Select Committee on Environment, Food and Rural Affairs Minutes of Evidence

Examination of Witnesses (Questions 560 - 579)



  Q560  Chairman: Let us move on to the question of probability versus intensity. One of the phenomena, again looking at it as a layman, is that all of our flood protection work is done on a probabilistic basis, preparing for 1:10, 1:30, 1:75 and so on but what this summer seemed to suggest was that we had the amount of rain in these probability possibilities visited in a very short space of time. I think what I am trying to understand is: do we have to adapt our thinking about the way that rainfall arrives? Are we going to see more intense events? Should that, therefore, be built in to the probability calculation, which in our earlier session for example has an impact on the ranking order of flood protection schemes? I am going to guess now. I do not know what 1:10 is but if a 1:10 arrived in five minutes instead of 24 hours, at that particular point there might be an awful lot of water to shift. It is just trying to understand the relationship between the two and do we need to change our predictive models to take this into account. Perhaps you could give me a little synopsis or education on that subject?

  Mr Noyes: I will deal with how we use probabilities in weather forecasts and I will hand over to John about what that means in a climate change scenario. I am sure colleagues on my left will also chip in. When we issue a forecast for the coming days, we use probabilities increasingly. That is particularly useful for exceptional weather events because it is helpful to convey to the people who have to respond—the emergency services, Environment Agency and so forth—a degree of confidence about what is going to happen. That could be snowfall, high temperatures, severe rainfall events. That is slightly different I think to where you are coming from in regard to return periods and probabilities of whether this thing will be more frequent in the future. I think it is useful just to clarify the difference. They are both used in the context of these events.

  Professor Mitchell: Just to clarify, I think the question you are asking is slightly different from the one I am going to answer, so please forgive me, but it is relevant. In terms of weather forecasting, the way we deal with uncertainty is to run a set of forecasts. The reason we need to do that is that we know both our models are imperfect and our initial data is imperfect. What we do is to run a set of forecasts or we can do a set of forecasts for slightly different nascent conditions, and that gives you a spread of outcomes. The usefulness of that is, I think, two-fold: it identifies if there is a small possibility of a very extreme event which will allow advance warning; and also whether the spread is narrow or wide gives you an indication of how confident you are in the forecast. Those are close to being related to the actual probability of those outcomes happening. It is a probabilities forecast in a very real sense. When it comes to climate change, we have uncertainties in models and we have no way of actually ranking those. What we tend to do is look, as we have for example done in UKCIP, and Dr West will say more about that, to using a variety of models and look at the spread those models give. That is really just a frequency distribution because we have not been able to assess which is a better model and which is not a better model. The way we use probability in climate change is different to the way we use probability in weather prediction. I think the question you were asking is whether in a 24-hour period it makes a difference if a given amount of rainfall falls over 24 hours or it ends up in 20 minutes. In terms of the model's ability to forecast in very short timescales, that will certainly stretch the model. The time-step in the model is typically 20 minutes to an hour and so you could actually evaluate rainfall totals, but I think you would have to question whether on that timescale the model is accurate enough. We do routinely keep six-hour totals, so we can look at a finer timescale. As you go to the finer timescale, I suspect the accuracy becomes more suspect just because of the shortcomings in the model and the fact that the model resolution is limited. If you can indulge me just a bit more, one of the issues that we have, I think, with the Environment Agency is in looking at rivers, and some of those are in quite small catchments. The forecast models at the moment are run at 12 kilometres and 4 kilometre resolution; 12 kilometres is much too coarse for a lot of the catchments and for some catchments a 4 kilometre scale is still not fine enough. We plan in the next two years to run a 1.5 kilometre model over the UK, which would allow for directly coupling in rainfall forecasts to the sorts of models that Dr Marsh's unit produces. You could actually have those combined rainfall and riverflow forecasts running in real time. There is still a question of scale but one could then, hopefully in the five years or so and after that is we get the enhancement to computing, run ensembles at this very fine resolution which would allow the Environment Agency to look at the risk of very high rainfall in small catchments. In the future, there are things that we can do better, both through better modelling and coupling that modelling into the river routing models and the flow models where the expertise is not in the Met Office but probably it would have to be done on the operational side. There is the potential in the future to bring those two things together. That would allow us to look at shorter timescales because I guess the timescale of Dr Marsh's model is hours. You could look in real time into the future at what the potential river flow was.

