Abstract
Abstract. 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 the environmental protection and economic and social well-being for both the watershed and the entire Yangtze River 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 recognized as 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" from 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 on a large spatial scale. Our review suggests that historic forest harvesting or land cover change has caused significant water yield increase due to reduction of forest canopy interception and evapotranspiration caused by removal of forest vegetation on both spatial scales. The impact magnitude caused by forest harvesting indicates 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 yield in the Minjiang River watershed. In addition, different types of forests have different magnitudes of evapotranspiration (ET), with the lowest in old-growth natural coniferous forests (Abies faxoniana Rehd. et Wils.) and the highest in coniferous plantations (e.g. Picea asperata Mast.) among major forest types in the study watershed. This suggests that selection of different types of forests can have an important role in ET and consequently water yield. Our synthesis indicates that future reforestation and climate change would likely produce the hydrological effects in the same direction and thus place double the pressure on water resource as both key drivers may lead to water yield reduction. The findings can support designing management strategies for protection of watershed ecological functions in the context of future land cover and climate changes.
Highlights
It is commonly accepted that in large, forest-dominated watersheds forest change or land cover change and climatic variability are two main drivers for hydrological changes at watershed scales
Cui et al.: A case study of large watersheds in the upper reaches of Minjiang River watershed in China watershed and a comparable but treated watershed) is commonly used to remove the climatic influence so that the effects of land cover or forest change on hydrology can be quantified
Zhang et al (2012) conducted a retrospective study to quantify the relative contribution of forest harvesting and climatic variability to hydrology based on the long-term historic data (1953–1996) on climate, streamflow and forest change history in Zagunao sub-watershed, situated in the upper reach of the Minjiang River watershed
Summary
It is commonly accepted that in large, forest-dominated watersheds (greater than 1000 km2) forest change or land cover change and climatic variability are two main drivers for hydrological changes at watershed scales. Cui et al.: A case study of large watersheds in the upper reaches of Minjiang River watershed in China watershed and a comparable but treated watershed) is commonly used to remove the climatic influence so that the effects of land cover or forest change (e.g. harvesting or plantation) on hydrology can be quantified Such an experimental approach is not suitable for large watersheds because of the difficulty in locating a comparable control watershed for an impacted watershed. The program was led by the Chinese Academy of Forestry (Shirong Liu) with involvement from more than 20 research institutes across China The purpose of this summary paper is (1) to provide the research findings on ecohydrological processes at the forest stand level; (2) to present the results on the effects of forest change or land cover change and climatic variability on hydrology at the large watershed scale; and (3) to integrate all results at both spatial scales to discuss management strategies for long-term water and watershed sustainability. Our review mainly focuses on the large watershed scale, inclusion of the results from the small forest stand scale will help the interpretation of the results obtained at large spatial scales
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