Effect of subsurface flow on the isotopic composition of soil water in headwater basins

The behaviour of subsurface water and stable isotope composition of soil wa ter was monitored in three headwater basins, Kawakami, Koshibu and Yodagiri , underlain by andesite, shale and granite, respectively. Hydrometric and t ensiometric observations were conducted and throughfall, soil water, ground water and discharge water were sampled. Stable isotope composition was dete rmined on all water samples in the three basins. In the hillslope of the Ka wakami and Koshibu basins, vertical unsaturated flow dominated and the delt a(18)O of deep soil water was similar to discharge water. In contrast, in t he Yodagiri basin, saturated storm flow in response to rainfall occurred th roughout the soil profile and delta(18)O content of soil water in the hills lope varied markedly. The vertical soil water movement might cause soil wat ers to become homogenized with respect to delta(18)O in the Kawakami and Ko shibu basins.