Impact of snow and glacier melt on the water security of users of the Ganges, India

Ewout Zwolsman, an MSc student from Wageningen University, assessed for the HighNoon project the contribution of snow- and icemelt to the water demand in the Ganges basin as part of his final Masters thesis.

The first challenge was to investigate the total water demand in the catchment. Based on LPJmL, a well known global vegetation and land-athmosphere fluxes model, five different water uses were distinguished. These include domestic, electricity, irrigation, livestock and manufacturing. The LPJmL is known to take only one growing season into account, while in reality there are at least two in large parts of the Ganges basin. As a result, the irrigation demand required adjustment for a second growing season. Seven scenarios with varying in water demand from 450mm towards 2250mm were implemented. Based a comparison of model output with measured runoff data, the 900mm scenario was chosen to account for the second growing season. Irrigation accounts for 90% of total water demand (see figure).

The next step was to compare the demands with the timing and magnitude of the melt water contribution. Output from three models (VIC, JULES and LPJmL) was used to estimate the snowmeltrunoff for the basin and different subcatchments. The three models correspond in spring and summer (though vary in magnitude of snowmeltrunoff), but differ in autumn and winter period. All three models show an increased contribution in spring (February – May), whereas during summer monsoon the contribution is very small. VIC and JULES also show a large contribution of melt water in autumn and early winter, up to 93.2% in December. These figures suggest a strong dependency on meltwater in several months of the year, especially during the Rabi growing season in January - March. As a result, changes in the amount of meltwater could affect agricultural production in the Ganges basin.

Average precipitation, irrigation water demand and the total water demand in the Ganges basin in mm