Drought Monitoring in Hong Kong using Standardized Precipitation Index (SPI)

MC Wu KW LiHong 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

Meteorological

Temporal ScaleShort Long

Spatial Scale

Fine

Broad

Agricultural

Hydrological

Meteorological drought - measured the amount of dryness and the duration of dry period

Agricultural drought - based on the impacts to agriculture

Hydrological drought - refers to impacts on water supply

Standardized Precipitation

Index (SPI) should be used as a

universal meteorological

drought index for more

effective drought monitoring

and climate risk management

(2009)

A user guide on SPI was issued

in 2012

WMO Recommendation

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)

Precipitation (mm)

Cu

mu

lati

ve P

rob

abili

ty

SPI

1.0

0.8

0.6

0.4

0.2

0.0

0 20 40 60 80 100 -3 -2 -1 0 1 2 3

1.0

0.8

0.6

0.4

0.2

0.0

Normal~(0,1)

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

SPI-24Extremely wet

Extremely dry

Characteristics of a drought eventSP

I

-1.0

(i) Minimum SPI ≤ -1.0 (ii) Duration: starts from SPI falling below zero and ends with SPI rising to zero(iii) Magnitude: the accumulated SPI between the drought duration (iv) Intensity = Magnitude / Duration

Duration

Magnitude (area)

0

Time

1 2 3

4 5 67

8

9

11

10

12

13

1415

16 1718

Drought episodes in HK based on SPI-24

1963

196206-196508(39 months)

189509-190306(94 months)

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

Thank you for your attention!