Separation of runoff components by stream temperature

Summary

It has been generally believed that, when a spring flood occurs, snowmelt discharges mostly to a stream as a surface runoff, and so the temperature of stream water is near 0 °C. Findings by the author in the northern part of Japan contradicted this belief, however, in that the stream temperature of a snowmelt flood was as high as 3–6 °C in the source area of a snow-rich basin, and soil had stored a sufficient amount of heat energy to warm the snowmelt according to measurements of soil temperature throughout the year.

Contrary to a rise in temperature of snowmelt, the temperature of water running off in a stream following a heavy rainfall in the warm period lowered to a temperature corresponding to that of soil at the depth of 2–3 m. This change in water temperature proves that both snowmelt and rainfall percolate mostly into the ground and then discharge as a subsurface runoff even at the time of a flood.