Estimation of evaporation rate from the forest floor using oxygen-18 and deuterium compositions of throughfall and stream water during a non-storm runoff period

Abstract

Rainwater, throughfall, stream water during a non-storm runoff period, and soil water were sampled about every 10 days at six nested basins within the Hitachi Ohta Experimental Watershed in Japan from 1998 to 2001. Basins HA and HB are covered with an 80-year-old artificial forest of Japanese cypress and Japanese cedar. HC and HV are covered with 12-year-old forest of the same species. HO and HX are covered with various aged stands of the same species. The samples were analyzed for the isotopic compositions of 18O and deuterium (D). The weighted means of δ18O (δD) in HA and HB were –7.35 (–46.7) and –7.38 (–46.8)‰, while they were –7.51 (–48.1) and –7.57 (n/a)‰ in HC and HV, respectively. They were –7.50 (–47.6) and –7.41 (–47.1)‰ in HO and HX, respectively. There was a relative difference of 0.2 (1.4)‰ in δ18O (δD) between 80-year-old and 12-year-old forest. The stream water during a non-storm runoff period is considered to reflect the effect of evaporation from the forest floor. The evaporation rates from the forest floor were estimated using δ values in throughfall and stream water using the Rayleigh distillation equation under equilibrium conditions. They were estimated to be 5.5 (9.1), 5.2 (9.0), and 4.9 (8.7)% of annual throughfall in HA, HB, and HX (mature forests), respectively, using δ18O (δD), and 4.0 (7.6), 4.1 (8.1), and 3.5 (n/a)% of annual throughfall in HC, HO, and HV (young forests), respectively.