ABSTRACT
ABSTRACT: Monthly precipitation samples were collected at Tsukuba, Ogawa and Utsunomiya City from 1992 to 2006, and isotope ratios of oxygen and hydrogen were determined for all samples. The isotope ratios of monthly precipitation have no remarkable trend in their seasonal change. With regard to d-excess, however, cyclic variations are observed, with relatively low d-excess values in warm periods and relatively high values in cool periods. A temperature effect on isotopes in precipitation is found during cool periods. In periods of snowfall, or precipitation from Baiu fronts and autumnal rain fronts, the isotope ratios in precipitation are relatively lighter than those of other periods. Long-term variations indicate that the annual mean air temperature is increasing and the amount-weighted mean δ18O values are decreasing gradually at all sites. The annual air temperature is increasing gradually because of global warming and urbanization. The decrease in stable isotope ratio could be caused by a change of meteoric conditions for precipitation (e.g. rainfall intensity).
