Spatial Variation Characteristics Of Soil Basic Hydro-physical Properties In A Watershed In The Natural Forest Region, Qinling Mountains, And Effect On The Runoff

This study was conducted in a typical watershed in the Huoditang natural forest region,Qinling Mountain range. Proportion of particle size, unit weight, water content andpermeability coefficient of soil were measured from soil samples which were collected at Ahorizon and B horizon respectively from sampling sites. The sampling sites were placed at theupper, middle and lower position of side slopes located in the upper, middle and lowerreaches of every one of3subwatersheds, respectively. These3subwatersheds were located inthe upper, middle and lower reach of the typical watershed, respectively. Being usedcomparison way and other ways to analyze the measured data, the spatial variationcharacteristics of soil basic hydro-physical properties in the watershed have been revealed.In the watershed, sand proportion of soil in A horizon is higher obviously than that in Bhorizon while silt and clay proportions are lower; the mean soil unit weight in A horizon ismuch lower than that in B horizon; the mean soil permeability coefficient in A horizon is1.88times as great as in B horizon.From upper position to lower position of side slopes in the subwatersheds, sandproportion increases eventually in both A and B horizons, silt proportion, however, decreaseseventually, clay proportion tends to decrease in A horizon while increases in B horizon; soilunit weight tends to increase in A horizon while to decrease in B horizon; soil permeabilitycoefficient tends to increase in both A and B horizons.From upper reach to lower reach in the subwatershedes, soil unit weight changes asreverse “V” shape in both A and B horizons while permeability coefficient changes as “V”shape.From upper reach to lower reach in the watershed, sand proportion tends to decrease inboth A and B horizons, silt proportion, however, to increase, and clay proportion changes as“V” shape; soil unit weight changes as reverse “V” shape in A horizon while decreaseseventually in B horizon; soil permeability coefficient change is the same as in the subwatersheds and consists with the change of clay proportion of soil in A horizon, however,not obvious in B horizon.Spatial variation of the proportion of particle size, especially of the clay proportion, andthe interaction of clay with organic matter play a very important role to influence hydrologicprocess in the forest watershed.There are more storms and the precipitation is higher in summer than any other seasons;the number of storms that occurred in autumn is in second position and the precipitation is alittle bit lower, but with more rainy days, the flow rate is relative high and stable; the annualprecipitation changes greatly in different years.That forest soil is a principal factor that affects discharge in forest ecosystem wasadvanced. When precipitation and rainfall intensity are all high, subsurface flow may occur insoil horizon A and B respectively. Subsurface flow in horizon A moves faster than that inhorizon B. When precipitation is high and the rainfall intensity is low, subsurface flow justmay occur in horizon B.The condition that subsurface flow occurs in horizon A is that horizon B is saturated orrainfall intensity is greater than its percolation capacity. When heavy rainfall occurs, therewill be two peaks on the hydrograph of the outlet of a drainage basin. The first one mainlycaused by subsurface flow occurred in horizon A, the second one is mainly caused bysubsurface flow occurred in horizon B. The former one has the characteristics of rising andfalling all quickly, and the magnitude of the peak discharge is related to the antecedent soilstorage, rainfall intensity and high-intensity rainfall duration.