Measurement and analysis of evapotranspiration and surface conductance of a wheat canopy

The characteristics of evapotranspiration and canopy surface conductance of winter wheat have been measured and analysed by using the Bowen ratio-energy balance (BREB) method. In order to understand better the effect of soil water status on evapotranspiration and conductance of wheat, the daily and seasonal variations of leaf stomatal resistance under different soil water conditions have also been monitored and analysed. The experiments have been carried out at Luancheng Experimental Station of Agro-ecosystem (LESA), Heibei, People's Republic of China. During the growing seasons after turning to green, most of the available energy (Rn - G) of the wheat field is partitioned into latent heat. In the daily course, generally, the Bowen ratio (H/LE) for winter wheat is less than one-third; sometimes, it can reach a value of around 0.4. The seasonal trend of LE/Rn shows a parabola curve. The ratio of LE/Rn varies from 71 to 93%, with the peak occurring in the blooming stage. G/Rn shows a declining logarithmic trend during the observation period, and reaches its minimum value of around 6-8% after blooming. According to the BREB method, the daily mean evapotranspiration rate from jointing to maturity is 3.9 turn day(-1), which is very close to the value 4.1 turn derived from the water balance equation. The daily average value for canopy conductance g(c) is about 7.8 mm s(-1). Seasonal trend of leaf stomatal resistance shows that the stomatal resistance in blooming and milking are lower than that in other growing stages. This is possibly determined by the characteristic of water requirement of winter wheat. The stomatal conductance is strongly dependent on soil water status in natural conditions. When the extractable soil water content (ESW) of the 0-40 cm layer is less than 50%, the leaf stomatal conductance g(st) is linearly dependent on ESW, whereas there is no relation between ESW and g(st) when ESW is more than 50%. Therefore, the ESW of 50% can be considered as a threshold value of extractable soil water content for controlling the leaf stornatal conductance, or as an indicator of incipient stornatal closure. The slope of the g(st)-ESW line varies through the growing stages, with the largest slope occurring in the milking stage. Based on the analysis of g(st)-ESW relations, we construct a simple but realistic model of stornatal conductance-soil moisture relations. Copyright (C) 2002 John Wiley Sons, Ltd.