42-Rainfall Contribution in Bhagirathi River near Gangotri Glacier Snout, Gomukh, Western Himalayas, India, Using Stable Isotope.

Abstract

Gomukh is the snout of Gangotri Glacier located at an altitude of 4000 m from where the snow/glacier meltwater stream known as Bhagirathi river emerges out. Stream and rainfall samples were collected for stable isotopes (8180 and 8D) analysis during ablation period (May to October) during the years 2004, 2005 and 2006 at site Bhojwasa, 3 km downstream of Gomukh. The isotopic characteristics of stream water are used to delineate the contribution of different sources in river Bhagirathi during ablation period. The isotopic characteristics of meltwater and rainfall were used to estimate the rain water contribution to Bhagirathi river. The variation in isotopic composition (6180) with time in river water shows the varied contribution of snow/glacier and runoff component in Bhagirathi river during the ablation period of 2005. This variation is quite obvious due to change in climatic conditions. Isotopic signature of stream water shows the snow dominated melt water up to third week of June and later ice/glacier dominate in the stream discharge.The significant variation in 8180 river water was observed during pre and post SW monsoon caused rain event occurred during July and September 2005. The isotopic composition of precipitation was much depleted (-30.3%) than the stream water (-14%o to -16%0) during the study period. Rainfall contribution to stream discharge was computed using two component model. The contribution was found maximum upto 40% of total discharge on particular rain of particular rainy day. The complete hydrograph separated out for three storm events in the months of July and September during the year 2005 revealed the rainfall contribution to the tune of 14% to 15% of the total stream discharge in each storm period. This accounts an aggregate 3% contribution of rainfall to total discharge of river Bhagirathi that occurred during ablation period of the year 2005.