still waiting for their lives to return back to normal told Look North that they are experiencing depression.

Another depends on the individual’s (and the community’s) coping strategies in terms of psychosocial resilience and physical mitigation strategies (Cashman and Cronin, 2008). Physical mitigation strategies usually involve engineering solutions such as the Thames Barrier, monitoring systems such as the Flood Alerts provided by the Environment Agency and land-use restrictions (potentially exacerbated by properties being built on floodplains). These physical solutions are far more obvious than psychosocial resilience, not just for flooding but for other hazards as well. Taylor (1999, in Cashman and Cronin, 2008) states that adaptations for community recovery from a disaster largely depends on simple explanations for the occurrence of an “inconceivable” event.

The Thames Flood Barrier (by Ian Capper)

It appears only when push comes to shove, when we lose properties, livelihoods, lives and money do we react to the hazard. Why must it be only when the hazard has occurred and caused all the damage that we say “we should do something about this.”

Another angle to this could be why can we not co-exist with the hazard? Kelman and Mather’s (2008) paper looks at how people living with volcanoes can become more resilient but I believe it rings true for flooding and other hazards as well. It details a ‘sustainable livelihoods approach’ in which they state that to live with the risk means accepting that the hazard is a usual part of life and that rather just surviving or reacting to the extreme event when it occurs, living with the risk allows the community to create and maintain habitats as well as livelihoods which might lead to the hazard becoming less of a danger and more integrated into day-to-day life.

We are a very resourceful species and certainly in this country have the knowledge of flooding impacts but why are we not more proactive about it? Members of the Department of GEES, University of Hull research various aspects of flooding, and other hazards. An aspect of this is education: I have always been passionate that all individuals at risk of hazards ought not be ignorant to that risk, and part of that is engaging communities in mitigating against our 'public enemy number one'.

There are psychological means to try and explain why people are ‘caught by surprise’ by a hazard, but in reality that should not be the case. I will give an example that I touched upon in my Masters' thesis.

It is a concept called 'saliency' – we make sense and prioritise our daily issues/threats. I do not think about flooding every day. Why should I? It not is looming over the horizon right now and I have other things to worry about. I have to think about managing my money and my disability. I worry about my grandmother. I hope my parents are coping with being parents to four newly adopted kids. I hope my best friend is doing alright in her new job. I concern myself with personal aspects of my life. Every person will be concerned about different things, based on what they value most. I value the wellbeing of my family and friends most. I myself, have been fortunate enough to not experience a flood so therefore, I will take no further action until the hazard is impending and will threaten the safety of my friends and colleagues.

Aerial view of Boston (by Richard Croft)

It is not all bad. The floods in Boston fostered social cohesion and a form of community resilience. In the face of adversity, neighbours who barely knew one another came together to use their own skills, knowledge and goodwill as a joint force to help ease and spread the stress of the situation and get everyone out of danger. After the event, the common ground these people shared is the experience of surviving the potentially life threatening situation. The social cohesion will now continue and hopefully live on to continue aiding one another when eventually the authorities leave and no longer offer the short-term recovery support. The aid could be physical: helping rebuild properties. People often fail to realise that counsel is just as important as physical aid. Local practices will experience an increased number of mental health related cases but sometimes, the social cohesion, the fact that the person was physically there, experiencing what they were experiencing, can be a useful and powerful recovery mechanism.

Social cohesion is also not a new concept to the government and researchers in this country. There are several governmental reports on it. An example is: “Guidance on the duty to promote community cohesion.” You just need to read the title to know how prominent it has been on the agenda. Even the House of Commons has a collection of reports on it and there are researchers looking at resilience against flooding here in the UK.

So if the government has been trying to be proactive in fostering social cohesion as a form of community resilience, why does it appear to be reactive? In all honesty, resilience is hard to measure unlike vulnerability and risk (even then they can be hard to measure). You will not know how effective the community resilience is or where to improve it until the hazard event occurs. I am researching resilience in volcanic environments and although there are indicators to help identify its presence, I do not know how resilient the communities are unless I observe it in a volcanic crisis. And that is a little dangerous.

I believe that the floods experienced in 2013, along with every other major flooding event this country has faced, have largely provoked a reaction to them. In the short-term, money is put in to physical mitigation against it and the short-term recovery programs. If this country is to overcome these ‘surprises,’ more effort needs to be made on being proactive, actively engaging in mitigation and adaptation between flood events. A lot could get done on the community scale in that time, yet it would require the support of everyone. However, people will turn mainly to what they believe will bring them security: the physical presence of flood defences.

