Sunlit and Shaded Maize Canopy Water Loss Under Varied Water Stress

Abstract

The precise estimation of transpiration from plant canopies is important for the monitoring of crop water use and management of many agricultural operations related to water use planning. The aim of this study was to estimate transpiration from sunlit and shaded fractions of a maize ( Zea mays L.) canopy, using the Penman-Monteith energy balance equation with modifications introduced by Fuchs et al. (1987) and Fuchs & Cohen (1989). Estimated values were validated by a heat pulse system, which was used to measure stem sap flow and by a weighing lysimeter. A relationship between incident radiation and leaf stomatal conductance for critical levels of leaf water potential was used to estimate transpiration. Results showed that computed transpiration of the shaded canopy ranged from 27 to 45% of the total transpiration when fluctuations in atmospheric demand and the level of water stress were taken in account. Hourly and daily estimates of transpiration showed agreement with lysimeter and heat pulse measurements on the well-watered plots. For the water-limited plots the precision of the estimate decreased due to difficulties in simulating the canopy stomatal conductance.

Assessment of Corn Water Uptake Based on Sap Heat Tracing Under Water-Stressed Conditions

“…Overestimation by the model, in the hottest hours of the days of measurement, as shown in Figure 3, was due to difficulties of modeling the canopy conductance to the vapor transport, as observed by Santos et al (1999). 2.…”

“…Despite the measurement water uptake based on the sap heat tracing being based on idealized heat transport theory e.g. (Swanson & Whitfield, 1981) which is quite complex, the use of probe configuration and a calibration factor (Cohen et al, 1981;Santos et al, 1998) is a practical alternative to the solution of those formal heat transport equations. Figure 3 shows hourly values of water uptake and estimated transpiration.…”

“…After 7 to 10 days the blocks were removed and new plants were chosen in order to avoid stem damage due to over heating. Comparisons of the heat pulse technique outputs were made by using simultaneous estimates performed by a modified Penman-Monteith energy balance equation (P-M model) (Santos et al, 1999). Figure 2A shows diurnal course of measured maize water uptake and estimated transpiration rate under conditions of high atmospheric demand.…”

“…Sap flow determinations using a heat pulse as a tracer are based on heat conduction and convection in a homogeneous and isotropic medium (Swanson & Whitfield, 1981). For conduction:…”

“…A modified Penman-Monteith energy balance equation for sunlit and shaded canopy water loss estimation (Santos et al, 1999) was used to make comparisons with the data output from heat pulse.…”