  Dr West: I think there is an issue with return periods and probability in terms of the threshold or the description of the event you are interested in. For a Boscastle-type event, it is a matter of perhaps in less than an hour how much rain falls in a small catchment. What I think we looked at last summer was the combination of that heavy rainfall over a long period, some months, and very intense rain. It is that combined probability that we need, if you like, to quantify. Different scales of event require different combinations of thresholds. For temperature, a threshold may be important if the temperature goes over a threshold for 20 minutes, or in terms of heat build-up it may be over several days. One of the ways we anticipate that users will interact with the next set of climate scenarios is that we want them to be able to describe what is an important threshold. For a seaside town, it may be sea level. For a flood management, it may be the amount of rain over four days. When they can define that threshold, and that requires them to look back in time probably at what has happened in the past to consider where they are vulnerable, then they can interact with these probabilistic scenarios and interrogate them in a way that gives them an answer to a question that is relevant to them. While not downplaying the value of increasing the accuracy and the amount of information in the climate scenarios, we feel it is very important that somehow we train decision-makers to interrogate these more intelligently than they perhaps do at the moment.

  Q561  Chairman: Do I interpolate from what you have said that you could have, if you like, a double probability: you could put two bits of probability together—the probability of a one in 75 year event or a one in 100 year event—and say that that event occurs with, if you like, a Boscastle intensity? In other words, you can say that we will have one of these every 100 years but there is a probability number that we can give to indicate whether it is going to be intense or not. Hopefully, I am not muddling the two things up too much there.

  Dr West: I think the answer is "yes". I think the probability is of your defined event, and in defining the event you will have to talk about intensity. If you are interested in how heavy the rainfall is at this instant, we can probably tell you that, but for any flood situation, it is how long it takes to get there.

  Q562  Chairman: When you define these one in some many year events, over what time period is the rainfall defined or measured?

  Dr West: I am going to ask the Hadley people to help with that. If we look at last summer's event, in terms of rainfall it is how much fell over a couple of months.

  Q563  Chairman: The thing that is still niggling at my mind is: if the events measured out over a 24-hour period but what we actually had was an event over two hours, it is still a 1 in 100 year event but it occurred over two hours and that is the bit that has put so much pressure on the existing systems to deal with the flood. What I am trying to understand is whether that compaction of the timescale of the 1 in 100 year event occurring is something which can also be modelled with any degree of accuracy?

  Dr West: I would suggest that so much rain falling in 24 hours may be a one in five year event. If it all falls in an hour, that is much more unlikely and therefore has a much longer return period.

  Q564  Chairman: True, but if we come back to the fact that the reality was that in these summer rains we had this very heavy concentration, when you look at Sheffield, an area of low flood risk probability, it was overwhelmed because a very unusual event occurred. Everybody is now casting round saying, "Can we predict when such events occur?" At the moment we have something that says a one in so many year event. I am not quite clear over what time period the rain is measured in terms of these. Professor Mitchell, you are nodding like a man who could give me an answer.

  Professor Mitchell: In terms of statistics, I think one can look at shorter-term rainfalls in observation. There are records of shorter-term variations than a day. A day happens to be a convenient unit of time which people use. I think the observational record exists for us to check on that. In terms of weather prediction models and whether they can predict on those shorter timescales, that is an interesting question. High resolution models of the sort that we have used on occasions recently, are certainly in a better position to do that than present-day weather forecast models. Where I would question it is when one starts looking at the models that we use for future climate, which are at 25 kilometre resolution at best, is whether you are smoothing out intense rainfall over the 25 kilometres squared, and so it may tend to underestimate those totals.

  Q565  Chairman: Dr Calver, did you have a contribution to help me?

  Dr Calver: With regard to recurrence intervals in hydrology, we associate that with the time period and so we would talk about the peak flow having a particular recurrence interval or we would say that it is a rarity event based on a 30-day period, or something like that. The time is an integral part of the number we come up with. There are one or two other comments that may be helpful to add on to the meteorology. Hydrology follows from meteorology but it also brings in other things like the ground surface, the type of surface, the antecedent conditions there, the topographic configuration and of course not least the flood defences. So the recurrence interval of a rainfall event when it becomes a hydrological event can be enhanced or reduced by the nature of the surface, the condition of it. If it is very wet, it may well be a higher recurrence interval event and vice versa. What do we do if we do not do real-time forecasting? Some of our research does go into the Environment Agency real-time forecasting, but we do a lot of hydrology based on Met Office and other output longer ahead into the future. We take the meteorology of climate models and they can, if you like, be used to "drive" hydrological models to derive possible future outcomes. In this case, there is that difficulty that John mentioned earlier, the difficulty of rainfall extremes. There is more accuracy expected of models in the future than now. For example, at the moment Defra's plus 20% allowance on peak flows for schemes over a 50-year scheme lifespan is based on that sort of hydrological modelling, following on from meteorological modelling. We can do it transiently (i.e. our model gives a time series through time) or we can jump to a future scenario for a certain date and we can do that in an ensemble set of model runs as well. I think in this context what is going to be a good analysis shortly to tell you about is river flood frequencies under climate change when we have the downscaling of UKCIP 08 scenarios—

  Q566  Chairman: I am sorry; downscaling of what?

