Geoscience Reference
In-Depth Information
3.2.1.2 Yearly Distribution of Rainfall with 3 Years Moving Average
Value and Its Relation with Landslide Events
Starkel (
1972
) for the
rst time, observed the geomorphic effects of an extreme rainfall
event in the easternHimalaya (Darjiling). Froehlich et al. (
1990
) investigated the same
area (Darjiling Himalaya) and found that shallow slides and slumps on steep slope
segments occur when 24 h rainfall reaches 130
150 mm or continuous three days
-
rainfall totals 180
200 mm Researchers (Ghosh
1950
,
2009
; Nautiyal
1951
,
1966
;
Dutta et al.
1966
; Basu and Sarkar
1985
; Basu and Maiti
2001
; Paul
1973
; Sengupta
1995
; Basu and De
2003
;Pal
2006
; Maiti
2007
; Sarkar
2011
) carried out a demand
oriented studies in Darjiling Himalaya and identi
-
ed the role of rainfall in causing
landslide occurrences phenomena. The rainfall data since 1979
2009 was collected
from Selim Hill Tea Estate, Tindharia Tea Estate and Shiviter Tea Estate. Analysis of
yearly average rainfall since 1979
-
2009 (Appendix B, Table B.1) also suggests that
the year 1980, 1981, 1984, 1985, 1989, 1990, 1991, 1998, 1999, 2000, 2003, 2005,
2006 and 2007 are the year of above average rainfall (317.75 mm). The devastating
landslide occurrence in the 1984 and 1985, 1987, 1988, 1999, 2000, 2003, 2005,
2006, and 2009 are associated with the preceding year increasing trend average
rainfall. So, the landslide phenomena in the Shivkhola watershed are related with the
cumulative effects of precipitation and the changing nature of ground-water condition.
-
3.2.2 Evapotranspiration
Evapotranspiration, the leakage from immediate system, regulates water balance
and moisture availability. To understand the water loss from the corresponding
watershed monthly evapotranspiration is calculated following Jensen et al. (
1990
).
The equation for well-watered grass or reference ET
0
(Penman
1963
) and
converted to SI units (Jensen et al.
1990
) is:
D
D þ c
R
n
G
Þþ
c
T
o
¼
ð
D þ c
6
:
43 1
:
0
þ
0
:
53v
2
ð
Þ
e
s
e
d
ð
Þ
ð
3
:
1
Þ
where,
λ
ET
o
reference ET for a well-watered grass expressed as latent heat flux density,
MJ m
−
2
day
−
1
,
slope of the saturation vapor pressure curve in kPa/
°
C,
Δ
ʳ
psychometrics constant in kPa/
°
C,
net radiation in MJ m
−
2
day
−
1
,
R
n
heat flux density to the soil in MJ m
−
2
day
−
1
,
G
v
2
average wind speed at a height of 2 m in m/s,
e
s
saturated vapor pressure at mean air temperature in kPa,
e
d
saturated vapor pressure at mean dew point temperature also e
s
×
mean
relative humidity).
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