Detection and mapping of daya features in the limestone plateau of Northwest Egypt

Abstract

This study delves into the previously uncharted territory of dayas, ephemeral wetlands dotting the arid limestone plateau of Northwest Egypt. As the first comprehensive investigation of these ecologically and geologically vital features, it addresses the critical lack of research on their origins, evolution, and spatiotemporal dynamics. Employing a multi-method approach that integrates high-resolution Sentinel-2 satellite imagery, multi-decadal Landsat time-series analysis, ALOS PALSAR digital elevation model (DEM) processing, and systematic field validation, we identify and delineate 5430 dayas encompassing 258.1 km 2 within a study area of 11,254 km 2 . The supervised maximum-likelihood image classification, validated against 346 stratified field control points, achieved an overall accuracy of 91% (kappa = 0.88). Morphometric analysis characterizes dayas as shallow saucer-shaped depressions with diameters ranging from a few meters to several hundred meters and depths of 0.75 to 5.5 m, etched into the limestone surface filled with loose sediments. 0.75 m is the minimum depth recorded among field-confirmed dayas during the March 2025 survey; the DEM-based automated detection employed a minimum sink-depth threshold of 0.5 m, which represents the lowest depth reliably resolvable at 12.5 m DEM resolution without incurring unacceptable false-positive rates from surface noise. A daya typology comprising bowl-shaped, small flat-bottom, and large flat-bottom classes is established, each reflecting distinct stages within a continuum from active to relict karst. Spatiotemporal analysis of Landsat imagery from 1984 to 2025 documents a 32.63% net increase in daya surface area, corresponding to a mean annual expansion rate of ∼0.8%. Their evolution appears to be a complex interplay of karstic processes, surface-water runoff, hydro-geomorphological interactions, and erosional forces, all influenced by the interplay of structural and climatic factors. This underscores the critical need for sustainable land management practices that acknowledge the dayas’ significance for environmental stability and potential aquifer recharge. In conclusion, this study not only sheds light on the origins and hydrological dynamics of these enigmatic wetlands but also emphasizes their crucial role in the Northwest Egyptian landscape. By advocating for sustainable land-use practices and conservation efforts, we can ensure the continued presence and beneficial influence of these unique dayas for generations to come.