Dryland Evaporative Flux in a Subhumid Climate: II. Plant Influences

Seasonal variation in three plant factors and in evaporation rates were determined for dryland cotton and grain sorghum in central Texas. Plant factors measured were fractional ground cover, dry matter production, and leaf area index (LAI). When an adequate supply of soil water was available in the rhizosphere, plant factors influenced evaporation rates during most of the first half of the groiwng season until a “threshold” canopy LAI was obtained. A threshold canopy LAI is defined as the minumum LAI necessary to obtain 90% of potential evaporation (Eₒ) from row crops when the soil evaporation (Eₛ) is small. The threshold LAI was approximately 2.7 for both crops studied. After the threshold canopy LAI was obtained, evaporation was practically independent of plant factors and equal to Eₒ, as estimated from daily net radiation, until soil water movement to plant roots began to limit evaporation. Eₛ strongly influenced the actual evaporation rates obtained when plant canopy LAIs were below 2.7, To evaluate the influence of plant factors on the resulting plant evaporation (Eₚ) independent of Eₛ, it was necessary to attempt a rational separation of Eₛ, and Eₚ. Eₛ was assumed to be equal to the net radiation equivalent of evaporation below the plant canopy when the soil surface was wet. After the soil surface dried sufficiently to limit water movement to the surface, Eₛ was estimated from the measured soil water content near the surface. Eₚ was estimated using the measured evaporation and Eₛ. The ratio Eₚ/Eₒ was found to be related to the LAI according to the equation Eₚ/Eₒ = —0.21 + 0.70 LAI¹/², 0.1 ≤ LAI ≤ 2.7 This nonlinear relation indicates that Eₚ is larger per unit of leaf area for smaller plants than for larger plants with LAIs approaching 2.7. Sensible heat originating over partially dry surface soil between plant rows was a significant source of the total energy contributing to Eₚ The physiology stage of growth did not greatly influence the amount of energy used in evaporation as grain sorghum plants approached maturity when an adequate supply of soil water was available. The results imply a possibility for improving water-use efficiency of dryland crops by using higher plant populations and closer row spacings to decrease Eₛ if the soil water supply and seasonal rainfall is adequate to produce an economic yield.