Let the authorities deal with that, but get the community involved as well, it will foster another form of community resilience: social networks. Giving the community a sense of empowerment and confidence to prepare will help them mentally cope with the arrival of another flood event and hopefully, lead the country into being proactive and not reactive against our main natural hazard threat.

Whittaker J., Handmer J. and Mercer D. (2012) Vulnerability to Bushfires in Rural Australia: a Case Study from East Gippsland, Victoria. Journal of Rural Studies. Vol. 28. Pg. 161-173.

Cashman K.V. and Cronin S.J. (2008) Welcoming a Monster to the World: Myths, Oral Tradition and Modern Societal Response to Volcanic Disasters. Journal of Volcanology and Geothermal Research. Vol. 176. Pg. 407-418.

Kelman I. and Mather T.A. (2008) Living with Volcanoes: the Sustainable Livelihoods Approach for Volcano-Related Opportunities. Journal of Volcanology and Geothermal Research. Vol. 172. Pg. 189-198.

Storm Surge 2013 : One Year On - Part Two : What we Learnt

by @cloudskinner

This is the second post of our mini-series about the 5 December 2013 storm surge, and its legacy for the Humber region in particular. Last week's post highlighted some of the research that had been undertaken after the surge, and why this is important for understanding the future flood risk in the Humber, especially in the context of climate change which is predicted to bring bigger and more frequent storms, as well as a steadily rising sea level. This post looks at what we have learnt about the storm surge a year after the event, and summarises the presentations given at the 2014 Humber Conference held at Hull's Guild Hall in mid November, organised by the Humber Nature Partnership. You can view individual presentations using the links on the presenters' names.

Dr Susan Manson from the Environment Agency (EA), a co-author on the research highlighted last week, began the conference by providing some of the key scientific details about the storm surge. It has commonly been held that the devastating 1953 storm surge was the baseline for these events in the Humber - the 2013 storm surge is currently the largest on record, but there have been five other events larger than 1953 in between. The fact that the devastation and loss of life has never been as extensive is true testament to the defences and plans that have been invested in since that time. For the recorded tide from the EA gauge at Immingham, which has been recording since 1963, 2013 was the highest ever water level, and by some margin.

Immingham Dock Oil Terminal - The water level recorded here on 5 December 2013 was the highest on record (by "Chris")

Susan gave some figures about the surge. 116 flood warnings were issued. 1,170 properties were flooded around the Humber estuary, but the defences protected a staggering 156,000 further from the surge. It normally takes a tidal crest more than an hour to propagate from Spurn Point, at the mouth of the estuary, to Blacktoft Jetty along the River Ouse, but the surge covered this distance in just 15 minutes – described as “like a wall” by some.

40 km of defences were overtopped but most of them held, with only two points where the flood defences themselves were breached (south of Cleethorpes on the south bank, NE Lincolnshire, and near Skeffling, on the north bank, E Yorkshire). The EA has been busy strengthening and repairing defences as fast as they can. Philip Winn of the EA described how they are using X-Rays to check the integrity of defences to ensure they are up to standard, and how the defences at Alexandria Dock have been improved (all of the flooding in Hull City Centre emerged from overtopping the 1 km stretch here). Phillip also described how the EA are looking to the future, reconsidering the Humber Strategy drawn up before the flood and going to the Government with a request for £1bn to upgrade the estuary’s defences to a 1 in 200 year standard.

Flood defences being repaired shortly after the storm surge - Chowder Ness, near Barton-upon-Humber, on the south bank, N Lincolnshire (by Jonathan Thacker)

But for many people the misery of the storm surge continues. One of the worst affected places is the small town of South Ferriby on the south bank. The defences overtopped and flooded the majority of the houses there and depositing large quantities of silt and mud inside. One of my old school friends described on Facebook how she sat on the stairs watching the water rise as her children slept upstairs. For many it was over 6 months until they could move back into their houses.

The Cemex factory was very badly damaged and a full year after the flooding it still has not returned to production. Kevin Groombridge of the firm described how the flooding did not just bring water, but also sediment and salt. These clogged machinery and corroded the electrics of the site, which were all at ground level. Having never flooded in 75 years they nearly did not heed the flood warnings from the EA, but the Director of the site insisted the workforce move. It is possible that he saved numerous lives by that decision and thankfully that is just speculation.

