Agriculture Reference
In-Depth Information
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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
and midland
Nov.-April
0.3-0.8
5-8
0.5-3
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
Ce ntral highland
and south
Central highland
Nov.-April
0.4-1.0
30-50
10-25
Southeast
Nov.-April
0.4-1.0
6-50
5-10
Mekong River
delta
Dec.-April
0.3-0.9
5-50
5-10
[352
w here the annual drought index is very high. Buduko (1958) developed a
dr ought index, K :
Line
——
-1.0
——
Norm
PgEn
R
Lr
K
=
[27.2]
w here R is solar radiation balance, r is rainfall, and L is energy needed for 1
m m of evaporation. Using this index, percentage of months with different
in tensity of drought was determined for different regions of Vietnam (table
27.5).
[352
H umidity Index
Selianinov (1958) developed the following drought index, K :
r
0 . 1 t > 10°C
K
=
[27.3]
where is r is rainfall during the crop-growing period, and t > 10 o Cis
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.
C onclusions
Every year the agricultural area (especially of rice, the main food crop
of Vietnam) that is lost due to droughts exceeds 10,000 ha. The time
of commencement and termination of drought is different for different
regions in the country. Drought intensity is higher in southern Vietnam.
While summer droughts usually occur in the central regions, winter-spring
droughts occur in northern and southern Vietnam. Drought indices to
 
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