Abstract: The freezing of water-conveying open channel in winter is a common natural phenomenon in cold regions. Once ice carapace appears in the middle route of the South-to-North Water Transfer Project, the safe flow reduces to only 30%-50% of the design flow. As temperature is the main factor driving the formation of ice carapace, the evaluation of air temperature level in winter became a key problem for water conveyance in freezing period. The interannual variations of air temperature along the main canal is revealed according to the daily average temperature and daily minimum temperature data of eight meteorological stations along the line of the Project in winter from 1951 to 2021. A concept of air temperature chain is proposed and the general mathematical expression is given based on the Lagrange particle tracking method in line with the physical process of air temperature affecting water temperature and then further producing ice carapace along the line. The National Standard Method, January’s Average Temperature Method, and the proposed Air Temperature Chain Method are used to evaluate the winter temperature level and construct the temperature order in 71 years. Results manifest that: 1) The air temperature along the main canal decreased gradually from south to north at a rate of 0.48 ℃/(100 km), and in particular, Baoding station displayed an accelerated downward trend, which made the greatest contribution to the formation of ice carapace. 2) The temperature in winter was fluctuating upwardly in general at a rate of 0.37 ℃/10 a. Severe warm winter appeared in the recent three decades and severe cold winter in the first three decades. The rising of air temperature is conducive to alleviating the formation of large-scale ice carappace. 3) The proposed Air Temperature Chain Method is superior to the January’s Average Temperature Method and the National Standard Method in predicting the formation of ice carapace. Better prediction result can be achieved by combining different time scales. Under the background of continuous global warming, the joint evaluation by January’s Average Temperature Method and Air Temperature Chain Method with 6-day data at two north stations can be adopted. 4) The thresholds of winter temperature grade obtained by the Air Temperature Chain Method with 6-day data at two north stations are given as follows. Strong warm winter: T_C≥-4.0 ℃; weak warm winter: -5.7 ℃≤T_C＜-4.0 ℃; normal winter: -7.4 ℃＜T_C＜-5.7 ℃; weak cold winter: -9.1 ℃＜T_C≤-7.4 ℃; strong cold winter: T_C≤-9.1 ℃. The average value of long series T_C is -6.5 ℃. The research finding is expected to offer a new reference for predicting the formation of ice carapace in the main canal.

Key words: middle route of South-to-North water diversion project, air temperature chain, winter temperature, ice carapace, grade evaluation, cold winter, warm winter