Variation of evapotranspiration with stand age and climate in a small Japanese forested catchment

Evapotranspiration (ET) was measured in both a young (stand age 4–7) and a mature (stand age 62–66) forest basin covered with a mixed stand of Japanese cypress (Chamaecyparis obtusa) and Japanese cedar (Cryptomeria japonica). To obtain ET, runoff data were analyzed on a monthly basis using a short-time period water-budget method. Canopy interception (I) was also measured in the mature (stand age 72–75) forest basin. Annual ET changes in the young forest basin showed a clear upward trend, and ET had higher values in hot summer (1994) than in the other summers. These results were simulated by a model based on the Penman–Monteith equation, and results agree with field measurements. This model predicts an ET-stand age relationship, which shows a peak in ET at 20 years, reflecting an LAI-stand age relationship data obtained from the literature. The response of ET to hot (1994), cool (1993), and mean (1981–1994 average) summer conditions was simulated for both the young and the mature forest basins. In hot summer ET was large for the young forest in comparison with a mean summer but the same level in the mature forest. In cool summer, however, ET was smaller in the mature forest than that of a mean summer but the same level in the young forest.