Seasonal Change of Evapotranspiration and Influence of Plant Density on Evapotranspiration.

Accurate determination of evapotranspiration or evaporation from ground surface is very important to analyze precisely groundwater flow in the unsaturated zone near the surface. Also this determination is important for detailed analysis of moisture movement and pollutant migration near the ground surface because evaporation give a boundary condition of flow. However, there are not so many knowledge on the evapotranspiration from the actual field. The present authors reported that the evaporation and evapotranspiration from ground surface can be measured using a box type technique with high accuracy and good applicability. In this study, seasonal evapotranspiration changes were successfully measured from an actual grass field. Two places were selected in these measurements, the first is vegetated place and the second is sparsely vegetated place. Evapotranspiration was measured in these places in the all seasons during a year. It was found that the maximum evapotranspiration from the vegetated place in the summer season is about four times of it in the same place in winter season in same year and evapotranspiration from the vegetated place is about three times of it from the sparsely vegetated place in the winter season. The effect of solar radiation or sunlight on evapotranspiration was clarified. It was found that, the radiation has large effect on the evapotranspiration rate. The evapotranspiration rate increases with the increase of solar radiation. Also, the effect of plant density on the evapotranspiration rate was studied. It was found that the evapotranspiration increases linearly in case of white clover plant and the transpiration for this type of plant could be estimated, separately, from the evapotranspiration. Evapotranspiration could be simply and easily determined in all seasons using the box type technique.