Distribution Properties Of Soil Macropores And Its Influence On Hydrological Process In The Huoditang Forest Region, Qinling Mountains

The southern slope of Qingling Mountain is the main water source for the Middle RouteProject in South-to-North Water Diversion. The Huoditang forest area situated in the center ofthe medium-altitude mountainous area, and the forest ecosystem had a goodrepresentativeness in the area. So the research of the influence of forest ecological system onthe runoff is of significance. Soil is a main component of the forest ecosystem, which playedan important role in runoff process. Conducting some integrated research on soil macropore,physical properties, hydrological characteristics, and the root, organic matter content indifferent types of forest community, to analyze the distribution characteristics of soilmacropore and its influence on soil hydrological characteristics; based on studying thehydrological processes in the small watershed in the area, we analyzed the mechanism ofaction of soil on the runoff process, contrasting and mechanism analyzing. That providedsome scientific basis to correctly cognition the influence of forest ecosystem on runoff. Theresults show that:（1） The equivalent radii of soil macropores in the Huoditang Forest ranged from0.3to3.8mm, and the macropores with equivalent radii>1.5mm made up only5.37%of the totalmacropore amount. Macropore density was great in upper soil layers but less in the lower,and this vertical distribution on soil profile was of significant differences. The order ofaverage area proportion of0–60cm layer was mixed forest> Pinus tabulaeformis forest>deciduous broadleaf forest> Pinus armandii forest. Based on principal componentanalysis, soil organic matter content and root distribution were the main influence factorson the distribution of soil macropore, and others were boulder, soil acid-base property andgrass coverage.（2） In the area, the physical properties and hydrological characteristics of soil are all great inupper soil layers but less in the lower, and this vertical distribution on soil profile wasabvious. There was a significant correlation between the macropore density with radii>0.7mm and noncapillary porosity water-holding amount （P<0.01）. Average macroporedensities of different equivalent radii were positively related with the saturated hydraulic conductivity and the density with radii>1.5mm contributes84%of the variability of thesaturated hydraulic conductivity. The porosity ranged from1.6to13.3%. The saturatedhydraulic conductivity increases with the porosity when it is less than5%. Macropore,especially that with radii>1.5mm, played a critical role in the soil water conservationfunction of forests.（3） Forest soil is a principal factor on the runoff process in forest ecosystem, macroporesplaying an important role in that. By influencing the infiltration and water-holding,macropores played an important role in rainfall-runoff process in watershed. In forest,rainfall intensity is usually less than the infiltration rate, so subsurface flow may be themain content. There were much organic matter and macropores in soil horizon A, whichwas conducive to the infiltration and retention of water; but less organic matter andinfiltration rate in soil horizon B. So there were two relatively impermeable interfaces.When precipitation and rainfall intensity were all high, subsurface flow may occur morequickly in horizon A than B. Deeper permeable of macropores sometimes also made theflow of horizon B accelerated. When heavy rainfall occurred, there would be two peaks onthe hydrograph, because of out of sync flow of horizon A and B; when rainfall intensityand rainfall was small, because of the infiltration and water-holding of soil macropore, theprocess slowed, appeared unimodal.（4） In the Huoditang forest, subsurface flow might also become the main source ofprecipitation runoff, while the upper soil was not saturated. When I₁<P<I₁+S1, and I₂>0（P,precipitation; S₁, seepage into the macropore at the soil surface; I₁, infiltration into matrixfrom the surface; I2, infiltration into matrix from macropores.）, surface macropore andtiny pore all absorbed rainfall. Rapid flow formed in macropores of surface soil,controlling the production of subsurface flow and directly influencing on the flood processof V notch weir.