Hydrograph separation and precipitation source identification using stable water isotopes and conductivity: River Ganga at Himalayan foothills

Abstract

Abstract:The observed retreat of several Himalayan glaciers and snow packs is a cause of concern for the huge population in southern Asia that is dependent on the glacial-fed rivers emanating from Himalayas. There is considerable uncertainty about how cryospheric recession in the Himalayan region will respond to climate change, and how the water resource availability will be affected. As a first step towards quantifying the contribution of glacier-melt water, hydrograph separation of River Ganga at Rishikesh into its constituent components, namely (i) surface runoff, (ii) glacial ice-melt and (iii) groundwater discharge has been done in this paper. A three-component mixing model has been employed using the values of υ 18 O and electrical conductivity (EC) of the river water, and its constituents, to estimate the time-varying relative fraction of each component. The relative fraction of the surface runoff peaks (70-90%) during winter, due to the near-zero contribution of glacial ice-melt, essentially represents the melting of surface snow from the catchment. The contribution of glacial ice-melt to the stream discharge peaks during summer and monsoon reaches a maximum value of ¾40% with an average of 32%. The fraction of groundwater discharge varies within a narrow range (15 š 5%) throughout the year. On the basis of the variation in the d-excess values of river water, it is also suggested that the snow-melt and ice-melt component has a significant fraction derived from winter precipitation with moisture source from mid-latitude westerlies (also known as western disturbances).

Tracer-based analysis of spatial and temporal variations of water sources in a glacierized catchment

“…A comparison of the isotopic composition of glacier meltwater in the Saldur catchment with samples taken in other parts of the globe reveals the variability of dominant climatic conditions. Saldur glacier melt was more depleted compared to the Mafengu River, China (Yang et al, 2012), similar to the Ganga River catchment in the Himalayan foothills (Maurya et al, 2011) and in the Langtang and Dudh Kosi basins in Nepal Himalaya (Racoviteanu et al, 2013). However, it was heavier than that in the Wind River Range in the American Rockies (Cable et al, 2011) and a Central Andean catchment (Ohlanders et al, 2013) and, not surprisingly, much heavier than that found at the Imersuaq Glacier, West Greenland (Yde and Tvis Knudsen, 2004).…”

“…2a) indicating that waters originating from upstream sources mixed to give composite stream water (Dalai et al, 2002;Maurya et al, 2011). The slope of 7.9 of the δ 18 O-δ 2 H relationship of stream water in the Saldur River was similar to that of rainfall (Fig.…”

“…3) and especially to that of snowmelt ( Table 4), indicating that these water sources underwent similar fractionation processes. In contrast, groundwater and stream water in the tributaries showed lower slopes of the δ 18 O-δ 2 H relationship compared to the Saldur River waters and to rainfall samples (and therefore a departure from the LMWL, not shown) suggesting post-precipitation evaporation during the groundwater recharge process (Maurya et al, 2011), as discussed in Sect. 4.8.…”

“…Moreover, water isotopes, coupled to other geochemical tracers, such as electrical conductivity (EC), have the potential to identify end-members (i.e. the dominant sources to runoff) and compute their contribution to streamflow (Maurya et al, 2011).…”

Isotopic composition of daily precipitation along the southern foothills of the Himalayas: impact of marine and continental sources of atmospheric moisture

“…The strength and frequency of WD over the last three decades has increased in western Himalayas and decreased in the central Himalayas. On the basis of the variation in the deuterium-excess (dexcess) values of Ganges River water at Rishikesh, it was suggested that a significant fraction of the snow-melt and icemelt components are derived from winter precipitation with a moisture source from the mid-latitude westerlies (Maurya et al, 2011). The impact of climate change on the frequency and magnitude of precipitation events in the tropics is still a matter of debate (Held and Soden, 2006;Wentz et al, 2007;Allan and Soden, 2008).…”