  Dr Calver: It is to make the scale of the outputs for the hydrological models so that we can take into account some of the surface variability. Hydrology works in a very variable domain in space and in time. The advantage over what Defra is working from at the moment for its project appraisal guidance will include that we may—when I say "we" I mean the hydrological community—and I stress may, be able to make distinctions in different parts of the country. We will include some sort of quantitative analysis errors. They may arise from emission scenarios, from climate models, from hydrological models, et cetera. We would probably call them uncertainties, if you understand what I mean. What we also hope to do is compare those flood frequencies in the future with what one would expect anyhow from natural variability. An important point to make here is that many errors in assigning recurrence intervals, flood frequencies, et cetera, are not that different in percentage terms from some which are predicted in climate change

  Q567  Chairman: Just to interpret this flow of future hydrological information in terms of outputs, in other words I am a decision-maker and I am salivating at the thought of the interrelationship between hydrological models and meteorological information, and I am sensing that I am going to get some better information about what might happen on the ground in the future, what kinds of things can I expect from this improvement?

  Dr Calver: May I just say that it is better in the sense that more things will have been taken into account, but we often find when we quantify uncertainties that that does not automatically mean that they are narrowed. The sort of outcome you get is a time series of flow in various rivers across the country. One could interpret that as what we call a flood frequency curve and say the magnitude of the peak flow in the river against how often on average you are likely to get that flow. Hydrology and meteorological models are intimately linked. They are done in different ways in a real time sense, but please ask the Environment Agency for details in the longer planning sense. There is also feedback from the hydrology of the surface into meteorological models.

  Dr West: I think it is helpful to recognise that climate models deal with average values over perhaps 25 kilometres squared and very often the decision-maker is interested in that point, that time of day. What Ann referred to as downscaling is using the current relationship between the current climate and current weather on very short timescales, on very local timescales, and saying, "We assume that that relationship will be the same in the future". So you can produce a simulation in the future from which you can say, "Yes, if this river rises at this rate, this piece of land will be flooded", but is an abstraction based on assuming that the relationship between the weather and climate stays the same as it is now. So that will be available, but it requires somebody with the sense to take that information to hydrologists and convert climate information on a grid square basis into real flow information.

  Q568  Chairman: Who is going to do that?

  Dr Calver: There are various potential people who can do that, whether in the public sector research establishments or universities.

  Q569  Chairman: Is it something that should be done?

  Dr Calver: I understand it will be done, as far as I know.

  Dr West: It will not be done as part of what is described as UKCIP 08, which we will launch in October, but it allows people who want that done—Defra flood management, the Environment Agency, any consultant—to take that information and apply it to a particular situation. For example, Leeds City Council might want to know about the local flow. Somebody has to do that; it is not going to be provided for free as part of UKCIP 08 but the tools to do that will be the best available.

  Q570  Chairman: What does UKCIP 08 stand for?

  Dr West: Our programme is the UK Climate Impacts Programme. Working with the Hadley Centre and funded by Defra, we are producing in October a new set of future climate scenarios for the UK which, as John described, for the first time will have probabilistic type information. Instead of saying, "That is the number at that time period in that place", here is a range of numbers. What you do with those numbers requires you to have some knowledge of what is important. If you just want to report what the climate is at that time, you might pick a mid-value. If you are very risk-averse, you might want to know what is the level that might be exceeded at a 90% probability. Your attitude to your own threshold will determine how you look at the data. If you do not have that attitude to risk and knowledge about a threshold, the information is still there but you will not be making maximum use of it.

  Q571  Patrick Hall: If I understood what Dr Calver was talking about, it was about river flood risk.

  Dr Calver: It is, and I am totally aware that there are other types of flood risk.

