Stable isotope signature along hydrological processes in a temperate forest, Japan.

Abstract:

Long-term observation of stable isotope signature along hydrological processes has been ongoing in Kiryu Experimental Watershed (KEW; 5.99 ha). It locates at Shiga prefecture, Central Japan (34°58’ N, 136°00’ E). KEW is covered by forest, mainly consists of Chamaecyparis obtusa Sieb. et Zucc. (Japanese cypress) planted around 1959 for forestry purposes. The climate is warm temperate, with rainfall distributed year round, peaking in summer, but producing little snowfall in winter. The mean annual precipitation is 1664.0 mm. The mean annual air temperature is 13.6 ℃.

The local meteoric water line (LMWL) was fixed based on the data rainwater samples observed in since 2001 to 2018 as:

δ²H = 7.6 δ¹⁸O +12.2 (r2=0.87).

Along the hydrological process, the fluctuation of isotope value was getting smaller; soil water below the A0 layer sampled using tension-free lysimeter, and saturated throughflow showed similar range of isotope values. The fluctuation in groundwater was smaller at deeper groundwater layer, and that in streamwater was intermediate. All of these isotope values are plotted along the LMWL. However, isotope values of soil water and branch water, both were extracted with cryogenic vacuum distillation, are plotted below the LMWL, especially δ2H are smaller.

In many recent studies, this kind of pattern in delta-diagram is considered as supporting ‘Two-water world hypothesis”. However, some studies have pointed out that the cryogenic vacuum distillation is no appropriate to extract the soil- and branch water. In this study, we will discuss about this hypothesis including technical matter in a warm temperate forest.