Wheat Canopy Resistance Determined by Energy Balance Techniques 1

Canopy resistances are important to evaluate plant environmental responses and difficult to measure in situ. This study was designed to evaluate a residual energy balance method for obtaining canopy resistances. A field study was conducted under natural rainfall conditions at Davis, CA during 1980 to 1981 on the Yolo silt loam soil (fine‐silty, mixed, nonacid, thermic Typic Xerorthent) with Anza wheat ( Triticum aestivum L.) seeded at 120 kg ha −1 with 80 kg ha −1 N supplied prior to planting. One‐half of the plots received unobstructed solar radiation, the other half had solar radiation reduced by a 55% shadecloth. From shortly after emergence to maturity, net radiation, air temperature, canopy temperature, incoming solar radiation, vapor pressure deficit, and windspeed were continually recorded. Canopy resistances were calculated from a residual of the energy balance. Canopy resistances obtained under optimal available soil water for both radiation treatments exhibited exponential decline with increasing solar radiation. Midday values of canopy resistance in the unshaded area were typically 20 to 25 s m −1 and 40 to 50 s m −1 in the shaded plot. Canopy resistance was 20 s m −1 under conditions which would be considered near potential evapotranspiration and, increased linearly with decreasing available soil water with a change in canopy resistance of 0.4 and 2.1 s m −1 /% available soil water for the unshaded and shaded, respectively. These were normalized by computing the ratio of canopy resistance under non‐optimal soil water contents to the canopy resistance influenced only by solar radiation. This allowed both solar radiation treatments to fit the same relationship and revealed that canopy resistance was not affected until 20% of the available soil water was removed. The residual energy balance method provides a valuable tool to quantify canopy resistance and will be useful to compare genotypes or management practices.