Planting trees shift water flow

We all know that Planting trees reduce atmospheric carbon dioxide. According to two new studies, planting forests in areas that currently don’t have trees — a process called aforestation — can reduce the local availability of water. One key measure of water flow is ‘base flow’. Base flow is often seen as the minimum supply of water on which people can safely rely. But in basins that contain small rivers, forestation can reduce base flow by up to 50%. Less base flow means less water for populations.

Two studies were conducted on seven paired basins — seven with native grasslands and seven that had been planted with forests — in the province of Córdoba, Argentina. With their deep roots and tall canopies, trees absorb and transpire more water than do grasses, resulting in drier streams. A second study presented Uruguay, came to similar conclusions. A team led by Wayne Skaggs of North Carolina State University in Chapel Hill, in collaboration with the US Forest Service, afforested one of a pair of watersheds. The researchers observed an 18–22% drop in base flow in the afforested watershed compared with the watershed that had been left as grassland. “As the trees get larger, the effect will be somewhat greater,” says Skaggs.

The latest study tell us that the ideal balance between afforestation and water needs is for one-quarter of the river basin to be planted with between 400 and 500 trees per hectare. “It is possible to prevent drastic effects” on water availability. Choosing tree species wisely might help as different species use water at different rates. Whereas fast-growing ever green pines consume large amounts of water, deciduous trees such as poplar and walnut use less, especially in winter. Also, planting only some portions of the watershed “might achieve the balance of providing wood products for the people without the impact on the basin’s water balance”.