Evaluation of evaporation rate from forested soil surface using stable isotopic composition of soil water in a headwater basin

The stable isotopic composition of rain water and soil water was analysed and the soil surface evaporation rate was evaluated using the isotope value of soil water taken from a forested headwater basin in central Japan. Rainfall, throughfall, soil water, groundwater and discharge water were sampled at intervals of one month to determine the isotopic compositions of deuterium (D) and oxygen-18 (18O) therein. In addition, tensiometric observation of soil water was performed continuously. The stable isotopic ratio of soil water was homogenized between the soil surface and a depth of 100 cm by means of active soil water movement above 100 cm in depth. The mean [dgr ]18O in deep soil water below a depth of 100 cm was 0.24[permil] higher than the volume weighted mean [dgr ]18O in throughfall. Consequently, the mean evaporation rate from the soil surface was calculated as 5% of the evapotranspiration rate using the difference between the [dgr ]18O in throughfall and in deep soil water, based on the Rayleigh equation under equilibrium condition.