30 years of forest hydrology changes at Coalburn: water balance and extreme flows.

The Coalburn experimental catchment, located in the Kielder Forest in northern Britain, was established in 1967 to study the hydrological impacts of upland coniferous plantation forestry. Results of 30 years' study (1967-96) are presented; they cover the transformation of the catchment from rough grazing through drainage and planting with conifers in 1972-73 and the subsequent forest development to canopy closure. In the early years of forest growth, the pre-planting forestry drainage dominated the hydrology and the observed changes were quite different from those normally associated with forestry; catchment evaporation was reduced, stream stormflow response times were shortened and dry weather baseflows were enhanced. These effects were sustained for an unexpectedly long period-up to one half of the forest plantation cropping cycle-before being reversed by the increasing influence of the growing forest. These results indicate that significant areas of young plantation forests may function hydrologically in ways very different from what is generally assumed from studies of mature forests. For large plantations, a mixed age forest structure may have hydrological as well as environmental advantages.