[AHW22-P05] Estimating evapotranspiration using diurnal fluctuations in total soil moisture under Japanese forest plantations
Keywords:Understory Vegetation, Total soil moisture (TSM), Evapotranspiration (ET), Soil water, Stable Isotopes
Our research project is conducted in a hill slope environment at Karaswayama mountain in Tochigi prefecture. In this study, we inspect diurnal changes in total soil moisture content (TSM) to estimate ET at a plot scale level. We observe fluctuations in water content on daily basis by installing several soil moisture sensors at different depths of 5, 10, 20, 30, and 50 cm. Soil water samples are also collected at 5 to 80 cm depths. We are performing this research on two study sites with a total of six plots. The first site (P1) is covered by Japanese Cypress plantation where strip thinning have been conducted. It consists of four plots two of which are below middle-canopy (MC) condition, and we remove the understory vegetation of one plot. The other two plots are below under-canopy (UC) condition and we also remove the understory cover of one of the plots. The second site (P3) is covered by Japanese cedar plantation where spot thinning have been performed. This site consists only of two plots one at MC condition and another at UC condition, and we keep the understory vegetation cover of both plots intact. The water table displays daily fluctuations influenced by ET depletion of soil moisture content. To compute ET, we use the night time slope of TSM between 1200h midnight and 0400h to estimate the subsurface flux. Then we calculate evapotranspiration based on a diurnal hydrologic balance. In addition, we perform the isotopic measurements to partition ET within each study plot.
Our primary examination using TSM method indicates consistent results of ET levels. Using past soil moisture data from P1 study site, we calculated ET and compared our results to previous measurements of evapotranspiration performed by Sun et al. 2017, which used lysimeter method. Our analysis demonstrates a positive correlation coefficient ranging from 0.25 to 0.86. Hence, we inspect the applicability of this method in our study sites to determine its suitability in estimating ET without prior information on vegetation type or detailed measurements of soil texture and site hydraulic properties. Nonetheless, the accuracy of the measurements requires for soil moister monitoring to remain continuous and of consistent values.