Stochastic and perturbation techniques to assess the
influence of climate change-induced multi-seasonal
drought on water resource vulnerability at Weir Wood
Reservoir, North Sussex, UK
1
2,3
2
1
By Christopher Harris , Geoff Darch , Robert McSweeney , Phil Jones and Meyrick Gough
4
1. Climatic Research Unit, University of Anglia
2. Atkins Water and Environment
3. Tyndall Centre for Climate Change Research, University of East Anglia
4. Southern Water Services Limited
Introduction
With climate-change water stress expected to increase in the UK throughout the 21st century (Bates et al., 2008), effective management of water resources on long time scales is a major
challenge. This study shows that information from weather generators can produce a best estimate of future water resource vulnerabilities to multi-seasonal drought on fine spatial scales that are
more severe than using a perturbation approach. In this case study the Environment Agency Rainfall and Weather Impacts Generator (EARWIG) (Kilsby et al., 2007) is applied to Weir Wood
Reservoir, North Sussex, to signify multi-seasonal drought in a baseline period (1961-1990) and, using change factors from the PRUDENCE project, the 2080s. A range of 4 climate models and 2
SRES scenarios (A1FI and A2) are used. The perturbation approach uses the A1B SRES scenario and change factors from 5 ENSEMBLES models.
Results: Stochastic Approach
Using the weather generator approach, substantial climate change-induced increase in potential evapotranspiration (PET) rate is identified for the 2080s compared to the baseline period (19611990) (Figures 1 and 2). The result is a projected increase in hydrological multi-seasonal drought severity, for droughts associated with a 4 degrees or more mean annual temperature rise.
Droughts are ranked according to antecedent 3-winter rainfall and PET totals. Worst case 2080s inflows in 13th-ranked multi-seasonal droughts are reduced to a total of 47.33 cumulative cumecs
over 30 months (a decrease of 83.88% from their equivalent rank in the baseline period) (Figure 3). This is largely a result of winter PET rates becoming high enough to reduce effective winter
rainfall, on which the reservoir relies almost exclusively for recharge.
The most extreme future multi-seasonal droughts are from the HIRHAM RCM driven by ECHAM4, which creates very high PET and less of a shift towards wetter winters and drier summers than
the other models. Climate models driven by HADAM3H represent the higher end of the uncertainties in Figure 3, with projected 13th-ranked droughts in the 2080s of a similar hydrological
severity as 1919-22. Yields are also substantially reduced from baseline levels in the 2080s at Weir Wood reservoir, and are lower than the notable droughts in the historical record (Figure 4).
Results: Perturbation Approach
Baseline 13th rank
PET (mm)
Top Right: Figure 2
Simulated cumulative flow (blue line) and simulated PET
(pink line) for the 13th-ranked drought event in the
HIRHAM_H 2080 with high (A1FI) emissions scenario
dataset.
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Bottom Left: Figure 3
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Instrumental cumulative flow (blue line) and PET (pink
line) for the 1919-22 drought.
Total inflow at Weir Wood Reservoir in the 2080s,
including baseline inflows for reference. Uncertainty
ranges are the highest and lowest values of the models
used, points are the mean.
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Bottom Right: Figure 4
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Yield (Ml/d)
1970-3 observed
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Rank 1
Rank 7
Rank 13
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1919-22 observed
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Top left: Figure 1
Estimated yields at Weir Wood Reservoir during the 21st
century. The 2080s data driven by both ECHAM4
(HIRHAM_E) and HADAM3H (HIRHAM_H) project that
yields during major droughts are likely to be lower than
the minimum yields in the droughts of the 1890s (8.9
Ml/d).
Ba
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Baseline 1st rank
150
Stochastic 1st rank high
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Stochastic 1st rank medium-ighh
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Stochastic 13th rank high
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1919-22 perturbed
Total inflow (cumulative cumecs)
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Stochastic 13th rank medium-igh
1970-3 perturbed
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Cumulative Flow (cumecs)
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PET (mm)
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Cumulative Flow (cumecs)
When the perturbation method is used, only small increases in hydrological drought severity at Weir Wood in the 2080s are apparent, and some models project an increase in inflow compared to
the historical droughts with which they are associated (Figure 3). Increased PET rates are significantly less pronounced in the perturbed drought events than the stochastic drought events. Indeed,
all winter rainfall remains effective and the reservoir is recharged to some extent even in the most extreme drought (the perturbed 1919-22 drought). This discrepancy between the two methods
may be a result of the stochastic droughts being specifically selected as 3-winter low rainfall events. In contrast, the historical droughts may not necessarily have three consecutive low rainfall
winters, instead perhaps including low summer rainfall totals, which are not significant to the recharge of the reservoir.
H
irh
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Climate Scenario
Discussion
As a tool to create entirely synthetic data, the weather generator is able to capture variability and change in droughts in the latter 21st century better than the perturbation technique. Crucially,
the periods of high PET within the synthetic dataset that are the stimuli for the major droughts of the 2080s are not apparent in the perturbed data. The increases in PET need to be further
investigated to determine exactly why they are occurring at such a greater rate in the weather generator approach than the perturbation approach. It may be the case that differences in the
methods of PET calculation account for some of the disparity.
Conclusions
With increases in global mean temperature potentially reaching and surpassing 4 degrees over the coming century, PET rates will increase more rapidly that we have observed in the past century.
The results of the stochastic approach presented here show PET dominating the increases in winter rainfall in the 2080s. PET and AET (actual potential evapotranspiration) may be somewhat
overlooked in multi-seasonal drought discourses, and relying solely on precipitation totals as indicators of drought events (which may show a gradual decrease in drought severity at Weir Wood
over the period covered by this project) will become impractical in the near future. Furthermore, with perturbation methods the current 'norm' for projecting future drought vulnerabilities, we
run the risk of underestimating the scale of hydrological droughts in our reservoirs as temperatures potentially increase to 4 degrees and beyond in the latter half of the 21st century, particularly
in south-eastern England.
Given the large-scale and high cost decisions that are made within water resource management, longer time horizons must be taken into account in decision making. Inconclusive evidence for
increased hydrological drought at Weir Wood in the 2020s is as a result of the full effects of anthropogenically-induced temperature rise on evapotranspiration rates not being felt until the midto-late 21st century. There is a risk of maladaptation if decisions were only based on scenarios looking 20-30 years ahead, as in current water resource planning methods.
For correspondence, please contact:
Christopher Harris
[email protected]
Geoff Darch, Senior Consultant, Atkins Water and Environment
[email protected]
Robert McSweeney, Environmental Scientist, Atkins Water and Environment
[email protected]
www.atkinsglobal.com/climate_change
References
Bates, B.C., Z.W. Kundzewicz, S. Wu and J.P. Palutikof, Eds. (2008) Climate Change
and Water. Technical Paper of the Intergovernmental Panel on Climate Change, IPCC
Secretariat, Geneva, 210 pp.
Kilsby, C.G., Jones, P.D., Burton, A., Ford, A.C., Fowler, H.J., Harpham, C., James, P.,
Smith, A., Wilby, R.L. (2007) A daily weather generator for use in climate change
studies. Env. Mod. And Software. 22, 1705-1719.
Plan. Design. Enable.