Investigating the Long-Term Effect of Livestock Grazing, Slope Location, and Sampling Depth on Soil Moisture Characteristic Curves

Soil moisture characteristic curves (SMCCs) play a crucial role in understanding and managing soil-water relationships, which is vital in various fields including agriculture, civil engineering, environmental science, and hydrology. This study investigates the impact of livestock grazing, slope location, and sampling depth on SMCCs in the Gonbad watershed, western Iran. Livestock grazing practices were compared between free grazing and controlled grazing, along with three slope positions (shoulder, back, and foot-slope) and two sampling depths (0−12.5 cm and 12.5−25 cm). The van Genuchten model was employed to model the SMCC, incorporating Mualem's assumption. Water content at different matric suctions and van Genuchten model parameters were assessed using a factorial design. Integral root mean square difference (IRMSD) and surface difference (SD) parameters were calculated to evaluate the impact of different treatments on the SMCC, as a whole. The controlled grazing treatment at the back-slope recorded the lowest residual water content, measuring 0.151 cm³ cm⁻³ in the 0.00- to 12.5-cm depth. Conversely, the highest value of 0.234 cm³ cm⁻³ was observed in the free grazing treatment at the shoulder, specifically in the 12.5- to 25-cm depth. The results indicate that the permanent wilting point (PWP) value at the second depth exceeded that of the first depth. Specifically, at the second depth, the PWP value in the shoulder region of the slope under free grazing (0.235 cm³ cm⁻³) surpassed that of the corresponding position under controlled grazing (0.192 cm³ cm⁻³). The findings revealed that the highest α parameter value (0.438 cm⁻¹) was observed at the first depth of the back-slope position under the controlled grazing treatment. Results indicated significant effects of grazing, slope position, and sampling depth on soil properties and SMCC parameters. Notably, controlled grazing led to higher values of IRMSD (0.178 cm³ cm⁻³) and SD (0.672 cm³ cm⁻³), indicating enhanced moisture retention capabilities compared with free grazing. The study provides valuable insights into optimizing grazing practices for sustainable soil health and water management in pasture ecosystems.