Impacts of forest changes on hydrology: a case study of large watersheds in the upper reach of Yangtze River Basin

Quantifying the effects of forest changes on hydrology in large watersheds is important for designing forest or land management and adaptation strategies for watershed ecosystem sustainability. Minjiang River watershed located in the upper reach of the Yangtze River Basin plays a strategic role in environmental protection and economic and social wellbeing for both the watershed and the entire Yangtze Basin. The watershed lies in the transition zone from Sichuan Basin to Qinghai-Tibet Plateau with a size of 24 000 km2. Due to its strategic significance, severe historic deforestation and high sensitivity to climate change, the watershed has long been one of the highest priority watersheds in China for scientific research and resource management. The purpose of this review paper is to provide a state-of-the-art summary on what we have learned from several recently-completed research programs (one of them known as "973 of the China National Major Fundamental Science" with funding of $3.5 million USD in 2002 to 2008). This summary paper focused on how land cover or forest change affected hydrology at both forest stand and watershed scales in this large watershed. Inclusion of two different spatial scales is useful because the results from a small spatial scale (e.g. forest stand level) can help interpret the findings at a large spatial scale. Our review suggests that historic forest harvesting or land cover change has caused significant water increase due to reduction of forest canopy interception and evapotranspiration caused by removal of forest vegetation at both spatial scales. The impact magnitudes caused by forest harvesting indicate that the hydrological effects of forest or land cover changes can be as important as those caused by climate change, while the opposite impact directions suggest their offsetting effects on water yields in the Minjiang River watershed. In addition, different types of forests have different magnitudes of ET with old-growth natural coniferous (Abies) forests being the lowest and the coniferous plantations (e.g. Spruce) being the highest among major forest types in the study watershed, suggesting that selection of different types of forests can have an important role in ET and consequently water yields. Our synthesis indicates that future reforestation and climate change would likely produce the hydrological effects in the same direction and thus place double pressures on water resource as both key drivers may lead to water yield reduction. Implications of the findings are also discussed in the context of future land cover and climate changes.