Study Of Spatial And Temporal Variability Of Top Soil Pore Characteristics Of Red Soil During Rainfall

Pore characteristic is distributing of pore size, porosity and the pore shape. It directlyaffects the runoff and infiltration in soil erosion process. And it has a close relationshipwith runoff and permeability, so it is important to know top soil pore characteristic. In thispaper, a typical red soil derived from Quaternary red clay was used to study the spatialand temporal variability of top soil pore characteristics and the main results are asfollowed:First, the pore quantity change of soil transect during rainfall: the section of 0-2.5mmdecreased sharply. And the section of 2.5-5mm followed. The pore quantity of section7.5-12.5mm decreased much less. Besides, the quantity of 0.05-0.2mm and 0.2-0.35mmincreased mainly in the section of 0-2.5mm and 2.5-5mm at 3 minute. At about 30min, thepore quantitity of each section became stable. Pore quantity of D (equivalent diameter)＞0.5mm took up a dominant status while D＜0.05mm decreased least.Second, the pore acre changes of soil transect during rainfall: the pore acrepercentage of all the D value of the soil transect decreased sharply. Only acre of D＜0.35mm increased a little. The soil pore acre percentage of D＞0.5mm took up adominant status in all the section of soil, while the pore of 0-5mm for D＞0.5mm wassmaller than other's pore acre after the rainfall. The change of 0-2.5mm was similar withthat of 2.5-5mm in the rainfall. That was all because such three section soil suffered lessfrom the surface and raindrop, which were only affected by sole seepage effect of water.Third, the pore shape changes of soil transect during rainfall: irregular pores took upa biggest acre as well as the reducing extent. Round pores accounted for least acre andsmallest reducing extent. Long pore took up small acre at any time. After 30 minutes,irregular pores of section 7.5-12.5mm accounted for big acre. Soil sample of D＜0.5mmtook up much smaller acre; round pore reached nearly a half, while accounting for almost70 percent in the section of 0-2.5mm. Long pores nearly disappeared in the section of0-2.5mm, so did other sections.Fourth, the pore quantity changes of soil vertical level during rainfall: the porequantity of level of 0-5mm began to reducing extent; after 6 minutes' raining and lasteduntil 30 minutes. The most obvious changing extent had D value ranging from 0.05mm to0.2mm and from 0.2mm to 0.35mm. The pore quantity of level of 5-10mm reducedgradually. Both reducing rate and extent were smaller than level of 0-5mm Among thepores of all the D value, the pores of 0.2-0.35mm were much more than level of 0-5mmThe level of 10-20mm had more pore quantities than 0-5mm and 5-10mm, reducing least inthe rainfall. After 30min's raining, the quantities still keeped 3800 around.Fifth, the pore acre changes of soil vertical level during rainfall: there was alsodecreased sharply in soil pore after 6minutes' raining and this phenomenon lasted until 30minutes. The pore acre of level-L1 reached minimum which were 5 percents at the 30 minutes. Among all the descending pore acre, the changing extent for D＞0.5mm was themost obvious. The pore acre level of 5-10mm reduced gradually, whose reducing speedwas less than the leve of 0-5mm and stable all the time. In the raining time, there is noobvious change for pore acre of the level of 10-20mm in the early 6 minutes, while a slowreducing after 9 minutes. Until 30 minutes the acre still maintained 20 percents around.The change of the pore acre of level of 10-20mm was less evident than others.Sixth, the pore shape changes of soil vertical level during rainfall: round pores tookup the least proportion and long pores and irregular pores took up much bigger. The soilpore of D＞0.5mm accounted for most while that of D＜0.05mm accounted for little. Inthe period of raining, long pores took up more than a half in the 5-10mm and 10-20mm.Irregular pores in the level of 5-10mm and 10-20mm, accounted for bigger than in the levelof 0-5mm. In the whole rainfall process, there was no big change of the pore shape andaperture in the leve of 10-20mm. That was because level of 10-20mrn suffered not muchshape the rainfall.Seventh, the comparing analysis of soil acre and runoff and erosion processmanifested that the total soil acre had a decreased sharply at 3-6minutes. Pore of D＞0.35mm had a decreased sharply. That was also the reason for soil slope runoff. The timeof slope runoff was about 8 minutes later than the change of pore acre. Soil pore acre kepta decreased sharply all the time and tended to be stable around 40 minutes. There was acoherence between the trend of soil pore acre and slope runoff strength. As the change ofsoil pore acre derived from soil grains transfer, they were manifestations of soil runoffstrength, erosion and eroded grains (Hairsine, 1991). Therefore, the change process ofslope runoff strength, erosion and eroded grains characteristic can be reflected by analysisof soil pore acre.