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Nepal Hazard Risk Assessment
Premonsoon:
During premosoon season, moisture index is positive over mid and high mountains of the east, central
and western development regions except Jomsom district (moisture surplus over the northwest part of
the country is not considered due to lack of representative station over that region: Figure 3.8d). Out of
this, moisture surplus is highest over the eastern mountains and Lumle region (SMI > 20). Moisture
index is negative over Tarai and Siwalik belts out of which Soil moisture deficit is highest over the
Tarai/ Siwalik range of mid and far western development regions (SMI < -20). There is moisture deficit
over rest of the far western, Midwestern and western parts of western development regions.
Postmonsoon:
This is the season of lowest rainfall in western regions of Nepal. Moisture index is predominantly
negative (deficit) in this season. Except for Lumle region and far eastern portion of the country moisture
index is deficit in rest of the country (Figure 3.10d). Except for Lumle positive values of SMI is confined
below 5 in surplus regions where as over more than half of the country SMI is below -5.
(b) Comparison of decadal and climatological moisture index
Using the values tabulated in Table 3.8, decadal and climatological SMI maps are plotted. These maps
are analyzed to assess the decadal fluctuation of SMI at different region in different seasons. This study
is limited for the period of just over 3 decade so it doesn’t give a very clear picture of the decadal
evolution of SMI but later studies can build on it and monitor the SMI variation over the long term in the
context of climate change. In following section spatial pattern of three decadal moisture indices was
compared against climatological moisture index for all seasons.
Winter:
Figure 6 shows that there is moisture surplus in largest portion of the country in the period 1988-1997
while moisture deficit in largest portion of the country in the period 1988-2007 with the intermediate
condition in 1977-1987 period. In 1977-1987 and 1988-1997 period highest positive amplitude of
moisture index was observed in north-western part of the large portion of the country (SMI > 8)
compared to 1998-2007. Likewise, moisture surplus is noted over the far western, large portion of mid
western and parts of western region in 1977-1987, which extends further eastward and covers most of
mid western and mountainous region in east and central region in 1988-1997 period. In 1998-2007
period moisture deficit penetrated further westward and moisture surplus was confined to small region in
far and mid western regions. Moreover, area with highest deficit (SMI < -4) covered larger geographical
extent over Tarai and mid mountain in eastern region. These finding are again in agreement with decadal
variation of probability of drought occurrence in winter as computed from SPI (Table 3.5), where the
probability is least in 1988-1997 (which is almost equal that in the first decade) but increases sharply in
the final decade.
Premonsoon:
Comparison of three decadal and climatological spatial pattern of premonsoon moisture index doesn’t
depict considerable differences however some subtle differences are noted mainly over the western parts
of the country (figure 7). In 1977-1987 decade there was mild moisture surplus in the northwest portion
of the country which is absent in the following two decades, on the other hand out of three decades,
magnitude of moisture surplus is highest in 1988-1997 decade over the Lumle region. The areal extent of
severe moisture deficit was largest in the 1998-2007 decade and covered most of Tarai and Siwalik belts
of far and mid western regions. These findings are not consistent with decadal variation of probability of
drought occurrence in premonsoon as computed from SPI (Table 3.5), where the probability is least in
1998-2007 and highest in 1988-1997.
Monsoon:
No significant change in moisture index is seen between the three decades and the climatology during
monsoon season (Figure 3.9). This is apparently because of large magnitude of the moisture index and
only small fluctuations in decadal mean. This is in line with the relatively constant probability of
occurrence of monsoon drought for different decade and climatology as computed from SPI (Table 3.8).
This is understandable as monsoon is the dominant system with excessive rainfall.
Postmonsoon:
This is the season with moisture deficit over almost the entire country with surplus in Lumle pocket and
small parts of eastern region (figure 9). The areal extent and severity of moisture deficit is highest in the
1988-1997 decade. Severity of moisture deficit is least in 1977-1987 decade and aerial extent of
moisture surplus is also larger in this decade. The decade of 1998-2007 lies in between in terms of aerial
extent and severity of moisture deficit. However, moisture surplus was of highest magnitude in the 1998-
2007 decade over Lumle region. These finding are again in agreement with decadal variation of
probability of drought occurrence in postmonsoon as computed from SPI (Table 3.5), where the
probability is least in 1977-1987 but increased sharply in the 1988-1997 and finally dropped to almost
the first decade value in the final decade.
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