Forest transpiration greatly speeds streamflow recession

Streamflow recessions from a 42‐ha forested watershed in New Hampshire are greatly affected by transpiration. In the dormant season, recession flow continues for many days at rates around 1 mm/day, whereas, in the growing season, Streamflow rate declines in several days to less than 0.05 mm/day. Transitions between the two regimes occur in June and late September or early October, corresponding to the leaf out and color change phenology of the area. Elimination of transpiration by cutting and herbiciding a 16‐ha watershed changed the rate of its summer recession from the rapid decline associated with transpiration to the slow recession characteristic of no transpiration. The recession without transpiration is probably maintained by slow drainage of unsaturated soil, since groundwater exists only during snowmelt on these steep watersheds. Rapid recession occurs when transpiration over the whole watershed removes soil water that would otherwise drain and become Streamflow.