Impact of the Darjeeling-Bhutan Himalayan front on rainfall hazard pattern

Multiscale interaction between monsoonal circulation and the local topography causes the southern front of the Darjeeling-Bhutan Himalaya to receive one of the highest annual rainfalls (3000-6000 mm) and most frequent heavy rains (up to 800 mm day(-1)) along the whole southern Himalayan margin. An examination of the patterns of annual rainfall, rainfall concentration, overland flow generation and slope instability indices in the Darjeeling-Bhutan Himalaya for 1986-2015 indicates that the mountain front disturbs rainfall gradient between the Bay of Bengal and the Tibetan Plateau. The results show that the precipitation concentration indices are lowest at the Himalayan front where the annual rainfall and the number of rainy days are highest. The Himalayan front has the highest predisposition to produce overland flow compared to adjacent foreland and the mountain interior. The average probability of the rainfall initialising the shallow landslides increases from 0.6% for a 1-day rainfall threshold of 144 mm to 6.1% for a 4-day rainfall threshold of 193 mm in the study area. The highest probability (up to 10%) of 2-day and longer low-intensity storms at the mountain front indicate that its area is threatened with particularly larger and deeper landslides. The multivariate regression analysis reveals statistically significant linear relationships of rainfall hazard indices with elevation and the distance to the mountain front in the mountain foreland and Himalaya, respectively. Regionally, the Darjeeling Himalaya reveals lower values of rainfall hazard indices, in comparison to the Bhutan Himalaya.