11 Apr 2022
11 Apr 2022
Status: this preprint is currently under review for the journal TC.

Estimating degree-day factors based on energy flux components

Muhammad Fraz Ismail1,2, Wolfgang Bogacki2, Markus Disse1, Michael Schäfer2,3, and Lothar Kirschbauer2 Muhammad Fraz Ismail et al.
  • 1TUM School of Engineering and Design, Technical University of Munich, Germany
  • 2Department of Civil Engineering, Koblenz University of Applied Sciences, Germany
  • 3Faculty of Agriculture, Yamagata University, Tsuruoka, Japan

Abstract. Melt water from snow and ice dominated mountainous catchments is a valuable source of fresh water in many regions. Seasonal snow cover and glaciers act like a natural reservoir by storing precipitation during winter and releasing it in spring and summer. Snowmelt runoff is usually modelled either by energy balance or by temperature-index approaches. The energy balance approach is process-based and more sophisticated but requires extensive input data, while the temperature-index approach uses the degree-day factor (DDF) as key parameter to estimate melt merely from air temperature. Despite its simplicity, the temperature-index approach has proved to be a powerful tool for simulating the melt process especially in large and data scarce catchments. The present study attempts to quantify the effects of spatial, temporal, and climatic conditions on the DDF, in order to gain a better understanding which influencing factors are decisive under which conditions. The analysis is physically based on the individual energy flux components, however approximate formulas for estimating the DDF are presented to account for situations where observed data is limited. A detailed comparison between observed and estimated DDF values yielded a fair agreement with BIAS= 0.2 mm °C-1 d-1 and RMSE=1.1 mm °C-1 d-1. The analysis of the energy balance processes controlling snowmelt indicates that cloud cover and under clear sky snow albedo are the most decisive factors for estimating the DDF. The results of this study further underline that the DDF changes as the melt season progresses and thus also with altitude, since melting conditions arrive later at higher elevations. A brief analysis of the DDF under the influence of climate change shows that the DDFs are expected to decrease when comparing periods of similar degree-days, as melt will occur earlier in the year and albedo is then likely to be higher. Therefore, the DDF cannot be treated as a constant parameter especially when using temperature-index models for forecasting present or predicting future water availability.

Muhammad Fraz Ismail et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2022-64', Roger Braithwaite, 12 May 2022
  • RC2: 'Comment on tc-2022-64', Lander Van Tricht, 14 Jun 2022
  • RC3: 'Comment on tc-2022-64', Rijan Kayastha, 15 Jun 2022
  • RC4: 'Comment on tc-2022-64', Álvaro Ayala, 23 Jun 2022

Muhammad Fraz Ismail et al.

Muhammad Fraz Ismail et al.


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Short summary
Fresh water from mountainous catchments in the form of snow and ice melt is of critical importance especially in the summer season for people living in these regions. In general, limited data availability is the core concern while modelling the snow and ice melt components from these mountainous catchments. This research will be helpful in selecting realistic parameter values (i.e. degree-day factor) while calibrating the temperature-index models for data scarce regions.