Determination and validation of corn crop transpiration by sap flow measurement under field conditions

Transpiration of an adult corn crop was measured with the sap how method using the stem heat balance technique. The 1 ha-field was located in the Parisian region and the measurements were taken over two consecutive years (1993 and 1994). The experiments were carried out during the active growing season, leaf area index being approximate to 4.0. In order to individuate the "mean plant" a preliminary analysis was carried out to determine the frequency distribution of plant stem diameters as measured on 34 randomly chosen samples. According to this analysis, the mean corn plant diameter was around 20mm for both years. As a consequence, the sap flow gauges were installed on plants having this diameter. The sap flow transpiration data was compared with the actual evapotranspiration measured by the Bowen ratio method on the same experimental field at the hourly and daily scales. The comparison between sap flow transpiration and actual evapotranspiration showed that the values are more widely spread at the hourly scale than at the daily scale due to the kinetics of plant water content during the day. Sap flow transpiration was 88-90% of actual evapotranspiration: this lack of about 10-12% is of the same order as soil water evaporation. In conclusion, the sap flow transpiration method could be considered a valuable method to use for measuring transpiration of certain crop plants in the field, especially for small plots. This method is substantially improved if sap flow measurement is carried out on plants screened by the analysis of plant diameter distribution in the field.