Agricultural Drought in Vietnam - Monitoring and Predicting Agricultural Drought: A Global Study - page 349

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

weather in the Mekong River delta is moderately warm most of the year.
Therefore rice can be cultivated at various times during the year, provided
sufficient irrigation is available. In this region, severe drought occurred dur-
ing April-June of 1983, 1992, and 1998, and during October-December
of 1958 and 1992 (Bui, 2000).

Probability of Agricultural Drought

Ta ble 27.3 presents the probability of monthly droughts in different re-
gi ons of Vietnam. In general, probabilities are high during January-April,
m oderate during October-December, and low during June-September, ex-
ce pt in north and south-central regions where drought probabilities are
hi gher than other regions. In addition, drought probabilities are higher for
so uthern regions than for northern regions.
In addition to monthly drought, studying dekadal (10-day) dry periods
is important for better monitoring of crop conditions. If rainfall during
a dekad is less than 30 mm, a dekadal dry period is considered to have
oc curred. Probability of dekadal dry periods was computed using a method
pr oposed by Frere and Popov (1982). The probability distributions of
se cond and third dekad dry periods for some major weather stations are
sh own in figure 27.2 (Nguyen, 1998, 2000, 2001).

[349

Line
——
-4.8
——
Norm
* PgEn

Drought Indices

[349

Annual and Monthly Drought Indices

M onthly drought indices are computed by Nguyen (2000):

K m = E m / P m [27.1]

w here K m is monthly drought index; E m is monthly evaporation; and P m
is monthly rainfall. The annual drought index is derived from the annual
ev apotranspiration 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
no rth mountain, central, and central highland regions) where the drought
in dex is < 0.3.
Vietnam has been divided into three groups of regions depending on the
dr ought characteristics (table 27.4). The first group comprises the north
pa rt of Vietnam (north mountain and midland and Red River delta regions)
w here the annual drought index is low and where moderate droughts occur
oc casionally during the dry season but severe droughts occur rarely. The
se cond group comprises the coastal part of north-central and south-central
re gions, where the annual drought index is not high, but droughts occur
du ring early- to mid-summer season. The southern part of this region is
m ore prone to severe droughts. The third group comprises the south of
V ietnam (central highland, southeast, and Mekong River delta regions),