Transcript Drought Monitoring in Hong Kong using Standardized Precipitation

Drought Monitoring in Hong Kong
using Standardized Precipitation
Index (SPI)
MC Wu KW Li
Hong Kong Observatory
Historical Droughts in Hong Kong
• Serious droughts in
1963 & 1967
• Water restriction supply in 4 hours
every 4 days
Rainfall in Hong Kong
Average
2399 mm
Highest
3343mm
(1997)
Lowest
901mm
(1963)
Water Resources
• Building Reservoirs – Plover Cove (1968), High
Island (1978)
• Importing water from Dongjiang River in
Guangdong province
Year
Volume of importing water from
Dongjiang River
(billion cubic meters per year)
1960
0.227
1964
0.68
1976
1.09
1982
2.2
2012
8.2
Allocation of Dongjiang Water
Currently,
around 80% of
Hong Kong’s
total fresh water
demand are
supplied by
Dongjiang water
What is Drought?
• Different perceptions for different people living in
different climate zones
– Sahara desert: less than 10 mm / year
– Cherrapunji (northeast of India): more than 10000
mm / year
• Three main types based on its impacts on
different aspects:
 Meteorological drought
 Agriculture drought
 Hydrological drought
Types of Drought
Temporal Scale
Short
Fine
Spatial
Scale
Long
Meteorological drought - measured
the amount of dryness and the
duration of dry period
Meteorological
Agricultural
Agricultural drought - based on the
impacts to agriculture
Hydrological
Broad
Hydrological drought - refers to
impacts on water supply
SPI - a probability index
• Based on the cumulative probability of rainfall amount for any time
scale
• Fitted to a gamma distribution
• Transformed into standard normal (mean = 0 and s.d. = 1)
Cumulative Probability
1.0
1.0
Normal~(0,1)
0.8
0.8
0.6
0.6
0.4
0.4
0.20
0.0
20 40 60 80 100
Precipitation (mm)
0.2-3
0.0
-2
-1
0
SPI
1
2
3
SPI classification
SPI values
Category
+2.0 and above
Extremely wet
+1.50 to +1.99
Very wet
+1.0 to +1.49
Moderately wet
-0.99 to +0.99
Near normal
-1.0 to -1.49
Moderately dry
-1.50 to -1.99
Severely dry
-2.0 and less
Extremely dry
Advantages of SPI
• Simple – only rainfall is required as input
parameter
• Normalized – can be used to compare the
drought severity for areas with different
climates
• Various time scale – can be used to assess
different types of drought or for different
planning purposes
SPI in different time scales
• 3-month SPI (SPI-3)
– Provides information on short term and seasonal variation in ppt
• 6-month SPI (SPI-6)
– Characterizes medium-term trends in ppt
• 12-month SPI (SPI-12)
– Reflects the longterm ppt patterns
• 24-month SPI (SPI-24)
– Characterizes persistent drought
SPI for monitoring persistent droughts
in Hong Kong
Extremely wet
Extremely dry
SPI-24
Characteristics of a drought event
(i)
(ii)
(iii)
(iv)
Minimum SPI ≤ -1.0
Duration: starts from SPI falling below zero and ends with SPI rising to zero
Magnitude: the accumulated SPI between the drought duration
Intensity = Magnitude / Duration
SPI
Duration
0
-1.0
Magnitude (area)
Time
Drought episodes in HK based on SPI-24
189509-190306
(94 months)
196206-196508
(39 months)
13
4 5 6
1
2
3
9
7
8
16
10
12
11 1963
14
17
18
15
The 18 episodes in Hong Kong
between 1885 and 2012
Collaboration
• SPI-24 for HK as well as the cities in Dongjiang
River Basin such as Heyuen can provide useful
reference information for the future
management and planning of water resources
in Hong Kong
• Hong Kong Observatory is collaborating with
the Water Supplies Department in utilizing
climate information and products for drought
monitoring and water resource management