The Cemex Cement Factory at South Ferriby (by David Wright)

Kevin described how immediately after the flood a ‘Blitz-spirit’ emerged among the staff, and how the factory manager had to buy new office furniture, laptops, stationery and even diesel generators on a credit card in order for them to continue working. Literally everything on the factory site had been destroyed.

Agriculture was also badly hit. Andrew Wraith of Savills UK, an Agribusiness, described some of the impacts that has struck them. On their Yokefleet Estate they have 34 residences and 22 of those flooded, and 1000 acres of their 2500 were flooded to a depth of 4 ft., and a green pea factory was flooded. Some of this land was flooded for 2 -3 weeks but was alleviated by pumps. A major issue they faced was soil erosion caused not by the flooding but the speed of the water draining away.

They lost many crops, both planted and stored, and Andrew put the cost of these losses in seven figures. But Andrew also said that they considered themselves lucky – when the surge hit they anticipated that their crop loss would be almost 100% but it was actually a loss of 5-10% of the yield. He put this down to the dry conditions prior to the surge allowing for effective drainage of the damaging salt water. The timing of the surge was also fortunate, as had it had been in the spring or the summer they would have felt a two year impact on yields.

Behind Hull's Tidal Barrier on 5 December 2013 - It the water level came within 40 cm of overtopping (by @Tom_Coulthard)

And it is this sense of being lucky that I want to end this post on. If you were one of the residents in South Ferriby, out of your home for the better part of a year, you will not feel lucky, and you weren’t. If you live in one of the 156,000 properties protected by the EA’s defences, a product of decades of work and investment, you were also not lucky but fortunate that we have invested in our excellent EA. But in many ways the Humber estuary could be described as being lucky as stories emerge of near misses and close calls. It was probably only the decision by one Director at Cemex that saved the lives of their workers. The tidal barrier at Hull came within 40 cm of overtopping and putting at risk hundreds of properties along the River Hull – if the surge was timed with the high tide, rather than 2 hours apart, it could have made the difference and spilled over. If the weather in the prior days had been wet then the salty flood water would not have drained as quickly as it did, this fact saving much farmland and properties from further damage.

In all the defences of the Humber were put under considerable strain by a massive, unusual, and largely unprecedented event, but came out on top. Just. Those responsible for them should be praised that they withstood the barrage, and that there was no loss of life. But we should not become complacent – we may never witness another event of that scale again in our lifetime, but as our climate warms, becomes stormier, and the sea level rises, the chances of another, or larger, storm surge in the Humber increases. We need to continually work to keep our defences ready.

Storm Surge 2013 : One Year On - Part One : Modelling the Surge

by @cloudskinner

This is the first post of a four week mini-series looking back at the storm surge of 5 December 2013. The surge caused extensive flooding along the East Coast of the UK but our focus has been on the area immediately around the Humber, and you can read our reaction shortly after the storm surge in this older post. Over the next few weeks we will be discussing the research that has been ongoing since the event, how it affected and continues to affect local residents and businesses, the community resilience that has been built and finally we consider the damage done to Spurn Point and its potential future.

This week the focus will be on a paper recently published by myself, colleagues at the University of Hull, the Association of British Ports (ABP) and the local Environment Agency (EA), which stemmed directly from the storm surge. The paper is free to view until 28 March 2015, after which you will require a subscription to Estuarine, Coastal and Shelf Science to view.

Hull's flood defences overtopping on 5 December 2013 (by @tom_coulthard)

Estuaries are very complex environments. There is a lot going on, beginning with the inputs of often several rivers, and the sea in the form of tidal flows. The relative influence of these on when and where the water and sediment moves in the estuary depends on the tidal cycle and the discharge levels of the rivers. It is a to and fro tug of war between these for influence within the estuary.

If that was not complex enough, there are secondary flows within the estuary. River water is fresh and sea water is salty, making the two flows a different density along with water that is mixture of the two in between. The two water types are often different temperatures too, again resulting in different densities and inducing flows from more dense to less dense regions. All of the flows are influenced by Coriolis forces, the deflection of water flow caused by the rotation of the Earth. The shape of an estuary also influences flow, and in combination with the influences above, estuaries like the Humber often show two channels along the bottom - one resulting from tides coming in and one from tides going out. Finally, overlain on these are the winds, waves and pressure influences of the weather.

This makes estuaries very complex and turbulent, and this turbulence can form a layer of thick sediment laden water to form along the bed - this basal mud layer clings to the bottom and effectively lubricates water flows along the estuary and shields the bed from erosion and deposition.  The salinity of the water also causes fine sediment to clump together in a process called flocculation which makes them behave like larger sediment particles.