  Q572  Patrick Hall: I just want to ask about another type of flooding which we have called earlier today flash flooding. Because of the likelihood of greater and more often intense rainfall episodes due to climate change, does it matter what the condition of the ground is in terms of flash flood risk? I have heard people say that the ground is so dry, it just ran off; and I have heard people say in other circumstances that the ground was absolutely soaked so that it had nowhere else to go and it just ran off. Does it matter what the ground is like in terms of intensity and flash flood conditions?

  Dr Calver: Yes. It is probably the rainfall intensity coupled with the duration that is more important but, yes, because otherwise a potential flood will not happen. Things like the ground conditions are modifiers beyond that.

  Q573  Patrick Hall: But both have been cited as explanations as to why a particular—

  Dr Calver: I am sorry, the wet and the dry. I think it is a bit like dry blotting paper that is very difficult to blot but, once it gets a little bit wet, it sucks up ink better. I am going to ignore the very dry conditions and say that if it is very wet, any rain will flow off; if there is capacity to take the water in, some of it will go into the soil. I believe there is an analogous position to the very dry blotting paper case, to do with soil pore water pressures.

  Q574  Patrick Hall: I think I can follow that.

  Mr Noyes: If you happen to live in a clay area, in the summer months if you have had a dry summer, the clay will form a very hard surface, which is almost like concrete, and so the water will run off quickly. Equally, if the clay is saturated in the winter, then the same thing is going to occur. There is an intermediate state between the two.

  Q575  Patrick Hall: That shows how complex these matters are.

  Dr Calver: The science is the spatial variability of what the rainfall is falling on and therefore the hydrological response. Of course, when you come to urban areas, there are other complexities and perhaps—others may wish to comment differently—these very intense rainfalls might be most danger in urban environments.

  Ms Yeates: It very much depends on the urban environment: the height of the gutters, the shape of them, the size of the sewers, how much has been paved over, how much green space there is. There are people that do this sort of modelling and this work looking at both what happens in the current situation, particularly focusing on areas that have already been affected where they have data, and then taking the climate information and looking forward to what might happen in the future, but it is very much dependent on the site and what the area is actually like in the first place., They have examples where what currently is a road or somebody's backyard suddenly becomes a very fast-flowing river during the rainfall period. If the rain stops a couple of hours later, it just becomes a pile of mud which is what was left after that rainfall.

  Q576  Paddy Tipping: What are people doing about it?

  Ms Yeates: The Pennine Water Group has I think moved to Sheffield University now.[13] They are one of the key researchers in this area. I believe Richard Ashley is still leader of that group.

  Q577  Paddy Tipping: So they are doing work on the natural environment and the interaction with the built environment?

  Ms Yeates: They are very much focused on the built environment and looking at what happens when the rain falls on to that built environment, recognising that if you have a road, obviously you are not going to get rainfall soaking into soil but you are going to get it running off into gutters, and looking at those sorts of aspects of it. Instead of saying, "We have a river basin and we have these areas to soak up water", they have different types of areas to soak up water and they are looking at how to manage that instead.

  Mr Drew: I am sorry I missed the early part of your comments. Can I look into the world of science fiction? There is something that I wanted to ask you. Is it complete science fiction at the moment actually to believe that one day if we were to get prognosis of a real threat to an area then we could either move that weather front in some way or possibly dilute the water? There would have been no problem if the rainfall that we had in Gloucestershire had either been slightly moved off over areas where there was not the same level of population or, more particularly, diluted so that the rainfall came down over a greater area.

  Chairman: That is climate modification.

  Q578  Mr Drew: That is a good term.

  Professor Mitchell: Climate modification or weather modification: I am quite sceptical about our ability to do that. I have just come back from China where I was told that there is something like 20,000 people employed to modify weather. They shoot something like silver iodide into clouds to modify the cloud structure. I think, even if we could do it, it is a very risky procedure because if you get it wrong—

  Q579  Mr Drew: If you get it wrong, you are sacked.

  Professor Mitchell: If you move it from Gloucestershire and it lands in Warwickshire, you can imagine all sorts of lawsuits occurring. Scientifically, I think the jury is still out. Why the Chinese invest so much in it, I am not sure. They obviously believe it works, but I think other countries are much more sceptical.

13   Note by witness: The Pennine Water Group (PWG) is an EPSRC funded Platform Grant centre dedicated to research into water and wastewater. It is based at the Universities of Sheffield and Bradford and is headed by Professor Richard Ashley as Managing Director and Professor Adrian Saul as Research Director. The PWG aims to advance engineering and scientific knowledge across all aspects of potable water, stormwater and wastewater service provision and management of associated assets. See for further information. Back

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