Asia’s Five Major Watersheds’ Resources Threatened by Climate Change

Asia’s has five major water basins all above 2,000m above sea level: the Indus, Brahmaputra, Ganges, Yangtze, and Yellow river basins. Together, these basins provide for rivers that provide water supply for more than 20% of the global population. Climate Change will bring changing trends in temperature and precipitation which will affect the amount of water available for people. The hydrolic affects leading to a change in water availability will vary greatly from watershed to watershed, but overall, there will be a decrease in water equivalent to a supply for 4.5% of the total population served. The Indus and Brahmaputra basins will have the highest water discharge decreases, and due to increased rainfall, the Yellow is projected to have a slight increase in discharge.–Darien Martin
Immerzeel, W., Beek, L., Bierkens, M., 2010. Climate change will affect the asian
water towers. Science AAAS 328, 1382–1385.

               
          To project water supplies for 2046-65), Immerzeel et al. measured the amounts of discharge, and water supply for each of these basins, and investigated the effects of melt water volume from ice and snow on discharge downstream. They then predicted future changes in ice coverage, and used this data to project the discharge and resulting water supply from each water basin. Other components affecting water discharge were factored in, such as amount of precipitation, before predicting water resource availability.
          In order to assess how ice and snow amounts effect discharge volumes, Immerzeel et al. used a Normalized Melt Index which is: snow and glacier discharge volume divided by downstream discharge. Upstream discharge was found using a snow melt runoff model, and downstream discharge was found using by subtracting the amount of upstream glacier discharge that had evaporated. It was found that the Indus and Brahmaputra received the largest percentages of their discharge from glacier melt; especially the Brahmaputra with glacier melt of 151% of downstream discharge. To determine ice storage, a gravity model combined with precipitation trends was used. Both ice storage, was tested to predict past discharge amounts, and predictions were accurate in comparison to past numbers.
Next, they predicted ice volumes and discharge for each basin in 2046 to 2065 using general circulation models, which include a number of climate change factors. These results were inconclusive, predicting a decrease in ice for the Ganges basin, and an increase for the Indus. Lastly, upstream water availability was assessed beside water demand in terms of crop yield and energy to project effects on future food supply.
          Results found that discharge from all rivers would increase for a short period when glaciers shrunk, but would then decrease in all basins except for the Yellow River, which had an overall increase of 9.5% water discharge upstream. Decrease in glacier area and water discharge was countered but not reversed (besides in the case of the Yellow River), by increases in annual precipitation predicted. The decrease in discharge may cause the Bramaputra and Indus, and Ganges rivers to become seasonal. The Indus and Brahmaputra will be most affected by climate change due to the high percentage of their water supply coming from glacial melt.

          This will create an overall stress on food supply. Population on the Yangtze is the largest, the Ganges is most densely populated, and the Indus, Ganges and Yangtze have the most agriculture to support. The Yellow River may provide for an increase in food availability from increased water supply, but the Indus and Brahmaputra are in danger due to their large irrigation networks fed largely by glacial melt. 

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