Modelling Runoff and Impact of Climate Change in Large Himalayan Basins

The runoff regime in the basins of the rivers Ganges (917’444 km2) and Brahmaputra (547’346 km2) is modelled from precipitation, remotely sensed snow covered areas and temperatures. The runoff cycle roughly corresponds to a calendar year. In view of the small proportion of snowmelt, it is mainly governed by the distribution of rainfalls resulting in flow peaks in the summer and recession flow in the winter. The accuracy of runoff simulations is acceptable in view of the available data and because the SRM model was for the first time used in basins of this order of magnitude. In addition, for the given climate change scenario (T+1.5 ̊, summer precipitation increased by 10%), the already high risk of floods in July-September is slightly increased. INTRODUCTION This study was conducted in the framework of the ESA-DUP SPIHRAL project with the aim to simulate runoff in the basins of the rivers Ganges and Brahmaputra, to evaluate contributions of snowmelt and to assess the effect of a climate change on runoff. The snow coverage from periodical satellite mapping and temperatures were provided only as monthly averages, precipitation only as monthly totals. Another problem was the extrapolation of precipitation by adequate altitude gradients between the sea level and 8848 m a.s.l. covering a variety of climatic zones. The results should be regarded in the light of these difficulties. In Alpine regions high accuracy runoff modelling is possible due to more detailed data available, as reported earlier (Seidel and Martinec., 1992). BASINS OF THE RIVERS GANGES AND BRAHMAPUTRA Published data on catchment areas and runoff vary to some extent. In an earlier publication (Chow, 1964) the combined area of Ganges and Brahmaputra river basins is given as 2’053’000 km2 and the annual runoff as 630’252.106 m3, with the remark “subject of revision”. The Water Encyclopedia (Troise and Todd, 1990) gives 1’058’852 km2 and 589’000.106 m3 for Ganges and 934’586 km2 and 625’000.106 m3 for Brahmaputra. Islam and Sado (2000a) indicate a combined area of 1’720’000 km2 for the rivers Ganges, Brahmaputra and Meghna. In this study, the area of the Ganges river basin was evaluated as 917’444 km2 and for Brahmaputra as 547’346 km2 (Baumgartner, 1999). The situation of the basins is shown in Fig. 1. In spite of the smaller catchment area, Brahmaputra has more runoff than Ganges. This discrepancy may be better understood by inspecting the area-elevation curves in Fig. 2 and Fig. 3. As much as 75% of the Ganges basin is below 500 m a.s.l. compared with about 25% for the Brahmaputra basin. Taking into account the precipitation altitude gradient, the precipitation amounts extrapolated for higher parts of the basin are greater for Brahmaputra than for Ganges. ICIWRM-2000, December 19-21, 2000, Roorkee, INDIA 1 HYDROMETEOROLOGICAL DATA The SRM model used in this study requires temperature, precipitation and snow covered areas as input variables. These data were available for the year 1995. Temperatures for the computation of daily snowmelt depths were provided only as monthly averages. A comparison of the number of degree-days obtained from daily fluctuating values and from monthly averages carried out in a Swiss Alpine basin indicates a certain loss of degree-days if monthly averages are used. As a compensation, temperatures around 0 ̊C can be increased as listed in Table 1. Precipitation was provided only as monthly totals which is inadequate for day-to-day runoff computations. In the SPIHRAL Interim Report (Baumgartner, 1999), monthly values were empirically desaggregated by clustering the events around the dates of measured runoff peaks, taking into account the statistically determined numbers of precipitation events in each month. The daily precipitaion amounts thus obtained were taken over for this study as well as the altitude gradients. Another adjustment of precipitation data was necessary in the Brahmaputra basin. While Brahmaputra has generally more precipitation than Ganges, the September total at 1250 m a.s.l. is only 9 mm against 231 mm in the Ganges basin. Evidently major rainfalls were Figure 1. Situation of the basins of the rivers Ganges (G) and Brahmaputra (B)