Estimation and analysis of the Antecedent Temperature Index–Melt Rate (ATIMR) function using observed data from the Kırkgöze-Çipak Basin, Turkey

•An effective temperature index snowmelt model for the Upper Euphrates River flows.•Estimation & analysis of the ATIMR function at different elevations and aspects.•A methodology for generalizing the point-based functions to use in complex terrains.•Trend in the tail part of the ATIMR is due to the metamorphic structure of wet snow.•Interception of water due to compact layers in the snowpack effective on melt rates. The ability to manipulate multiple variable temporal and spatial atmospheric and earth-born parameters with a single air temperature value affecting the melting and accumulation processes of the snowpack is a convenient approach for the design of water policies and the effective use of water resources. Some of the temperature index models require snowpack’s melt rate as a function of the accumulated number of heating degree-days determined using a model with the Antecedent Temperature Index–Melt Rate (ATIMR) function, in which the modeler can adjust the melt rates during the season. The use of ATIMR function provides a reasonable method; however, the literature on the nature of the default ATIMR function (developed in 1991 and currently in use) is largely unclear and not understandable or practicable. For that reason, in this study, the point-specific and domain-based ATIMR functions specific to the region were determined physically in a logical manner using error propagation analysis. Thus, it was found that R-square values up to 0.99 precision can be obtained. The Hydrologic Engineering Center’s hydrologic modeling system (HEC-HMS) was used and the results were found to be characteristically and statistically effective and valid for various Snow Water Equivalent (SWE) simulations. In addition, it was revealed that the unexpected behavior of a rapid decreasing-increasing trend observed in the tail part of the modeled ATIMR function resulted from the metamorphic processes in the wet snowpack. Therefore, it is crucial to analyze snow metamorphism without simplistic assumptions of dry snow due to the interception of water in the snowpack, particularly in case of having ice and crust layers for the subject region (i.e., Radar Station at 2891 m. asl). Within this study, three-year data sets were obtained from three different meteorology and snow observation stations that have been specially constructed and operated at different elevations and aspects in Kırkgöze-Çipak Basin. The snowmelt-dominated basin is considered to be the source of the headwat