How Would London Deal With Drought?

This essay is based on the Greater London Authority’s Climate Change Adaptation Strategy, which can be found here

The London Climate Change Adoption Strategy was published in 2008 by the Greater London Authority (GLA) and outlines the likely effects of global climate change on London, with particular reference to risk assessment and understanding, long-term management, emergency planning and public policy issues. Drought prediction and water management in the Thames Valley is given emphasis as a very serious concern for the future sustainability of London. Several authors, (notably: Hennessy et al. 1997; Blenkinsop and Fowler, 2007 and 2007a; Hirabayashi et al. 2008; Hulme et al. 2002 and May, 2008) have attempted to quantify the effect of greenhouse gas induced climate change on precipitation frequency and intensity in the near and distant future under various scenarios. The general consensus is that both the frequencies and the length of dry spells are likely to increase in the future. However, drought variability over the past century has changed very little (Hughes and Saunders, 2002; Hisdal et al. 2001 and Easterling et al. 2000) indicating that perhaps more understanding of both drought and climate change processes, as well as the intrinsic relationship between the two, is required.

Hisdal et al. (2001); Hughes and Saunders (2002) and Easterling et al. (2000) all conclude that whilst there is no indication that drought frequency and severity have increased across Europe over the last 100 years it is important to understand that the quality, drought parameters and spatial and temporal resolutions of the data strongly influences these results. Recently, several authors have used statistical and remote sensing models to illustrate and predict the frequency and severity of extreme events, such as drought, under various climate change scenarios. Notably, Hulme et al. (2002) predicts that whilst winter precipitation under all IPCC Special Report on Emissions Scenarios (SRES) scenarios (low, medium-low, medium-high and high) is likely to increase by a maximum of 30% by 2080, summer, autumn, spring and the overall annual average rainfall is likely to decrease considerably. Their findings are summarised in the table below.

SRES Emission Scenario
Summer Precipitation (% change)
Winter Precipitation (% change)
Annual Average Precipitation (% change)
Low
-20 to -30
+15 to +20
0 to -10
Medium – Low
-30 to -40
+15 to +20
0 to -10
Medium – High
-40 to -50
+25 to +30
0 to -10
High
> -50
+25 to +30
0 to -10

Whilst precipitation frequency during winter is predicted to increase, its intensity (amount of rainfall, per unit of time, per unit of area) is also set to increase, as shown by the figure below, from May (2007). This is significant as more intense rainfall events are likely to be more localised, shorter, have larger raindrop sizes and facilitate more rapid run-off processes which may not necessarily contribute to groundwater recharge.

Similar findings were reported by Frei et al. (1998). A warming of 2°C was predicted to increase the frequency of heavy (> 30mm day-1) precipitation events by 20%. Correspondingly, Easterling et al. (2000) concluded an increase in both one day and ‘multiday’ intense precipitation events would be ‘very likely’ across Europe by the end of the 21st Century.

Under all SRES scenarios, Hulme et al. (2002) also predict a significant increase in annual, summer and winter temperatures, with the greatest increase occurring during the summer months. This increase, coupled with the warming effect of the urban heat island (UHI) would serve to further exacerbate evapotranspiration and increase public water demand, putting further pressure on London’s water resources.

Water Resources, Demand and Drought Management in London
An annual average of 690 mm of rain falls over the Thames catchment. Of this, 455 mm (66%) is lost by evapotranspiration. Of the remaining 235mm, 129 mm (55%) is abstracted (the highest proportion of any catchment in England) and 105mm (45%) is allowed to flow back into rivers. 80% of London’s water is stored in reservoirs around the city after being extracted from the Thames and the River Lee. The remaining 20% is abstracted from groundwater stored in the chalk aquifer below the city. Groundwater recharge, which replenishes both the aquifer and the river network, occurs during the winter, when rainfall is at its highest and evaporation is at a minimum. Londoners, due mainly to increased prosperity and lower occupancy densities, consume 18 litres more water per day on average (168 litres person-1) than people in the rest of the country, yet the Thames region has a considerably lower water availability (265 m3 person-1 year-1) than the rest of England and Wales (1,334.1 m3 person-1 year-1). Although this clearly illustrates the value of the limited water resources in London, a further 600 million litres per day is lost through leakage caused by the aging water network, subsidence of the clay strata on which the network is laid, vibrations from transport and pipe corrosion.

