Assessing areal and volumetric reservoir dynamics using remote sensing and in-situ measurements to explore drought relationships in the Marmara Region of Türkiye

Water resources are vital for ecosystem integrity, economic activities, and potable supply, yet the Marmara Region increasingly faces climate variability, rising demand, and recurrent droughts that challenge sustainable water management. This study provides the first regionally consistent, long-term assessment of reservoir dynamics in the Marmara Region by integrating remote sensing, in-situ volume records, trend and change-point analyses, and hydrological drought indices within a unified framework. Google Earth Engine-derived NDWI reservoir surface areas from Landsat imagery were validated using volumetric measurement from the General Directorate of State Hydraulic Works (SHW). Mann-Kendall trend tests, the Palmer Hydrological Drought Index (PHDI), and change point detection were applied to identify significant shifts, assess long-term dynamics, and drought impacts on water storage. Results reveal strong correlations between surface area and volume (r = 0.74–0.98, p < 0.05 n = 120 monthly observations per dam reservoir) and highlight pronounced declines during severe droughts, notably in 2014 and 2021, with Terkos and Karaidemir reservoirs being the most affected. In general, most reservoirs exhibited declining surface area/volume and negative drought trends, with only Ömerli showing an increasing trend and Doğancı deviating by not exhibiting a drought trend. These findings highlight the joint effects of climatic variability and human pressures associated with irrigation, urban water supply, and energy production, underscoring the need for integrated remote sensing-supported monitoring and adaptive reservoir management to strengthen regional drought resilience.