# The Application of Computed Tomography to Study the Soil Porosity of Mountain Red Earth
## 2 Materials and Methods  
### Study Site and Sampling  
The study area is located in the southwestern corner of Songmao Reservoir, Laoya Mountain, Chenggong District, Kunming City, Yunnan Province, China (Figure 1). Songmao Reservoir, built in 1958, is situated in the upper reaches of the Laoyu River in the Chenggong District of Kunming City, Yunnan Province. The normal reservoir capacity is 10.9 million cubic meters, with a catchment area of 38 square kilometers. The annual average precipitation is over 1000 mm. This area belongs to the subtropical highland monsoon climate. The reservoir water surface is at an altitude of 2016.75 m, surrounded by mountains and dense forests, making it a typical red soil region. The research area is situated approximately 10 km east of Dianchi Lake, in the hilly region on the outskirts of the Kunming Basin. The area features small elevation differences and slopes, well-developed gullies, and relatively well-preserved but variably thick weathering crusts. In this region, extensive and deep regional ancient soils known as Mountain Red Earth are widely distributed across the Yunnan Guizhou Plateau, primarily concentrated in the eastern part of the Yunnan Plateau. These red soils are generally believed to have begun forming in the late Tertiary period, when a warm and humid environment facilitated a process of desalination and aluminum enrichment in the rocks. This process resulted in the formation of thick, high-aluminum red weathering crusts. Subsequent tectonic movements intermittently uplifted the plateau, further promoting the development of Mountain Red Earth [33]. The vegetation in the region predominantly comprises subtropical evergreen broad-leaved forests, mainly consisting of artificially planted Yunnan pine and Chinese fir. The vertical structure of the plant communities is relatively straightforward. However, in recent years, the Laoyu River basin has faced severe erosion of Mountain Red Earth due to natural factors and human activities. This has resulted in adverse vegetation retrogression in parts of the study area, making it challenging for vegetation to establish and grow. Urgent measures such as tree planting, grass seeding, and vegetation restoration are crucial to address these ecological challenges. The sampling site is located on the southwestern slope of Songmao Reservoir area, Kunming City, Yunnan Province, with geographical coordinates approximately 102.88\({}^{\circ}\) E longitude and 24.88\({}^{\circ}\) N latitude. In this area, there is development of secondary forests dominated by artificial plantations. The vegetation is primarily composed of eucalyptus trees. In order to maintain the original state of internal soil pores and facilitate sampling, an area was chosen that is relatively open and free from the influence of large tree roots. During sampling procedures, particular attention was given to carefully removing the superficial humus layer, which is loose and not pertinent to the research focus of this study. Initially, a boundary of 100 cm \(\times\) 100 cm was excavated on the slope. After digging to the predetermined depth, the excavation slowly proceeded towards the core area of the soil column. When the excavation reached within 5 cm of the core region, the soil column was meticulously trimmed. The surplus of fine roots and debris on the side of the soil column was removed using a soil knife. To prevent water loss and disturbance to the soil body in its natural state during transportation, multiple layers of cling film and cushioning cotton layers were wrapped around the surface of the soil column at the site. The soil sample was then secured with a pre-prepared wooden board, and the exterior was tightly wrapped and sent to the detection unit for CT scanning. During sample collection for CT scanning, samples were also taken at 10 cm intervals for particle size determination.