The GLA report highlights some of the key areas of drought vulnerability in London and attempts to address these issues by identifying the risks and possible mitigation and adaption strategies available. Whilst addressing these issues is a commendable step towards sensible drought management, it is important to realize that these are merely theoretical concerns hypothesised on the basis of contemporary drought frequencies and intensities. Future drought events in London should be considered relative to the wider context of climate change across both the UK and the world. The GLA report, whilst going some way to documenting policy and planning initiatives, lacks the quantitative drought prediction capabilities in the vein of Hulme et al. (2002) and others.

If the predicted decrease of between 20% and 50% in summer rainfall in the UK due to climate change holds, this would put unprecedented pressure on London’s water network. Also, an increase in the intensity of winter precipitation would not facilitate the efficient replenishment of the groundwater store, further exacerbating drought conditions during the summer months. Whilst the UHI effect would undoubtedly intensify temperature extremes between the city and its surrounds, its effect on drought events seems uncertain. Increased hygroscopic pollution and convective uplift associated with urban canopy layer would serve to seed cloud formation and augment convective rainfall consecutively. The small increase (c.10%) in precipitation would be negligible when considered relative to the negative effect predicted to be caused by national and global climate change and would mainly affect downwind areas. Certain London-specific factors, such as the extremely high abstraction rate coupled with the below average individual water availability, the use of CSOs and river-fed cooling systems in power plants as well as leakage from aging pipes may also serve to intensify the social, economic and environmental implications of a drought event. The GLA report provides a concise policy framework for drought management but operates within the theoretical structure of past and/or present physical drought impacts. It also lacks any substantial quantitative drought prediction models and fails to identify a timescale for change which could be used to realistically assess and address the risks of drought in the City.

References:
Blenkinsop, S. and Fowler, H.J. (2007) Changes in drought frequency, severity and duration for the British Isles projected by the PRUDENCE regional climate models. Journal of Hydrology. 342 (50 – 71)
Blenkinsop, S. and Fowler, H.J. (2007a) Changes in European drought characteristics projected by the PRUDENCE regional climate models. International Journal of Climatology. 27 (1595 – 1610)
Easterling, D.R., Meehl, G.A., Parmesan, C., Changnon, S.A., Karl, T. R. and Mearns, L. O. (2000). Climate Extremes: Observations, Modelling and Impacts. Science. 289 (2068 – 2074)
Frei, C., Schär, C., Lüthi, D and Davies, H.C. (1998). Heavy precipitation processes in a warmer climate. Geophysical Research Letters. 25 (9) (1431 – 1434)
Greater London Authority (2008) The London climate change adaptation strategy: Draft report. London: Greater London Authority.
Hisdal, H., Stahl, K., Tallaksen, L. and Demuth, S. (2001). Have streamflow droughts in Europe become more severe or frequent? International Journal of Climatology. 21 (317 – 333)
Hughes, B. L. and Saunders, M. A. (2002). A drought climatology for Europe. International Journal of Climatology. 22 (1571 – 1592)
Hulme, M., Jenkins, G. J., Lu, X., Turnpenny, J. R., Mitchell, T. D., Jones, R. G., Lowe, J., Murphy, J. M., Hassell, D., Boorman, P., McDonald, R. and Hill, S. (2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Norwich:  Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia.
May, W. (2008). Potential future changes in the characteristics of daily precipitation in Europe simulated by the HIRHAM regional climate model. Climate Dynamics. 30 (581 – 603)
Met Office (2008). Microclimates. [Online] Available at: http://www.metoffice.gov.uk/education/secondary/students/microclimates.html. (Accessed on the 12 Feb 2011)

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