Severn Trent use of UKCP09 joint probability plots for climate change risk assessment As part of our climate change risk assessment, we have used joint probability plots from the UKCP09 tool to help us understand the in-combination effects of climate change on Water Service activities. Individual climate driver projections from UKCP09 show that future summers are expected to be warmer and drier, and winters are expected to be colder and wetter (relative to the 1961 1990 baseline period). But it is difficult to assess exactly how much the individual weather variables impact on our activities, and it has not been possible to define simple trigger levels. Instead. it is the incombination effects of the climate change variables that pose the most significant impact on our water supply capability. In the absence of simple thresholds for the individual variables, we have used past events to understand the implications of the climate change scenarios for future water supply. For example, high domestic demand, is driven by both low rainfall and high temperatures, which combine to produce high Soil Moisture Deficit levels. Dry soils and warm weather trigger higher demand as water consumers begin watering their gardens. Under extreme summer temperature conditions customers demand can dramatically increase, and at the same time our water resources can be under stress due to low rainfall. Figure 1 shows the record of total demand for water across our region since 1989, while figure 2 shows a record of summer rainfall and temperature over that period from a weather station in our region. The long term trend in annual average demand for water is a declining one, but with periods of high demand during years with extreme hot and dry summers. For example, we recorded a record high demand for water during 1995-96, which was a year of high summer temperatures and extremely low rainfall. We also experienced a significant peak week demand for water in summer 2003 and summer 2006, which were both years with a high summer peak temperature. Our experience of managing water supply operations during years with extreme weather conditions gives us a strong foundation for planning to minimise the impacts that climate change would have on water supply.
Figure 1: Severn Trent Water total water into supply since 1989 Figure 2: April to August cumulative rainfall and maximum average daily temperature at Sutton Bonnington, Leicestershire.
We have analysed the data for the in-combination effects of the climate change variable for summer by the 2050s and 2080s, looking specifically at the likelihood of increasing daily maximum summer temperatures alongside the likelihood of decreasing mean summer rainfall. As described, we identified that 2003 and 2006 are examples of two recent years when high summer temperatures and low rainfall led to very extreme peak demand for water. We have plotted how the temperature and rainfall conditions in those years compare with the climate change projections on the joint probability plots. We have also included data for 1995, as this was the last drought year in our region. The rainfall data shown in Table 1 is taken from the weekly updates we receive from the Environment Agency of areal rainfall across the Severn Trent Region. The long term average figures were provided by the EA and cover the 1961 to 1990 Met Office standard period. Table 1: Three-month average rainfall against long term average in the Severn Trent Region Year Jun to Aug monthly average (mm) 3 month average against LTA (%) 1995 17-71 2003 46-22 2006 48-19 The temperature data, as shown in Table 2, covers the Central England area and is taken from the Hadley website. The long term average is the 1961 to 1990 Met Office standard period. Table 2: Three-month average temperature against long term average in Central England Year Jun to Aug monthly average ( C) 3 month average against LTA ( C) 1995 17.4 +2.0 2003 17.3 +2.0 2006 17.2 +1.1 These data have been plotted on joint probability plots (Figure 3 and 4) to provide an indication of how past conditions which were known to put a stress on our water supply system compare to projections of what the future climate. The conditions experienced during our last drought year in 1995 are too severe to be plotted within the axis of the joint probability plots. We have also identified key months which might be useful for comparison. The months with the greatest difference between monthly average and the LTA are shown in Table 3. However, the change in precipitation in these key months against the long term average
does not fit the scale of the probability plots, implying that the actual conditions experienced during these months are still extreme when compared to the UKCP09 projections. Table 3: Difference between key month actual and long term average in the Severn Trent region Year Key Month Month average temperature against LTA ( C) Month average rainfall against LTA (%) 1995 August +3.4-86.6 2003 August +2.5-74.6 2006 July +3.7-8.1 It should be noted that during July 2006 we experienced our highest peak monthly demand. Figure 3: 2050s Medium emissions summer temperature vs summer precipitation
Figure 4: 2080s Medium emissions summer temperature vs summer precipitation The conditions experienced during 2003 are closest to the most likely projection of temperature and rainfall under the 2050s medium emissions scenario. This implies that the extreme operating conditions experienced during 2003 could become the normal operating conditions by the 2050s. The levels of rainfall in 2006 are close to UKCP09 s most likely projections for both the 2050s and 2080s, however, temperatures are likely to increase across the summer to a level higher than experienced that year. This in-combination effect of hotter, drier summers would cause significant stress to our water supply and distribution infrastructure.