A physiographical study on the secular fluctuations and regional characteristics of rainfall-runoff processes

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Changes in the components of rainfall-runoff characteristics including precipitation, evapotranspiration, and runoff ratio over the last 100 years were comparatively investigated in two river basins as a case study. Annual precipitation in the study area has tended to decrease since the first half of the 1970s. On the other hand, the smooth trend curve indicates that annual evapotranspiration has increased over the long term, especially since the 1980s. Smoothed long-term changes in the difference between annual values of precipitation and potential evapotranspiration are analogous to those of annual precipitation. The average annual runoff ratio for each year has also shown a tendency to decrease during the last few decades. It is pointed out that the notable characteristics of current frequency-magnitude distributions in annual precipitation are both a reduction of intervals of hydrological extremes and an increase of range in each year. Under conditions of extreme meteorological events that increased air temperature in the summer of 1994, river water quality showed a remarkable change compared to its average value in a normal year. The most striking feature due to high temperature and small discharge was a very low value for the concentration of dissolved oxygen, especially in July and August. The facts identified in the present study provide a meaningful perspective of the possible consequences of global warming for rainfall-runoff processes, and are also useful basic data for evaluating the effects of future climate change on the aquatic environment.