Radiation balance and evaporation partitioning in a narrow-row soybean canopy

Seeding rate and row spacing of agricultural crops are managed to maximize yield but also have significant implications for energy partitioning and canopy microclimate. The objective of this study was to measure radiation budget components in a narrow-row soybean [Glycine max (L.) Merr.] canopy, determine soil water evaporation (E) as the difference between measured evapotranspiration (ET) and plant transpiration (T), and compare measured E and T with estimates from an energy balance approach. The study was completed in a production soybean field near Ames, IA, with 350,000 plants ha−1 in 0.38 m wide rows. The field was planted on 8 May 2004 and continuous measurements were made from 8 June to 27 September. Shortwave and longwave radiation components were measured above the canopy with hemispherical radiometers. Net all-wave and incoming and reflected shortwave radiation were measured beneath the canopy with line radiometers and downwelling longwave radiation was calculated from a radiation balance. ET was measured with an eddy covariance system and T was measured with sapflow stem gauges. On sunny days under full canopy conditions, nearly 90% of the shortwave and net radiation was attenuated by the canopy with over 80% of the available energy utilized by ET. E accounted for 8–12% of the ET under full canopy conditions although these percentages may be overestimates due to evaporation of dew present on the canopy. With such a large proportion of the available energy consumed by ET, fluxes of sensible heat (H) were very low and vertical temperature gradients across the soil–canopy–atmosphere interface were only 2–3 °C. A Priestley–Taylor energy balance approach used to estimate E and T tended to underestimate E and overestimate T. The E underestimate may be due in part to E including evaporation of dew. Conversely, T was overestimated by 25 and 14% on sunny days under full canopy conditions even with direct measurement of Rn interception by the canopy. Use of shortwave extinction coefficients to estimate Rn interception failed to improve T estimates significantly. The high-population, narrow-row planting strategy resulted in a dense canopy that, under full canopy conditions, resulted in very little light penetration or E.