Nature paper explains impact of forest management on water resources

Billions of people worldwide rely on streams flowing from forest lands for their water supply.

But the link between forests and the amount of water flowing in streams and rivers has been a hotly-debated topic for decades.

New research by the University of Saskatchewan (USask), published in Nature, has found that the amount of water that the landscape is able to retain is the crucial factor in predicting annual steam-flow increases when trees are cut down.

A study by USask’s Global Institute for Water Security (GIWS) concluded that the capacity of the soil in which trees grow to absorb and retain water is the key factor determining the impact of deforestation on annual streamflow.

“There is huge variability around the world in the impact of harvesting trees on water flow, which has perplexed water scientists for decades,” said co-author Jeff McDonnell, GIWS associate director. “These findings could help inform sustainable forest practices, helping planners to more accurately predict the impact of logging and tree planting on natural water resources.”

The study, Global Analysis of Streamflow Response to Forest Management, examines why, in some areas, felling trees has not increased the water yield, as expected. In other regions, deforestation or thinning trees has led to a greater annual increase in water in streams than predicted.  The study concluded that the depth of soil and porous rock beneath the trees are the crucial factors.

The team of hydrologists assembled data from past published studies that had been carried out on 502 research basins or watersheds looking at the effects of forest planting and removal on streamflow.

The international research team looked at the number of trees felled and planted, local conditions, the tree canopy, and soil depth and type. They found the storage capacity of the soil in which trees were planted was the key factor determining the impact of deforestation on annual streamflow.

 “The amount of water storage under foot has a large role in determining how a stream will respond to above-ground harvesting. More storage below ground means more annual flow in the stream when the trees are removed,” said lead author, Jaivime Evaristo of the Copernicus Institute of Sustainable Development at Utrecht University in the Netherlands.

The research was funded by the Natural Sciences and Engineering Research Council of Canada.