Landscape Dynamics of Marly Slopes in the Lower Valley of Wadi Tamri (Morocco): An Integrated Approach Using Geomorphometry, Toposequences, and Remote Sensing

Abstract

The mountainous marly slopes of the Atlantic High Atlas host a unique, vulnerable ecosystem shaped by a complex interplay of biotic, abiotic, and human (anthropogenic) factors. Characterized by steep slopes, poor soils, and degraded Argan (Argania spinosa) forests, this landscape exhibits active and complex geomorphological processes, with the presence of diverse erosion forms (gullies and rills) indicating ongoing instability. The study focused on the dynamics of marly slopes in the lower valley of Wadi Tamri in the northern part of Agadir Ida Ou Tanane province. Using geomorphometric analysis, remote sensing, and field observations, we investigated the impact of various factors, including climate, geology, and human activities, on landscape evolution. Various applied geomorphometric indices were compared with the results of a hydrological evaluation model. The comparison was made using two digital elevation models with multiple dates (2007 and 2014). To assess land use changes within this sparsely forested, marly slope landscape unit, Landsat imagery from 1984, 2000 and 2022 was analyzed, supported by landscape point sampling and field validation to facilitate interpretation of the maximum likelihood classification algorithm. The results showed that the number, length, drainage and hydrographic density values of gullies decreased between 2007 and 2014. Bifurcation and length ratios vary according to stream order and demonstrate some evolution during this period. Evolutionary stages and scenarios are marked by the degradation of the forested landscape between 1984 and 2022. Conversely, the area of marly substrate within the landscape unit has increased by 891.30 ha between 2000 and 2022. Declining forest cover can expose marly surfaces to hydric erosion. This research provides a scientific foundation for further investigation of marly slopes and offers valuable insights that will serve as an example for regional soil and water conservation strategies.