Behavior of Subsurface Water Revealed by Stable Isotope and Tensiometric Observation in the Tibetan Plateau

抄録

Intensive observation of the pressure head in subsurface water and sampling of subsurface water were performed to investigate the subsurface flow process in the Tibetan plateau from July 24 to September 12-the monsoon season-1998. Pressure head of the subsurface water was monitored using tensiometers and subsurface water was sampled using suction lysimeters installed at multiple depths from 10 to 100 cm.The pressure head of subsurface water ranged from −10 to −100 cmH₂O, and zero flux plane was often observed above the depth of 30 cm. The groundwater recharge was very active during this period, thus the groundwater table rose up to the depth of 55 cm in the beginning of September. The δD and δ¹⁸O of shallow subsurface water varied markedly with precipitation and evaporation, whereas those of groundwater were stable. The mean δ¹⁸O of groundwater was 3.4‰ higher than the volume weighted mean δ¹⁸O of precipitation. The difference of δ¹⁸O between the groundwater and the precipitation would be caused by isotopic enrichment along with evaporation from the soil surface, and 27% of precipitation might be lost by evaporation from the soil surface.