Friday, July 23, 2010

How Relevant is Meltwater in River Basin Hydrology?

Much has been written about the disappearing glaciers that will lead to the drying up of the Himalayan-fed river systems. Which has also been disputed subsequently by our Indian scientists, who claim there is no sign yet of a Himalayan meltdown. However, as we know only too well, our river flows are indeed reducing at an alarming rate.

None of the above or related studies factored in, the percentage component of glacial melt in these riverine systems, or the extent of dependence on glacial run-offs and seasonal snowpacks. Nor was any attempt made to separate and analyse the hydrological elements in the cryospheric changes or the mechanisms at work.

So this peer paper in Science Magazine, makes for an interesting study, as it focuses on the core of a riverine system, the upstream basin, and its impact on the entire river basin. The paper is a comparative assessment of the hydrological processes in the upstream areas, vis-à-vis the five major river basins in the Asian sub-continent fed by the Himalayan glaciers - the Ganga, Brahmaputra, Indus, Yangtze and Yellow River basins. The three components analysed are a) importance of meltwater in river basin hydrology, b) cryospheric changes, and c) effect of climate change on water supply from upstream basins and on food security.

So changes in the upstream precipitation and melts have been analysed with respect to their effect on downstream basin.

While a negative trend of –0.22 ± 0.05 m year–1 was observed in snow and ice storage particularly for the Ganga basin, with no discernible trend for Brahmanputra, it was noted that melt parameters were different for both Ganga and Brahmaputra. Discharge by snow and glacial melt was found to be 10% for Ganga but higher at 27 % for Brahmaputra.

Upstream water supply is critical for sustaining the upstream reservoir systems. Any changes thereon affect the dams that store water for release to downstream areas, affecting irrigation and lives of people downstream.

Though the models applied for the study, show a decrease in mean upstream water supply (-17.6% for Ganga, -19.6% for Brahmaputra), this is partially offset by increased mean upstream rainfall (+8% for Ganga, +25% for Brahmaputra). Which still means a net decrease in the upstream water supplies that will most certainly affect downstream basin, notwithstanding the fact that "most climate models have difficulty simulating mean monsoon and the inter-annual precipitation variation "

Alluding to the IPCC report, the paper mentions

it was suggested that the current trends of glacier melt and potential climate change may cause the Ganges, Indus, Brahmaputra, and other rivers to become seasonal rivers in the near future.”

It moves on to argue that these rivers are already seasonal rivers, “because the melt and rain seasons generally coincide” but the “decrease in meltwater is partially compensated for by an increase in precipitation”.

While concluding that considerable cryospheric changes are expected, with a temporal shift in the upstream hydrograph patterns and a trend towards reduction in upstream precipitation, the mean monsoon and precipitation variation cannot be estimated precisely because of difficulty in climate model simulation.

The paper wraps up with

"We conclude that Asia’s water towers are threatened by climate change, but that the effects of climate change on water availability and food security in Asia differ substantially among basins and cannot be generalized. The effects in the Indus and Brahmaputra basins are likely to be severe owing to the large population and the high dependence on irrigated agriculture and meltwater".

This study is significant for emphasising the relevance of snow and glacial melt in upstream flows, the parameters of upstream river basin that directly affect the main river system. In other words, irrigation, cropping, aquifer recharge, drinking and other water requirements of man, that are essentially met in the downstream basin.

Although glacial thawing and retreat has been studied at random, the changes in snow cover patterns and seasonal melts also need to be embedded for empirical studies and applied to hydrological modelling. As this report says, scientists have discovered the Himalayan snow melting in winter as well.

It is an established fact that downstream water availability is dependent on upstream melt water and precipitation, though not uniform for all river systems. However, for rivers that depend substantially on glacial melts and seasonal snowpack run-off, such studies are quiet relevant for effective water-resource planning and short-term cryogenic feedback, to enable lower basin impact mitigation.