Groundwater flow is one of the mechanisms through which trees keep rivers flowing even the dry season. Besides this, the recharge of groundwater tables also benefits people because India’s groundwater situation is growing dire. In 2011, almost 30% of India’s districts had a groundwater situation that was either semi-critical, critical or overexploited. This is up from just 8% in 1995. “If current trends continue, in 20 years about 60% of all India’s aquifers will be in a critical condition” says a World Bank report.
While more sustainable irrigation practices need to be brought in to completely reverse these trends, trees are also an important component in the solution. There are many examples of how groundwater availability has declined when trees are cut down from an area. In Bengaluru, for example, Thindlu or Doddabommasandra lake used to be surrounded by thick vegetation and had a high groundwater table. But when construction started coming up in the region, vegetation was removed. This led to the drastic decline in the groundwater table. The lake was once home to several hundreds of bird species that spent part of their migratory cycle there. But today, the lake is completely dry. The residents in this area face water shortage with groundwater levels falling to 700 feet below the surface.
The Koramangala Games Village in the city is no better. With the removal of vegetation and the proliferation of concrete structures, rainwater no longer seeps into the ground. The water table has thus dropped to 600 feet below the surface, and residents are bearing the brunt.
This groundwater or base flow component of river flow is especially important in the context of climate change. Several studies have shown that the Indian subcontinent will experience more extreme wet and dry spells. In this situation, the planting of trees will ensure that water sinks into the soil rather than flowing above ground and causing floods. This groundwater component also increases dry season flow, thus reducing the effect of dry spells.
Agroforestry techniques are now being used in many countries to increase food production, check soil erosion and reduce flooding. Computational models show that if reforesting is done in 20-35% of the river’s catchment, a 10-15% reduction is seen in flood peak heights after 25 years of forest growth.
Another important factor is that many native trees in India consume less water per kilogram of produce than rice and wheat – the predominant food crops grown in India today accounting for 59% of India’s agricultural area under food grains. For example, wheat requires about 1900 liters/kg and rice needs 3000 liters/kg. Mangoes, like wheat, need 1900 liters/kg – which is still 30% less than rice. But fruits such as oranges need 900 liters/kg – less than half what wheat consumes and less than a third of what rice consumes. Pomegranate needs even less – about 750 liters/kg.