Transpiration Quantified by Sap Flow Heat Gauges and the Penman-Monteith Equation.

In situ measure of crop transpiration can enhance field studies of crop water use and productivity. Sap flow heat gauges (five), controlled by an automated data acquisition system, were deployed in each of four replicated field plots of corn irrigated to minimize water deficits at Garden City, KS in 2004 and 2006; apparent water flux through each stem was calculated as a residual of a heat balance equation. Gauges were transferred to adjacent plants at bi-weekly intervals. Operational metrics of gauge performance include consistency of calculation of an empirical gauge constant, uncertainty of gauge energy balance components in relation to calculated flow, and relation of calculated flow to transpiration calculated following the Penman-Monteith (P-M) form. Apparent transpiration calculated from gauge data (T_SFHG) was linearly related to that calculated from the P-M, but with negative bias in offset. Coefficients of variation in T_SFHG within plots (21%) and among plots (24%) were numerically similar. Gauge installation impaired grain yield formation; gauges installed at anthesis reduced seed set among 33% of plants and reduced seed mass among these and an additional 13% of plants; yield formation was impaired for 13% and 7% of plants, when gauges were installed two and four weeks following anthesis, respectively.