Analytical geomorphodynamic assessment of six decades of gully expansion (1957–2020) in the Kaju–Gargaru watershed, southeastern Iran

Gully erosion is the dominant land-degradation process in the arid coastal plains of southeastern Iran [1]. In the Kaju–Gargaru watershed, Sistan and Balouchestan province, an expanding gully network threatens villages, roads, and flood-irrigated farmland. This study quantifies the trend of gully expansion between 1957 and 2020, characterizes gully morphometry, and determines linear and areal advancement rates across a 130,000-ha study area. The study area is a low-gradient, highly erodible alluvial plain developed on marl-rich formations, with saline–sodic, low-permeability soils under an arid climate. Gully networks were mapped for five dates following a multi-temporal approach [2]: 1957, 1968, and 1993 from ortho-rectified aerial photographs, 2009 from ALOS satellite imagery supported by field surveys, and 2020 from a dedicated UAV campaign. A fixed-wing eBee X platform carrying a 24-megapixel Aeria X camera surveyed the 32,000-ha critical zone, yielding an orthomosaic, DEM, and DSM at ~6 cm ground sampling distance; Sentinel-2 (10 m) and Google Earth imagery completed the coverage of the remaining area. Gullies were digitized in GIS, and validated in the field. Width, depth, and planform of 206 gullies around nine villages were measured from the UAV products. The gullied area grew from 2,371 ha in 1957 to 8,730 ha in 2020, a 3.7-fold increase that raised the affected share of the study area from 1.8% to 6.6% (figure 1). Mean linear head-cut retreat of the main gullies ranged from 30 to 150 m per year depending on the period (63-year mean ≈120 m per year), with individual retreat rates up to 612 m per year in 1968–1993 and a maximum single-gully advance of 1,024 m during 2009-2020 (Belshemel gully), placing these gullies among the fastest-retreating reported worldwide [3]. The gullies form digitate (finger-like) networks with predominantly U-shaped cross sections, depths of 0.2–8 m (mean ≈3 m), top widths of 0.9–56 m (mean ≈9 m), and width-to-depth ratios above 1 that increase toward the gully heads. Soil loss by gully erosion in the critical zone averaged ≈955,000 t per year (≈38 t ha⁻¹ yr⁻¹) over 1993–2020. Across all periods, the highest expansion rates consistently followed the pathways and magnitude of Kaju River floods rather than differences in soil type or vegetation, identifying flood routing as the primary control on gully advance; only physical flood-control structures arrested it.