Impact Assessment of Hydrologic Attributes and Slope Instability - Semi-quantitative Approaches for Landslide Assessment and Prediction - page 96

phenomena to each class of all the hydrological factors responsible for slope
instability, landslide potentiality index value (LPIV) is determined by means of
ratio between number of landslide location to each class and total number of
landslide location in the basin. So, the study of all these attributes is very much
signi

cant in analyzing the nature of slope susceptibility of the basin in term of
geo-hydrology.

3.2 Climatic Attributes and Landslides

The climate of the Darjiling district is especially unique because of its position in
relation to the Tibetan land mass, the wide differences in altitude, the powerful
effects of the monsoon against the Himalayan barrier and the peculiar con

guration
of the neighboring mountains which deflects winds and affect local climate (rainfall
and temperature) On account of the hilly nature of the terrain of Shivkhola
watershed, there are sharp variations in rainfall even between nearby sections of the
mountain ranges. The precipitation during south west monsoon constitutes about
80 % of the annual rainfall, July being the wettest month. The variation in the total
rainfall from year to year is not much. On an average, there are about 120 rainy
days in a year. Temperature generally touches the highest level in May. January is
the coldest month of the year. Over the northern parts of the watershed the atmo-
sphere is highly humid throughout the year. In the north-western hilly tract the
relative humidity between 90 and 95 % during the rainy season. But in the low
lying tracts to the east and south-east, the relative humidity is slightly less. The
driest months are March and April when the relative humidity varies between 45
and 60 % (Table 3.1 ). The present study involves the analysis of some basic
information available either from field work or some reliable secondary sources.

The climatological information are collected from Tea Research Center (Kurseong),
Shiviter Tea Garden and Agriculture and Soil Conservation Of ce (Kurseong,

Darjiling).

3.2.1 Study on Rainfall

3.2.1.1 30 Years Monthly Average Distribution of Rainfall

and Assumption of Catastrophic Rainfall (mm) Year

'

Drainage basin is a proper spatial scale for analyzing hydrological parameters like
input of rainfall and resultant output of discharge in a systematic interactive
combination with other topographic and geometric attributes (Chorley 1969 ;
Strahler 1957 ). Amount of rainfall is one of the triggering factors for slope insta-
bility because it affects surface run-off, in

ltration, depth of the saturated soil and
thus influences soil-moisture condition. In ltration and evapotranspiration are