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 lever in the young forest. (C) 2000 Elsevier Science B.V. All rights reserved.