ENERGY-TRANSFER OVER CROP CANOPIES - SIMULATION AND EXPERIMENTAL-VERIFICATION

The exchange of energy between the atmosphere and the surfaces of crops and soils stabilizes the thermal regimes at these surfaces, allowing sustained biological activity to occur. In this study, a simple Eulerian submodel of energy exchange was constructed from published algorithms in order to reproduce the dynamics of water and energy exchange between the soil-crop surface and the atmosphere as part of a larger agroecosystem model. Hourly output from the submodel was compared with data recorded over a soybean (Glycine max. L. Merr.) canopy at Mead, Nebraska on two dates during which soil water status differed. Recorded diurnal trends of leaf water potential and leaf stomatal resistance were reproduced in simulated diurnal trends of canopy water potential and canopy stomatal resistance on both dates. Under water stress, simulated canopy water potential was 0.1-0.2 MPa higher, and simulated canopy stomatal resistance approximately 25% lower, than recorded leaf values. Simulated fluxes of net radiative energy, and of latent, sensible, and soil heat were within 50 W m-2 of recorded values. Differences between simulated canopy and recorded air temperatures were consistent with those calculated at other sites under comparable atmospheric conditions. Inclusion of these algorithms in the agroecosystem model allowed a more comprehensive validation of the simulated transport of water and energy through the agroecosystem than would otherwise be possible.