It is commonly thought that to model the processes in an estuary then you need to account for all of these processes, but doing so is incredibly computationally expensive. It is possible to do, but even on expensive and powerful machines it often takes several days to model a single tidal cycle. Trying to use them to predict the future of an estuary several decades in the future would be almost impossible. Our approach was to use a simpler model, CAESAR-Lisflood, which has been widely used for a similar purpose on rivers for over a decade, to try and model the Humber Estuary successfully without all of this detail.

Animation showing the CAESAR-Lisflood model simulating the 2013 storm surge and associated flooding.

It was during this process when the storm surge struck and the focus of our research switched. We had already tested the model's ability to reproduce tidal flows - rapidly and at small timescales - so we soon tried applying the data recorded by ABP during the surge. This showed that the model could also reproduce the location and extents of the flooding on that night. This was using the latest information on the Humber's flood defences provided by the EA. The quickness of the model to process the data would make it suitable for producing numerous possible scenarios based on live and forecast data, and potentially help predict the extent of future flooding before it occurs.

This work is ongoing. Next week I will highlight how local residents and businesses were affected by the flooding, as discussed at the Humber Conference of December last year. If you wish to view this paper you can do so here.

Skinner, C. J., Coulthard, T. J., Parsons, D. R., Ramirez, J. A., Mullen, L., and Manson, S., 2015. Simulating tidal and storm surge hydraulics with a simple 2D inertia based model, in the Humber Estuary, UK. Estuarine, Coastal and Shelf Science. 155, 126-136 doi:10.1016/j.ecss.2015.01.019

Reconstructing Bronze Age environments at Hobbister, Orkney

by Michelle Farrell (@DrM_Farrell)

Last Monday (2^nd February) it was World Wetlands Day, and consequently my Twitter feed was full of stunning photographs of different types of wetland. Much was made of their role in alleviating flooding by acting as giant natural sponges which soak up water, as well as their biodiversity value and ability to store vast amounts of carbon. But despite all the wetland appreciation that I witnessed on Monday, there was very little mention of their importance to archaeologists and palaeoecologists.

Wetlands have a whole archaeological sub-discipline devoted to them. Wetland archaeologists are drawn to these damp, muddy environments because the waterlogged, anaerobic conditions inhibit microbial activity and often result in exceptional preservation of artefacts made from organic materials such as plant fibres, hair, wood and leather. These artefacts rarely survive on dryland sites, meaning that wetlands often preserve an additional level of detail relating to the everyday lives of our ancestors. Wetland archaeological sites also preserve plant and insect remains, which give us insights into the function and economy of the sites. Additionally, wetlands contain an archive of information relating to their own environmental history. Past changes in vegetation can be reconstructed from pollen grains, and the remains of single-celled organisms called testate amoebae provide information about past climates.

Wetlands were also important to people in the past. Across north-west Europe, deposits of precious metalwork were made in both wetland and dryland environments during the Bronze and Iron Ages. Artefacts deposited on dryland tend to be interpreted as valuables that were either lost or hidden with the intention of retrieving them in the future. Given that it would have been difficult to retrieve items from wetlands once they had been deposited, these objects are commonly thought to be votive offerings. In the past wetlands may have been viewed as wilderness and as being resistant to domestication, and it may be that these deposits represent an attempt to appease supernatural powers associated with these environments during times of perceptible environmental change. There is considerable palaeoenvironmental evidence for a shift to a wetter climate during the Bronze Age, particularly in upland regions of Britain. Deposition of valuable metalwork was perhaps an attempt to domesticate and control the changing landscape during this period of wetter climatic conditions.

To date, evaluation of this hypothesis has been hampered by a lack of palaeoenvironmental data relating to the findspots of votive deposits - and in many cases, the exact locations of the finds are not recorded. In Orkney in 2006, when I had just begun my PhD research with the aim of reconstructing Bronze Age vegetation and environmental conditions in the islands, peat cutters at Hobbister in Orphir uncovered a beautiful example of a late Bronze Age socketed axehead. Was it a votive deposit, and was there any palaeoenvironmental evidence for changing conditions at the time of deposition? An archaeological survey of the site had revealed various structures interpreted as the remains of a prehistoric field system, as well as several probable Bronze Age burial mounds. The discovery of a potential Bronze Age landscape buried by peat meant that the site would be useful for my PhD research, even if it turned out that I wasn’t able to say much about possible reasons for the axehead deposit.

