Stable isotope ratios in precipitation and their relationship with meteorological conditions in the Kumaon Himalayas, India U. Saravana Kumar a, * , Bhishm Kumar b , S.P. Rai b , Suman Sharma a a Isotope Hydrology Section, Isotope Applications Division, Bhabha Atomic Research Centre, Mumbai 400 085, India b Hydrological Investigations Division, National Institute of Hydrology, Roorkee 247 667, India article info Article history: Received 29 July 2009 Received in revised form 21 April 2010 Accepted 10 June 2010 This manuscript was handled by L. Charlet, Editor-in-Chief, with the assistance of Peter Wolfgang Swarzenski, Associate Editor. Keywords: Environmental isotopes Source of moisture Altitude effect Kumaon Himalayas summary For the first time, environmental isotopic (d 2 H, d 18 O, 3 H) data, predominantly based on precipitation sam- ples in the Kumaon Himalayas, India, have been used to understand the influence of various meteorolog- ical factors governing rainout processes in the region. Further, the data are also used to understand the orographic effects in precipitation and to estimate the altitude effect in stable isotopic ratios in precipi- tation, etc. The interpretation of the isotopic data revealed that the source of moisture for winter (October–Febru- ary) and summer (May) precipitation in the Kumaon Himalayas is mainly from the Western Disturbances whereas for the remaining period the source is monsoonal (southwest). The d 2 Yd 18 O relationship in the local precipitation during the monsoon season shows a distinct seasonal effect, with a slope of 7.6. The winter and summer precipitation samples measured higher environmental 3 H compared to southwest monsoon samples, thus indicating continental evaporated moisture. There is wide range of altitude effects (d 18 O variation per 100 m elevation: 0.30‰ [July–August]; 0.57‰ [September]) with the mean altitude effect being 2.61‰ and 0.36‰ per 100 m elevation for d 2 H and d 18 O respectively. These values are different from that reported earlier for the region based on the isotopic compositions of springs/rivers samples. The ‘altitude effect’ in successive precipitation is basically a temperature dependent phenome- non and is explained on the basis of adiabatic cooling related rainout process (dry adiabatic lapse rate, moist adiabatic lapse rate and saturated adiabatic lapse rate for moist air mass). The altitude effect is found to be more reliable in case of d 2 Y, as deuterium is least affected by secondary evaporation. The effect of secondary evaporation has been observed on the true ‘‘altitude effect”. Secondary evaporation of rainfall increases the oxygen isotopic ratios and the increase is directly proportional to the vertical dis- tance travelled by the raindrops through air. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The Himalayas, the youngest mountain range on the earth, gives rise to three of the world’s major river-systems viz. the Indus, the Ganges and the Brahmaputra. In spite of the hydrological impor- tance of the region, very few studies have been reported on stable isotopic (d 2 Y and d 18 O) characteristics in precipitation in the re- gion. Evidently, none of the studies on Himalayas (e.g., Bahadur, 1976; Ramesh and Sarin, 1992; Bartarya et al., 1995) has presented the actual observed altitude effect in precipitation. Thus the com- plexity in the analysis of the relationship between isotopic compo- sition of precipitation and elevation in parts of Himalayas has been reported by Poage and Chamberlain (2001). For the first time, environmental isotopic data predominantly based on precipitation samples, and some water bodies in the lake Naini Catchment, situated in the Kumaon Himalayas, India, have been used to understand the various meteorological factors gov- erning rainout processes in the region and the orographic effects in precipitation and estimate the altitude effect in precipitation using actual observations (air temperature, relative humidity and rainfall etc.). 2. The study area The study area (Fig. 1), situated in the Kumaon Himalayas (Lat. 29°24 0 N; Long. 79°23 0 E; Area: 4.7 km 2 ), encompasses an altitude range between 1937 and 2600 m msl. The mean annual rainfall in the study area is 2488 mm. The mean monthly rainfall, relative humidity and air temperature recorded at lake Naini site for 1995 are shown in Table 1. The rainfall amount, in the Himalayan ranges, generally decreases westward because of the increasing 0022-1694/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2010.06.019 * Corresponding author. Tel.: +91 022 25592418; fax: +91 022 25505151. E-mail addresses: uskumar1@rediffmail.com, vsk@barc.gov.in (U. Saravana Kumar). Journal of Hydrology 391 (2010) 1–8 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol