Citation: ¸ Sensoy, A.; Uysal, G.; Do˘ gan, Y.O.; Civelek, H.S. The Future Snow Potential and Snowmelt Runoff of Mesopotamian Water Tower. Sustainability 2023, 15, 6646. https://doi.org/10.3390/ su15086646 Academic Editor: Tommaso Caloiero Received: 19 March 2023 Revised: 10 April 2023 Accepted: 11 April 2023 Published: 14 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article The Future Snow Potential and Snowmelt Runoff of Mesopotamian Water Tower Aynur ¸ Sensoy *, Gökçen Uysal, Y. O˘ gulcan Do ˘ gan and H. Soykan Civelek Department of Civil Engineering, Faculty of Engineering, Eskisehir Technical University, Eski¸ sehir 26555, Türkiye; gokcenuysal@eskisehir.edu.tr (G.U.); hscivelek@ogr.eskisehir.edu.tr (H.S.C.) * Correspondence: asensoy@eskisehir.edu.tr Abstract: Mountainous basins are frequently called “natural water towers” because they supply essential water to downstream regions for irrigation, industrial–municipal use, and hydropower generation. The possible implications of climate change on water supplies have gained prominence in recent years, particularly in snow-dominated mountainous basins. The Euphrates River, a snow-fed transboundary river that originates from the Eastern part of Türkiye with several large dam reser- voirs downstream, was chosen within this scope. The study reveals the impact of climate change on two snow-dominated headwaters, namely Karasu and Murat, which have a basin area of 41,109 km 2 . The impact of climate change is assessed across runoff regimes and snow dynamics for future periods (2024–2099). Global Climate Model (GCM) data sets (CNRM-CM5, IPSL-CM5A, EC-EARTH, MPI- ESM-LR, NorESM1-M, HadGEM2-ES) were downscaled by Regional Circulation Models (RCMs), provided from CMIP5 EURO-CORDEX domain for climate projections under RCP4.5 and RCP8.5 scenarios. Future projections of runoff and snow variables are predicted by two conceptual hydro- logical models, HBV and HEC-HMS. The results indicate a dramatic shrink in snow cover extents (>65%) and snow duration (25%), a decrease in snow water equivalent (>50%), and a timely shift (up to a month) in peak runoff through early spring in the runoff hydrograph for the last future period (2075–2099). The overall assessment shows that operations of downstream water systems should be reconsidered for future changes. Keywords: climate change; snow dynamics; runoff projections; Euphrates River basin 1. Introduction Streamflow projections for a changing climate are crucial for improved strategic planning and management of water resources, including floods, droughts, sustainable irrigation, and hydropower, especially in highly vulnerable snow-fed regions [1–4]. Since snow accumulation and ablation are extremely sensitive to air temperature, the impact of atmospheric warming is anticipated to be significant in watersheds dominated by snow [5–8]. The fundamental effect of warming is a change in hydrological regimes from a regime dominated by snow to a regime dominated by rain [9,10]. Most regions of the Northern Hemisphere that depend on snow are expected to endure growing stress from low snow years [11], and experience changes toward earlier streamflow peaks [12,13]. Mesopotamia has long been a hotspot of agricultural activity and is frequently called the “cradle of civilizations”. This is partly because of the region’s mild climate and the fresh water provided by the Tigris and Euphrates, two significant rivers fed by snowfall. In this water-stressed region, severe droughts and a lack of water security have historically been important reasons for conflict [14,15]. The water resources of the Euphrates–Tigris basin are crucial for hydroelectric power generation, agriculture, and domestic consumption in Türkiye, Syria, Iraq, and Iran. On the other hand, the Middle East, located east of the Mediterranean basin, is one of the geographical areas most sensitive to climate change effects, according to the Intergovernmental Panel on Climate Change’s Fourth Assessment Sustainability 2023, 15, 6646. https://doi.org/10.3390/su15086646 https://www.mdpi.com/journal/sustainability