 

Blanket bog at Hobbister, Orkney

Commercial peat extraction at Hobbister, Orkney

I analysed two peat cores from the site – one from the deepest area of deposits to ensure the fullest possible record was recovered, and one from as close as possible to where the axe was found. Analysis of the peats revealed evidence for a mixed economy based on arable cultivation and livestock rearing. The field system probably formed part of an ‘infield-outfield’ system, where fields nearest to a settlement (‘infields’) were cultivated more or less continuously by adding fertiliser in the form of dung, turf and seaweed, while those beyond (‘outfields’) were only cultivated on a temporary basis, being manured only through the folding of livestock in the summer prior to cultivation. Beyond the outfields would have been common pasture for livestock grazing. At Hobbister the pollen evidence indicates that this would have largely consisted of heathland, and there is evidence from charcoal contained within the peat that this was managed by burning to improve the quality of the grazing by encouraging dense growth of new shoots of heather, which contain more nutrients than old-growth heather, and by allowing grasses to grow in the gaps created by fire.

The remains of plants preserved in the peat at Hobbister suggest that the surface of the bog became slightly wetter during the later Bronze Age, at around 1200-800 BC. If the bog became wetter at this time, it is likely that the surrounding area did too. The suitability of land for farming would have been highly dependent on local hydrology, and increased wetness may have rendered the soil incapable of supporting cereal crops. Although the pollen evidence suggests that cereal cultivation at Hobbister continued at least until the Iron Age, local people would have been extremely aware of the gradual encroachment of peat onto formerly more productive land, and it is distinctly possible that they tried to halt these changes through votive deposition.

Distinctions have been drawn between votive deposits made in different types of wetland, with the suggestion that rivers, with their opposing banks, may have been viewed as boundaries dividing communities, and that deposition here might have been a display of power and prestige to other social groups. Bogs, on the other hand, may have been the focus for ritual acts aimed at reinforcing social cohesion within communities (Fontijn 2002; Mullin 2012). Orkney has no major river systems, but the highly indented coastline may have played a similar role in dividing communities here. Hence the deposition of the Hobbister axe could be seen as an attempt by local people to maintain community integrity during a time of perceptible environmental change.

In summary, wetlands are awesome - they preserve so much information about our past that simply doesn't survive on dryland archaeological sites. Next year on World Wetlands Day, we palaeoecologists and archaeologists need to get in on the act and promote the value of wetlands for understanding our heritage!


References:
Fontijn, D.R. (2002) Sacrificial landscapes: cultural biographies of persons, objects and ‘natural’ places in the Bronze Age of the southern Netherlands, c. 2300-600 BC. Analecta Praehistorica Leidensia 33/34: 1-392 (download for free here)

Mullin, D. (2012) The river has never divided us: Bronze Age metalwork deposition in western Britain. Oxford Journal of Archaeology 31: 47-57

This post is based on my recent paper, available here:

Farrell, M. (in press 2014) Later prehistoric vegetation dynamics and Bronze Age agriculture at Hobbister, Orkney, Scotland. Vegetation History and Archaeobotany. doi: 10.1007/s00334-014-0507-6

Modelling Rain to River over Africa - A Paper Review

By Chris Skinner @cloudskinner

I recently had my first research article published - "Hydrological modelling using satellite rainfall estimates in a sparsely gauged river basin: The need for whole-ensemble calibration". It has been accepted by the Journal of Hydrology, with which I am very pleased, and it is available to view for free until the 27th February 2015 here. If you are reading this after that date, I'm afraid you will need a subscription to the Journal to view it.

The problem the project was hoping to address is the issue of a lack of equipment available in many parts of the world which records rainfall. There are several methods of doing this, which I explain in an older post, but the most common ways are to use a network of rain-gauges or radar, both of which are expensive to install and maintain. For many nations, the measurement of rainfall is not a priority enough to invest in these networks but they would benefit greatly from having reasonable estimations of how much rain has fallen - it allows them to monitor water resources, forecast floods and droughts, and even predict how many crops will grow.

