The simple, empirical degree‐day approach for calculating snowmelt and runoff from mountain basins has been in use for more than 60 years. It is frequently suggested that the degree‐day method be replaced by the more physically‐based energy balance approach. The degree‐day approach, however, maintains its popularity, applicability, and effectiveness. It is shown that the degree‐day method is reliable for computing total snowmelt depths for periods of a week to the entire snowmelt season. It can also be used for daily snowmelt depths when utilized in connection with an adequate snowmelt runoff model for computing the basin runoff. The degree‐day ratio is shown to vary seasonally as opposed to being constant as is often assumed. Additionally, in order to evaluate the degree‐day ratio correctly, the changing snow cover extent in a basin during the snowmelt season must be taken into account. It is also possible to combine the degree‐day approach with a radiation component so that short time interval (<24 hours) computations of snowmelt depth can be made. When snowmelt input is transformed to basin output (runoff) by a snowmelt runoff model, there is little difference between the degree‐day approach and a radiation‐based approach. This is fortuitous because the physically‐based energy balance models will not soon displace the degree‐day methods because of their excessive data requirements.

Following the development of rainfall-runoff models the attention of the international hydrologic program is now increasingly focused on the snowmelt-runoff. The present simple model is based on taking into account the variability of the degree-day factor, recession coefficient and snow coverage. It can be adapted to heterogenous conditions of snow accumulation and temperature in mountainous basins.

During 1965-1967 the snowmelt runoff in a small mountain basin in Northern Czechoslovakia was studied by measuring the tritium and oxygen-18 content of precipitation, snowpack, and runoff, in addition to an improved degree-day method which was used as a standard tool. The results showed that about two-thirds of the meltwater infiltrated to the soil, and the displaced water recharged during the previous years toward the stream channels. The tritium content of the baseflow and the subsurface runoff indicated that the mean residence time of the subsurface water in the basin was approximately 2.5 years with a standard deviation of 1.25 years. The volume of the subsurface storage was estimated to be 2.6 X 106 m s. The oxygen-18 content of snow samples was in agreement with the tritium results on the relative contribution of the meltwater and subsurface runoff to the total runoff from the basin. x At present with the Institute of Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland. ' At present with the Institut fiir Schnee-und Lawinenforschung, Dayos, Switzerland. bined with environmental isotope techniques to investigate all phases of the snowmelt-runoff process.An evaluation of data obtained in the winter and snowmelt seasons of 1966 and 1967 is presented in this paper. This study illustrates the application of isotope techniques to problems of snow hydrology and it is hoped the study will stimulate the use of these techniques. CHARACTERISTICS OF THE BASINThe Modry Dul representative basin is situated in the eastern part of the Krkonose mountains in northern Bohemia (15ø42'E and 50ø43 'N (Figure 1)). The surface area is 2.65 km •, of which about 70% is mountain meadows and 30% is spruce forest. The mean altitude of the basin is about 1300 meters above sea level with a uniform distribution of altitude between the lowest point (1000 meters) and the highest point (1554 meters). Geologic conditions are characterized by rely-110 ß ß ß ß ß ß BASEFLOW --11.5 ß ß ß ß ßß ß ß Oß O0 Oß 0 / o• o o //o ß ß ß ß ß -12.3

No abstract

No abstract