Effects of geomorphological processes and landforms on the soil and vegetation characteristics: A case study of Davarzan alluvial fan, Northeast Iran

Comprehensive understanding of the effects of alluvial fan geomorphology on the soil and vegetation properties can lead to better conservation and management of vegetation and soils. This study aims to analyse the effects of geomorphological landforms and processes of Davarzan alluvial fan on the properties of soils and vegetation. Aerial images were obtained by a DJI Phantom4 Professional drone to measure vegetation density (VD) and canopy cover (VCC). Subsequently, six topographic profiles from fan surfaces were provided by the 10-cm DEMs, prepared by aerial images. A total of 36 soil samples were gathered from different landforms (bar, swale, channel and interfluve) and positions (toe and apex) of fan surfaces. Soil properties including sand%, silt%, clay%, total organic carbon (TOC), soil hydraulic conductivity (K) and soil water content (W) were measured. Results show that TOC, clay% and silt% are higher at the old surface than the relict and young surfaces. Lower TOC, clay% and silt% of the relict surface (especially at its toe) can be attributed to its entrenched and crenulated surface with relatively steep topography, making it prone to greater soil erosion. Nevertheless, the means of vegetation canopy cover and vegetation density are higher at the relict surface than old and young ones, likely due to the fact that beds of deep channels developed on the relict surface have shaded conditions and are closer to groundwater. Data reveal that VD and VCC are higher at toes than apexes, at channels than interfluves of abandoned surfaces, and at swales than bars of young surface. Overall, the relative age, landforms, channel morphometry, geomorphological processes (i.e., weathering and aggradation) and positions (toe and apex) of alluvial fan are dominant factors affecting soil and vegetation characteristics.