Effects of historical land use on sediment yield from a lacustrine watershed in central Chile

Sediment yield in the San Pedro Lake watershed, inferred from sedimentation in the lake, can be related to land use changes shown on aerial photograph s taken during the period 1943-1994. In this watershed, which covers 4.5 km (2) of mountainous terrain in San Pedro County, central Chile, the area of native forest species decreased from 70 per cent in 1943 to 13 per cent in 1994. During this same period, the area of pine plantations increased from 4 to 46 per cent. To study effects of these changes, we took a core from th e centre of the lake and estimated sedimentation rates by Pb-210 dating, wh ich we checked with Cs-137 and pine pollen. The results show that sedimenta tion rate ranged from 5 mg cm(-2) a(-1) in the late 1800s to 60 mg cm(-2) a (-1) in the late 1960s. Those rates, together with assumptions about the pr oduction and delivery of the sediment, give corresponding figures for sedim ent yields with maximum values close to 1 t ha(-1) a(-1). Sediment yield be tween 1955 and 1994 closely tracks the total land use change that can be de tected, irrespective of land use type, on sets of aerial photographs taken four to 18 years apart. However, this measure of land use change, while con venient and successful as a predictor of historical erosion, may be unrelia ble because it probably excludes many changes that occurred in long interva ls between successive photographs. Copyright (C) 2001 John Wiley & Sons, Lt d.