A Map of the Senegal River Basin. The rain-gauge network used for the study covered this wide area, yet the hydrological modelling focused on the Bakoye catchment (area in the south-east, containing both the Bakoye and Baoule rivers). (Image by Kmusser) 

"Senegalrivermap" by Kmusser - Own work, Elevation data from SRTM, drainage basin from GTOPO [1], all other features from Vector Map.. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Senegalrivermap.png#mediaviewer/File:Senegalrivermap.png

My research focused on a section of the Senegal River Basin, a large area with only 81 rain-gauges across the wider region with which to estimate rainfall, a density of 1 gauge per 7,000km². The density of the network is the equivalent of covering the UK with only 27 rain-gauges. There would be no way we could capture the complexity of our rainfall with so few gauges, and in reality the UK maintain a network of over 4,000 gauges with a density of 1 gauge per 76km². In addition to this, the UK also maintain 15 rain recording radar stations, yet none are available to the Senegal Basin region.

Currently, the best way around this and to fill in the gap is to use information from satellites. For continuous observation of the same area it is not yet possible to directly observe rainfall, but it is possible to monitor other factors that indicate rainfall, particularly the temperature of clouds. Unlike the UK, this area of Africa generally only receives one type of rainfall, so there is a strong relationship between the temperature of the tops of clouds and whether they are raining - if the cloud is below a specified temperature it is assumed to be raining - the longer it remains below that threshold, the more intense the rain is thought to be. This is all calibrated against the very little rain gauge data that is available.

Example of a processed satellite image showing the number of hours each pixel of the image is below the specified rainfall (here it is -20c). The number of hours is known as the Cold Cloud Duration (CCD) and this is related to rainfall. The rain gauges used for the project are represented by the white circles.

Unfortunately, as this is not directly observing rainfall it can be wrong on occasion, and especially so when a user tries to make use of it at a smaller scale than the density of the rain gauge network. When the estimates are used as information for models, such as a hydrological model to estimate river flows, the errors in the rainfall estimates are passed on to that model. What we do know, however, is just how wrong the rainfall estimate could possibly be and this allows us to try and represent this.

The increasingly common way of representing this, what scientists call uncertainty, in the rainfall estimate is to take the value of how wrong it might be and to randomly produce several hundred different versions of the possible rainfall - each different but equally possible based on the information available to us. The users takes this ensemble rainfall estimate and feeds each member individually into the hydrological model and produces an ensemble of river flow estimates. Statistics can be used to suggest the probability of river flows for each step in the record.

View of the across the River Senegal in the Kayes region, downriver and north of the study site. (Image by Bourrichon)

This is where my research came into the process. Hydrological models need setting up before they are used and this is done via a process called calibration. You need a period of the record with rainfall data and recorded river flow data, and you vary adjustable values within the model (these are called parameters) until you get the closest match between the recorded river flows and what the model estimates. You then test this against another period of recorded river flow that was not used in the calibration to test its performance (known as validation or verification).

It is good practice to calibrate a model using the same data you intend to drive the model with. For example, the model will not perform as well if you calibrate it using rainfall data as estimated by rain gauges, but subsequently run the model using satellite rainfall estimates. This poses a particular problem for when you intend to run a hydrological model using ensemble rainfall estimates. This has previously been performed using either using the original satellite estimate (disregarding its uncertainty), or an average derived from the ensemble members. However, I tried the calibration using all the ensemble members individually, but assessing the model performance with them as a whole - this is called whole-ensemble calibration and named EnsAll in the paper.

Graph showing the mean error from the Pitman model, run separately with each ensemble member using data for the period 1997-2005. The model was calibrated using data for 1986-1996 using the whole-ensemble method (EnsAll), the daily mean rainfall estimate from all the ensemble members (EnsMean), and the theoretical mean of the ensembles (EnsExp). EnsAll clearly produces less error than the other two calibration methods on this measure.

The parameter values produced using the whole-ensemble calibration produced more accurate river flow estimates from the ensemble rainfall estimates than those produced by the other methods. In fact, the whole-ensemble calibration was the only method to produce 'reliable' estimates during validation, with the other methods proving no better than making an educated guess based on the rainfall record for the period.

I do hope that this provides you with a better understanding of the paper. If you want to know more of the technical details it is all in there, along with some nice statistics and some graphs to make Dave Gorman weep with joy. The research represents only a very small facet of the problem, which will require many more small facets to solve rather than one big one. I hope to produce a few more.

The paper can be viewed for free until 27/02/2015 using this link. The reference for the paper is - 

Skinner, C. J., Bellerby, T. J., Greatrex, H., and Grimes, D. I. F., 2015. Hydrological modelling using satellite rainfall estimates in a sparsely gauged river basin: The need for whole-ensemble calibration. Journal of Hydrology, 522, 110-122