ニホンナシ園における蒸発散速度の実態解明と日蒸発散量の推定

Abstract

In order to estimate the amount of soil water content of orchards in a region and to improve water management, observations of transpiration and evapotranspiration were made in a Japanese pear orchard. The observations were made over a perild of 4 years, from April to October of each year, using the energy budget (EB), soil water (SW), eddy correlation (EC), and trunk energy budget (TEB) methods. The measured daily transpiration or evapotranspiration was highly proportional to Penman potential evapotranspiration (PET) for almost any month and year. When the inter-year variation was neglected, the coefficient of this proportionality for evapotranspiration (CET) in each month ranged from 0.77 to 0.85. Thus, (0.80 × PET) was proposed as the simplest estimation for daily evapotranspiration. The inter-year variation in CET was larger during the summer season. In the summer, CET was lower in the years in which less precipitation occurred, and a midday depression in the rates of transpiration and evapotranspiration was observed when pF increased. Therefore, water stress was suggested as the cause of inter-year variation in CET. When the effect of water stress was excluded, CET was lower in April, May, and October, and its seasonal variation was around 20%. Since the ratio of transpiration to evapotranspiration was lower in April and May, seasonal depression in CET was suggested to occur due to depressed transpiration that occurred as a result of a small leaf area index (LAI). Thus, finally, we proposed the estimation of daily evapotranspiration more precisely by using the formula 0.97 × f (pF) × g (LAI) × PET, in which the decreasing function f (pF) and the increasing function g (LAI) were introduced.