Soil Hydraulic Parameters Of Forest Land And Sloping Farmland In The Hilly Area Of Central Sichuan Basin

In this study, a forest land and a sloping farmland located at the Yanting Agro-ecologicalExperimental Station of Purple Soil （10527E，3116N）, Sichuan, Southwestern Chinawere chosen as the study sites. The soil hydraulic conductivity of the forest land and the slop-ing farmland were determined with in situ methods （tension infiltrometer and Guelph infil-trometer） and laboratory falling head method. The differences in measured saturated hydraulicconductivity between the above two types of methods were compared. In addition, the soilwater retention curves （SWRC） of the forest land and sloping farmland were measured in asand box and a pressure chamber. The effect of the inclusion of soil volumetric water contentdata at low suctions on the SWRC was analyzed and the distributions of the soil pore sizeclasses were calculated. The main conclusions of this thesis are as follows:（1） The values of saturated hydraulic conductivity obtained with tension infiltrometerand Guelph infiltrometer were greater than laboratory measurements on ring samples, proba-bly due to the possible destruction of soil macro pores and the smaller soil volume repre-sented by the latter.（2） The difference in un-saturated hydraulic conductivity at depths of0¹0cm,10²0cmand20³0cm between the forest land and the sloping farmland was larger at the low suctions（h>-3cm）, while that at the high suctions （h <-3cm） was smaller. The saturated hydraulicconductivity values of the forest land at different depths were smaller by nearly an order ofmagnitude than the corresponding depth of the sloping farmland.（3） At the same depth and pressure head, the difference in mean pore radius between theforest land and the sloping farmland was small. The mean pore radius increased graduallywith the increasing suction （more negative）. For the forest land, the effective porosity value ofall three large pore size classes （i.e., r>0.5mm,0.25<r <0.5mm and0.125<r <0.25mm）at three depths did not exceed0.015cm³.cm^-3, which was significantly smaller than the cor-responding values of the sloping farmland. The effective soil porosity on the sloping farmlanddecreased gradually with increasing depth. In addition, the effective porosity values obtainedat the plough layer was the greatest, with the maximum value reaching0.23cm³.cm^-3, and thevalues of plow layer was appeared to be the lowest, being less than0.10cm³.cm^-3. （4） The results showed that the obvious ‘S’ shape of SWRC obtained from the datameasured using both a sand box and a pressure chamber was not observed for when the dataobtained without using the sandbox. The fitted value of soil residual water content （r） wasclose to the soil water content measured at a suction of15000cm only when the data obtainedat low suctions are included. Therefore, it is necessary to employ soil retention data measuredat both low and high suctions for soil water dynamics study, especially for relativelycoarse-textured soils as purple soil in Sichuan, China.（5） From the calculated soil pore size distribution, It show that the radii of the structuralpores showing a peak content in the surface soils （0¹0cm） of the forest land, the upper-slope, mid-slope and lower slope of the farmland were similar, being about41.28m. At10²0cm depth, the peak content was found at sizes of4.53m,52.64m,41.27m and49.56m-in radius, for the forest land, the upper slope, mid-slope, and lower slope of thefarmland, respectively. The radii of matrix pores showing peak content in the same layer wasfound similar under different land uses and slope position.