The influence of vegetation type on the hydrological process at the landscape scale

The relationship between vegetation and hydrological processes is still a critical issue in ecology and environment science, especially at the landscape scale. Mingjiang valley plays an important role in water and soil resources conservation and erosion control in the upper Yangtze River. In this paper, the influence of vegetation type on hydrological processes at the landscape scale was studied using remote sensing and spatial analysis in Mingjiang valley and its five catchments. First, the vegetation distribution was mapped with high accuracy using three scenes of Landsat thematic mapper (TM) imagery and the optimal iterative unsupervised classification method. Then the spatial precipitation and actual evapotranspiration (AET) database was developed by converting the point-based data of meteorological stations to spatial surface with spatial interpolation. Cross-tabulation spatial analysis was employed to study the relationship between vegetation and rainfall, evaporation, and runoff. The results show that dominant vegetation types are grasslands, forests, and shrublands in the Mingjiang valley, with the proportions of 37.44%, 29.97%, and 22.62%, respectively. The annual precipitation ranges from 560 to 720 mm in areas of conifer and mixed forests, shrublands, and grasslands. For broadleaf forests, croplands, and other vegetation types, the precipitation distribution ranges from 480 to 800 mm, indicating a broader variation than that for the dominant vegetation type. In high-precipitation regions of the valley, forest vegetation covers the largest area. The precipitation is positively correlated with vegetation cover. We found that AET has a nonlinear relationship with vegetation cover, but this relationship is complicated. Our results demonstrated that the relative evapotranspiration rate (ER) is negatively correlated with precipitation, and water remaining (WR) is positively correlated with precipitation in the landscape. From the hydrological records in the Mingjiang valley, the annual mean runoff is 502 m3·s–1, the mean annual runoff amount is 140 × 109 m3, and the annual runoff rate is 0.0213 m3·s–1·km–2. We found that percent forest cover is positively correlated with percent runoff. This supports the results of previous nonspatial investigation in the valley. From scale analysis, we found that most spatial patterns of climate and hydrological variations are scale dependent, e.g., precipitation, AET, ER, WR, and runoff vary at different levels of landscape scales.

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