Tree rings hold the key

THERE is always a dearth of data on amount of water in rivers. The information is essential for better management of water resources but observational data on stream flow goes back only a few decades in most cases. This has prompted many countries, including China and the US, to use proxy methods, like studying tree rings, for collecting river flow data. Width of these rings corresponds to the availability of water, making them a natural archive of river flow data.

Researchers from the Wadia Institute of Himalayan Geology in Dehradun, and Birbal Sahni Institute of Palaeobotany, in Lucknow, explored the possibility of using this method in India. They studied water availability in the Satluj river using tree rings of the Himalayan cedar (deodar). They found that the width of the rings of these trees was directly related to water availability in the river.

Satluj originates in the Tibetan plateau and flows down to Himachal Pradesh. Most of the water in the upper Satluj basin comes from glaciers. Downstream, the contribution of summer monsoon is more. For the study, the researchers collected wood samples from 21 deodar trees growing on rocky slopes in Purbani village of Kinnaur district. Such trees provide a better perspective of stream flow than those growing on the riverside as the latter grow in a water-replete soil and never experience water scarcity. Annual rings of the trees were studied using standard dendrochronological techniques.

The researchers also studied discharge of the Satluj at Bhakra dam between 1923 and 2004 along with rainfall and temperature data of the area between 1952 and 1994. The data shows the flow has started to decrease during winter and monsoon seasons since 1990. This is despite an increase in temperature which should have resulted in increased glacial melt. The trend has been attributed to thinning of the Himalayan glaciers.

This change in water discharge and future water availability in the Satluj can be studied through tree-ring based predictive models, say the researchers. “The models can also help figure out if the multitude of power projects on our rivers would be viable in the long run,” says lead author Jayendra Singh. The method has been extensively used in the US to develop a forecast for future water use in Colorado state. The researchers are now developing a network of tree-ring chronologies from Satluj basin to develop better river-discharge models.

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