Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 49

60

W1

W2

W6

W7

40

D1

D6

W3

W5 W4

D5

D7

D4

D3 D2

20

0

1080 1455 1900 Cal AD

1200 1000 800 600 400 200 0

Number of layers (counted from top)

Figure 2.13. Annual record of rainfall and drought, Shihua cave, Beijing, China
from a speleothem over the past 1100 years (from Liu et al ., 1997). D = drought and
W = wet.

consists of a light coloured zone at the base of the band, which is likely to have
been deposited by old water, and a dark part at the top of the band deposited by
fresh water. They developed a time series comparing individual layer thickness
with an historical index of drought and flood (Fig. 2.13)andtheinstrumented
record of precipitation from AD 1951 to 1980. Both records agreed on the years
of high rainfall providing confidence in the interpretation of the older part of
thestalagmite banding record. The thickness of individual layers was taken as a
proxy for the level of annual precipitation. They found several peaks in annual
rainfall and years of drought over the past 1100 years. Seven episodes of drought
were identified and spectral analyses highlighted several climatic cycles with
periodicities of 136, 50, 16--18, 11 and 5.8 years. There was also evidence for
possible millennial scale variability.

Conclusion

Palaeodrought records provide one of the more accurate and informa-
tive archives of the long-term nature of an extreme natural event. Annual layers
in tree rings, speleothems and lake sediments show that droughts, both in terms
of their severity and duration, can be highly variable over time. One of the clear
messages from these records is that 20th Century droughts are a poor reflection
of the nature of this hazard in previous centuries. Droughts of the late 13th and
16th Centuries in the USA for example were more prolonged and severe than
those of the 20th Century and Dean (1997) has recognised that droughts here,