ARS | Publication request: Radiation Balance and Evaporation Partitioning in a Narrow-row Soybean Canopy

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 26, 2007
Publication Date: July 27, 2007
Citation: Sauer, T.J., Singer, J.W., Prueger, J.H., Desutter, T.M., Hatfield, J.L. 2007. Radiation Balance and Evaporation Partitioning in a Narrow-row Soybean Canopy. Agricultural and Forest Meteorology. 145:206-214.

Interpretive Summary: How plants intercept sunlight effects how fast they grow and, for crops, how much they yield. Growers try different numbers of seeds per acre and row widths to find the best number and spacing of plants to maximize yield. In recent years, soybeans have been grown with more plants per acre in narrower rows. There is some concern that having so many plants too close together may have negative effects on greenhouse gas emissions or plant disease outbreaks. A study was completed to measure light capture by a high-density, narrow-row soybean canopy and how it effects water use. Results show that when the canopy is at its fullest the dense foliage captures almost 90% of the sunlight and that most of this energy is used to evaporate water from the plant leaves. Only a small amount of energy reaches the soil surface to evaporate water there. There was also little energy left to warm the canopy. A method to estimate the evaporation terms was inaccurate because it could not account the evaporation of dew from the leaves. This study is important to growers and scientists because it demonstrates that high plant population of soybean crops creates such a thick canopy that little light can pass through. This could create problems as, for example, plant diseases often prefer warm, moist canopies.

Technical Abstract: Seeding rate and row spacing of agricultural crops are managed to maximize yield but also have significant implications for canopy microclimate and turbulent exchange processes. The objective of this study was to complete a comprehensive radiation budget for a narrow-row (0.38-m) soybean [Glycine max (L.) Merr.] canopy, measure the available energy beneath the canopy, and estimate soil water evaporation (E) for comparison with E determined as the difference between measured evapotranspiration (ET) and plant transpiration (T). The study was completed in a production soybean field near Ames, IA with 350,000 plants ha-1 of Pioneer cultivar 92M70. The field was planted on May 8 and continuous microclimate measurements were made from June 8 to September 27. Net, shortwave, and longwave radiation were measured above the canopy with hemispherical radiometers. Net 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 six stem flow gauges attached to representative plants. 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 12 - 21% of the ET under full canopy conditions although these percentages may be overestimates due to evaporation of dew present in 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 degrees C. A Priestley-Taylor energy balance approach used to estimate E and T consistently underestimated E and overestimated T. The E underestimate by 0.1 - 0.5 mm may be due in part to E including evaporation of dew present on the canopy. Conversely, T was overestimated by 25% and 33% on sunny days under full canopy conditions even with direct measurement of Rn interception by the canopy. Further effort will be necessary to discern the cause for this discrepancy and will be needed if accurate simulation of ET partitioning is to be achieved.