Effect of Soil Surface Wetness and Air temperature on Transpiration in Wheat Field Covered Sparsely by Vegetation

Abstract

We constructed a detailed heat balance model for a sparse canopy, which allowed the interaction of heat flux between soil and vegetation, and set the values of various parameters for the model based on observation data, obtained from a wheat field covered sparsely by vegetation (plant height, 0.52 m; leaf area index, 1.2) between 1100 and 1400 h on July 12–14. Using this model, we examined the effect of soil surface wetness (volumetric soil moisture in the top 2 cm layer θ, 13.7 % or 22.3 %) and air temperature (23 °C or 27 °C) on transpiration from vegetation under a given condition (solar radiation, 850 Wm^-2; atmospheric radiation, 350 Wm^-2; specific humidity, 0.013 kgkg^-1). As a result, it was found that transpiration increased 1.41–1.46 times as air temperature rose by 4 °C under the same condition of soil surface wetness, and increased 1.14–1.17 times as soil surface changed from wet (θ, 22.3 %) to dry (θ, 13.7 %) condition at the same air temperature. The ratio of transpiration to evapotranspiration was 0.42–0.49 when the soil surface was wet, and 0.71–0.75 when the soil surface was dry.