Afforestation of catchments reduces water yield. On hydroelectric power catchments, land uses can be compared in terms of energy production. On the Maentwrog catchment, Wales, forestry appeared highly energyefficient before inclusion of HEP losses, with ratios of energy input to energy output of 1:16-21. Inclusion of HEP losses reduced ratios to l :OS-3. A study of 41 sites on catchments in North Scotland showed that at the majorlty, afforestation would lead to net energy gain. However, in discounted cash flow terms, most potential afforestation sites were unprofitable when HEP losses were included. The balance of advantage depends crucially upon the criterion chosen and site conditions. Afforestation at high elevations on catchments [...]