Soil and Canopy Energy Balances of a Row Crop at Partial Cover

Abstract

When crops are grown in a row configuration, heat and mass transfer within the soil‐canopy system influence the energy and water balance of the crop. Field experiments were conducted near Lubbock, TX, to examine the energy balance of the soil and canopy separately, in cotton (Gossypium hirsutum L.) under a variety of aerial and soil moisture conditions. Bowen ratio techniques were used to obtain the field energy balance, including total latent heat flux (LE). Latent heat flux from the crop canopy (LEc was determined from sap flow measurements of transpiration. Latent heat flux from the soil (LEs) was computed as the daerence between LE and LEc. These measurements were coupled with radiation measurements at the soil surface to partition the energy balance into soil and canopy components every 12 min throughout the day. Results indicate that detailed measurements of energy exchange within the soil‐canopy‐atmosphere system can be obtained without making simplifying assumptions about energy transfer. Daily energy balances were strongly influenced by sensible heat transport, and the radiation balance alone did not account for the magnitude or diurnal pattern of LES and LEC. When the soil surface was dry, the canopy simultaneously absorbed sensible heat originating from the soil and above‐canopy air, accounting for more than 21 and 12% of LEC respectively. After an irrigation, LES accounted for more than 50% of LES even when the leaf area index was greater than two, and 11 to 21% of daily LEC occurred at night. The soil surface absorbed sensible heat from the canopy after irrigation, which increased LE1 while decreasing LEC. Analysis indicates that within‐canopy radiative and convective energy transfer must be considered to accurately characterize LES and LEC in row crops during periods of partial cover.

A Three-Dimensional Index for Characterizing Crop Water Stress

“…There was more exposed soil in 2008 than in 2007. The irrigated plots had a maximum of only 56% GC, with the exposed soil surface contributing to the increased canopy temperature through sensible heat transfer to the canopy and surrounding environment [5]. Development of the VITT approach for determining crop water stress was intended to account for partial crop canopy cover [6] in semi-arid regions, like the Texas High Plains.…”

“…It is common for irrigated production systems located in arid or semi-arid regions to not reach full ground canopy cover as a result of their relatively high evapotranspiration (ET) demand and lack of sufficient irrigation. For effectively using remote sensing imagery in CWSI-based irrigation scheduling, there is a need to separate the effects of the canopy and soil spectral responses [4] and account for the significant contribution of exposed soil in the surface energy balance of the crop [5].…”

Evapotranspiration Partitioning Techniques for Improved Water Use Efficiency

“…Latent heat fluxes from the canopy and the soil are complex processes governed by energy exchange between the soil, canopy, and the aerial environment. Investigating evaporation and energy exchanges in the crop field requires energy balance of the soil and the canopy to be examined separately (Ham et al, 1991). Several methods are used to measure or determine evapotranspiration (ET) components simultaneously.…”

“…Several methods are used to measure or determine evapotranspiration (ET) components simultaneously. Ham et al (1991) showed that energy balances of canopy and soil in a cotton field could be determined by combining sap flow with BREB measurements of transpiration (T) and evapotranspiration respectively. Also, ET in a drip-irrigated vineyard was determined from separate measurements of T by sap flow gauges and soil evaporation (E s ) by micro-lysimeters at various positions (Yunusa et al, 2004).…”

“…The BREB method is considered to be fairly robust for measuring ET (Steduto and Hsiao, 1998b), and has compared favorably with other methods, e.g., soil water balance (Malek and Bingham, 1993), aerodynamic method (Malek, 1993), eddy covariance method (Dugas et al, 1991), and weighing lysimeter (Prueger et al, 1997;Tanner, et al, 1960). The validity of this method has been established over various vegetation stands (Ham et al, 1991;Heilman et al, 1994), and natural vegetation (Kalthoff et al, 2006). Ashktorab et al (1989) used a microBowen ratio system for energy balance determination close to bare soil, and reported E s readings within 10 % of the weighing lysimeter measurements.…”