Analyzing the long-term hydrological impacts of forest change

The long-term impact of forest cover changes on hydrological processes and their spatial scaling has rarely been studied. This paper investigates two questions: (1) Do forest cover changes have a long-term impact on hydrological processes? (2) Do the effects of forest cover change differ with spatial scale? Using a combination of historical records of land use change, time series remotely sensed data (1973-2007), stream flow data (1938-2006), and precipitation data (1930-2006), we study changes in forest cover and forest management, stream flow, and precipitation from two nested, large subwatersheds (hundreds to thousands km2) from the Homochitto Watershed in Mississippi (USA). To overcome data limitations, this study mainly focused on understanding long-term hydrological changes and related them to changes in forest cover and precipitation. Daily stream flow data were analyzed using spectral analysis and regression to describe the overall and seasonal trends from two subwatershed areas. This approach explained change characteristics of low flow, revealing long-term and cumulative hydrological impacts of climate and land cover changes. These processes observed in the two subwatersheds were compared to address spatial scaling issues. The results showed a long-term increase in low-frequency flow associated with an increase in forest cover. High levels of disturbance in the matured secondary forests (> 10 % of forest cover) brought a sudden increase in low flow, but the regeneration of previously disturbed forest areas reduced this increase. Overall, the hydrological responses in the two sub-watersheds showed a very similar pattern, regardless of differences in their geographical characteristics.