SOIL AND CANOPY ENERGY BALANCES OF A ROW CROP AT PARTIAL COVER

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 (LE(c)) was determined from sap flow measurements of transpiration. Latent heat flux from the soil (LE(s)) was computed as the difference between LE and LE(c). 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 LE(s) and LE(c). 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 LE(c), respectively. After an irrigation, LE(s) accounted for more than 50% of LE even when the leaf area index was greater than two, and 12 to 21% of daily LE(s) occurred at night. The soil surface absorbed sensible heat from the canopy after irrigation, which increased LE(s) while decreasing LE(c). Analysis indicates that within-canopy radiative and convective energy transfer must be considered to accurately characterize LE(s) and LE(c) in row crops during periods of partial cover.