Forest and water relationships: hydrologic implications of forestation campaigns in China
Reforestation and afforestation (referred to forestation thereafter) campaigns in the past two decades have resulted in great increases in both forest land area and forest ecosystem productivity in China. Although the ecological benefits of forests are well accepted, the hydrologic consequences of man-made forests by forestation are unclear. Debate and confusion on the hydrologic effects of forestation practices in China remain due to lack of convincing experimental data. This paper reviews worldwide research on the relationship between forest cover and watershed hydrology with special focus on hydrologic effects of reforestation. We limited our review to research conducted using the 'paired watershed' approach. We found most of the existing literature suggests that forestation has potential to reduce annual water yield and baseflow, but have limited effects on peakflow rates and flooding events. We found that the variability of the hydrologic effects is large due to differences in watershed hydrologic processes which are controlled by climate, soils, and the stage of vegetation development. We predict that forestation campaigns in China are not likely to cause large scale changes in streamflow water yield, baseflow, and flood peaks before the hydrologic properties of degraded soils are fully improved. However, baseflow and annual water yield may be reduced in small watersheds to affect local water supply. This situation may be especially true for the semi-arid Loess Plateau region and other areas of Northern China where water shortages are already common. We suggest forest hydrology research should focus on the impacts of forestation on hydrologic processes using a paired watershed approach. Comprehensive science-based evaluation of the positive or negative roles of forest on regulating regional water resources is critical to the current forestation endeavors in China.