Evaluation of stable isotopic composition of Nepalese rivers before and after the Mw 7.8 Gorkha earthquake

International audienceStable isotopic studies (H and O) have been continuously performing from the major large rivers like Kosi and Narayani as well as their small tributaries to understand the water cycle from yearly isotopic record in relation with direct precipitation. Here we present a set of data, mostly from the southern Himalayan flank, including daily sampling of rivers in Narayanghat, Khudi, Marshyandi, Trisuli and Kosi as well as precipitation water data since the pre-Gorkha Earthquake (Mw 7.8) of 25 April 2015. For the precipitation at a given location, data are marked by strong seasonal contrast (e.g. Zhang et al. 2001, Gajurel et al. 2006) probably related to source of moisture and amount effect.Strong seasonality in oxygen isotopic composition of river water (4-5‰) in the catchment of Narayani River in the Himalaya in 2010 is recorded. In contrast, the composition evolves much more stable with a slight tendency to increase during the monsoon in 2015. The Kosi data set of 2015 shows a variation in δ18Osmow from -12‰ to -8‰, and for the δDSMOW from -80‰ to -50‰ (Fig. 1). The data set situated on line δD = 7.7 × δ18O + 7.7‰) is close to the GMWL. The seasonal trend in δ18O generates an offset particularly between June-July and August that influence on river isotopic composition. The river isotopic compositions are progressively depleted in heavy isotopes during the monsoon reflecting the seasonal evolution of precipitation, however, it is buffered by the ground water reservoir. At the beginning and end of the monsoon, the contrast between precipitation and ground water is large and precipitation events generate scattered river variations depending on direct runoff to ground water supply, however, such trend is not always observed particularly after 2015 Gorkha earthquake