Drought Indices

Annual and Monthly Drought Indices

Monthly drought indices are computed by Nguyen (2000):

where Km is monthly drought index; Em is monthly evaporation; and Pm is monthly rainfall. The annual drought index is derived from the annual evapotranspiration and precipitation and it generally ranges from 0.3 to 3.0 (table 27.4; equation 27.3). However, there are some places (in the north mountain, central, and central highland regions) where the drought index is < 0.3.

Vietnam has been divided into three groups of regions depending on the drought characteristics (table 27.4). The first group comprises the north part of Vietnam (north mountain and midland and Red River delta regions) where the annual drought index is low and where moderate droughts occur occasionally during the dry season but severe droughts occur rarely. The second group comprises the coastal part of north-central and south-central regions, where the annual drought index is not high, but droughts occur during early- to mid-summer season. The southern part of this region is more prone to severe droughts. The third group comprises the south of Vietnam (central highland, southeast, and Mekong River delta regions),

Table 27.3 Probabilities of drought occurrence in different months for different regions of Vietnam

Probabilities (%) of drought occurrence

Table 27.3 Probabilities of drought occurrence in different months for different regions of Vietnam

Probabilities (%) of drought occurrence

Region

Weather station

Jan.

Feb.

March

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

North mountain

Laichau

64

47

33

6

0

0

0

0

2

29

35

34

and midland

Sonla

50

59

57

14

2

0

0

0

4

42

54

45

Langson

59

73

57

22

3

0

1

3

8

27

52

57

Hongai

50

72

61

35

9

0

1

0

3

27

39

55

Red River delta

Hanoi

66

83

77

11

6

3

0

3

6

17

47

47

Ninhbinh

64

79

63

38

0

2

6

0

0

4

29

60

North-central

Vinh

53

67

54

45

12

20

37

10

0

5

9

31

Tuongduong

39

57

50

26

4

11

15

4

0

4

68

41

Hue

9

45

64

54

39

39

42

31

0

2

0

4

South-central

Danang

29

79

86

82

58

50

43

28

0

0

3

7

Bato

0

58

70

63

5

0

15

11

0

5

0

0

Phanthiet

100

100

98

75

11

4

2

2

2

10

67

94

Central highland

Playku

100

96

86

30

2

0

0

0

0

20

59

96

Buonmathuout

100

100

85

38

0

0

0

0

0

6

40

83

Dalat

98

88

55

2

3

0

0

0

0

2

27

83

Southeast

Saigon

96

96

83

58

14

0

0

0

0

0

18

72

Mekong River delta

Cantho

92

99

93

65

7

3

3

4

3

3

14

71

Phuquoc

83

79

55

17

0

0

0

0

0

0

5

5

Figure 27.2 Probability distribution major weather stations in Vietnam.

of two consecutive dry dekads and three consecutive dry dekads at some

Figure 27.2 Probability distribution major weather stations in Vietnam.

of two consecutive dry dekads and three consecutive dry dekads at some

Table 27.4 Agricultural regions and their drought characteristics

Annual Monthly No. of Drought drought drought months with Group of regions Region season index index (K) K > 5 (%)

North

North mountain

Nov.-April

0.3-

-0.8

5-8

0.5-3

and midland

Red River delta

Nov.-March

0.3-

-0.8

3-5

1-3

Central coastal

North-central

April-Aug.

0.3-

-0.8

3-8

1-2

South-central

Feb.-Aug.

0.3-

-0.8

20-60

5-10

Central highland

Central highland

Nov.-April

0.4-

-1.0

30-50

10-25

and south

Southeast

Nov.-April

0.4-

-1.0

6-50

5-10

Mekong River

Dec.-April

0.3-

-0.9

5-50

5-10

delta delta where the annual drought index is very high. Buduko (1958) developed a drought index, K:

Lr where R is solar radiation balance, r is rainfall, and L is energy needed for 1 mm of evaporation. Using this index, percentage of months with different intensity of drought was determined for different regions of Vietnam (table 27.5).

Humidity Index

Selianinov (1958) developed the following drought index, K:

where is r is rainfall during the crop-growing period, and £ t >10oC is the total active daily temperature exceeding 10°C during the same period. Based on the value of K, one can categorize the intensity of drought. If K is less than 1.0, it indicates the beginning of drought. A K value between 0.4 and 0.5 means moderate drought, and a K value between 0.2 and 0.4 indicates severe drought. A value less than 0.2 means desertic conditions.

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