Managing forests for water yield: the importance of scale.

Examination of expected change in water yield for a large area where vegetation thinning has been proposed in the Sierra Mountains of California, indicates that the size of the area has an important bearing on annual runoff. Results indicate that average changes in annual runoff per unit area for large areas would typically be less than 0.4%. Such changes can only be quantified by extrapolation of paired watershed studies because direct measurement is not feasible.