The rainfall in the future will fall more intensely over a span of few days due to global warming. Can big dams help face this change?
Effect of Global Warming
It is true that the rain which is currently spreading for four months will be shrinking for less number of days due to global warming. Below is the picture of the distribution of rainfall 30 years and 10 years (Report of WWF is available here). It can be seen that the water in the 30 years was spread in four months and it has shrunk in 10 years.
This means that when the rain falls, it will very intense and soon it will move to the sea along the rivers. If the rainfall falls slowly, then more of the water is absorbed by the land and it will recharge the ground water. Due to the speed of the rainfall, water flows in the sea more quickly, and does not percolate inside the ground. Thus we will get less amount of water for irrigation. At present the ground water level is already falling. In the future, it will fall faster as there will be less recharge. Consequently, irrigation will be reduced. The production of our food grains will come down. The question is, may the solution of this lie in big dams like Tehri and Bhakra?
Problems with Big Dam
According to our assessment this will not happen. Most rainfall falls in our plains but not in the hilly areas. The average rainfall in Uttarakhand and Uttar Pradesh is as follows:
Uttarakhand – 53483 m2 (area) * 130.86cm (average rainfall per year) = 69 lakh cubic meters. Of this we assume 50% falls in the catchment of large dams and 50% in the plains of Uttarakhand. Thus the water that can be retained in the dams is 35 lakh cubic meters. Uttar Pradesh – 243286 m2 (Area) * 156.3cm (average rainfall per year) = 380 lakh cubic meters
Only 35 lakh cubic meters can be stored in dams, whereas 380 lakh cubic meters is going waste in the plains. Therefore, we must conserve the rainwater in the plains to face global warming.
The second problem is that when we store the rain water in the dams, flooding in the plains gets reduced. The common belief is that limiting the flood is in public interest. But it also has the opposite effect. When flood water spreads all around, then ground water is recharged in the field, which is used for irrigation by taking out from tube well in winter and summer. Therefore, when we stop the rain water in the dams, the floods are reduced in the plains and due to low flooding, the recharge of ground water is reduced and irrigation is also gets reduced. Thus there are two opposite effects of large dams. The positive effect is that the water stored is released in the winter and summer and irrigation is increased. The negative impact is that the flood water spreads less, and less water is recharged in ground water resulting the less irrigation. The combined effect of these two contradictory effects is not positive in our assessment.
We can increase the recharge of ground water by further storing water in tanks as is done with pynes in Gaya District of Bihar.
Usually the river flows like this:
When the water in the river increases then it is diverted into special canals and collected in the village ponds like this:
When the water of the river decreases, then the water stays in ponds and recharges the ground water like this:
Pictures of the Pyne are given below:
The third problem is of siltation in big dams in the hilly areas. Scientists estimate that Tehri Dam will be filled with silt in 130 to 170 years. After this the rainwater harvesting by this dam will become negligible. While, the impact of global warming will become more severe. The need to store water will increase while the storage capacity of the dams will be reduced. Therefore, water storage in dams can be helpful only in the short-term.
We believe that big dams will be unsuccessful because their catchment is less as compared to the plain areas and they will be filled with silt soon. Therefore, solving the problem of global warming through dams is going to be a big mistake. The only solution is to remove the dams like Tehri and allow the flood water to spread easily recharge the groundwater.
